Warning Systems – B737

Warning Lights

     

Master Caution and System Annunciator lights, left and right.

 

The Master Caution system was developed for the 737 to ease pilot workload as it was the first Boeing airliner to be produced without a flight engineer. In simple terms it is an attention getter that also directs the pilot toward the problem area concerned. The system annunciators (shown above) are arranged such that the cautions are in the same orientation as the overhead panel e.g. FUEL bottom left, DOORS bottom of third column, etc.

On the ground, the master caution system will also tell you if the condition is dispatchable or if the QRH needs to be actioned. The FCOM gives the following guidance on master caution illuminations on the ground:

Before engine start, use individual system lights to verify the system status. If an individual system light indicates an improper condition:
• check the Dispatch Deviations Procedures Guide (DDPG) or the operator equivalent to decide if the condition has a dispatch effect
• decide if maintenance is needed

If, during or after engine start, a red warning or amber caution light illuminates:
• do the respective non-normal checklist (NNC)
• on the ground, check the DDPG or the operator equivalent

If, during recall, an amber caution illuminates and then extinguishes after a master caution reset:
• check the DDPG or the operator equivalent
• the respective non-normal checklist is not needed

Pressing the system annunciator will show any previously cancelled or single channel cautions. If a single channel caution is encountered, the QRH drill should not be actioned.

Master caution lights and the system annunciator are powered from the battery bus and will illuminate when an amber caution light illuminates. Exceptions to this include a single centre fuel tank LOW PRESSURE light (requires both), REVERSER lights (requires 12 seconds) and INSTR SWITCH (inside normal FoV).

When conducting a light test, during which the system will be inhibited, both bulbs of each caution light should be carefully checked. The caution lights are keyed to prevent them from being replaced incorrectly, but may be interchanged with others of the same caption.

Keying of warning lights

 

  • Red lights – Warning – indicate a critical condition and require immediate action.
  • Amber lights – Caution – require timely corrective action.
  • Blue lights – Advisory – eg valve positions and unless bright blue, ie a valve/switch disagreement, do not require crew action.
  • Green lights – Satisfactory – indicate a satisfactory or ON condition.

 

 

Aural Warnings

Cockpit aural warnings include the fire bell, take-off configuration warning, cabin altitude, landing gear configuration warning, mach/airspeed overspeed, stall warning, GPWS and TCAS. External aural warnings are: The fire bell in the wheel well and the ground call horn in the nose wheel-well for an E & E bay overheat or IRS’s on DC. Only certain warnings can be silenced whilst the condition exists.

To test the GPWS, ensure that the weather radar is on in TEST mode and displayed on the EHSI. Pressing SYS TEST quickly will give a short confidence test, pressing for 10 seconds will give a full vocabulary test.

The GPWS pane. Click photo to hear the GPWS vocabulary test. (175kb)

AURAL WARNING PRIORITY LOGIC
MODE PRIORITY DESCRIPTION ALERT LEVEL
7 1 WINDSHEAR WINDSHEAR WINDSHEAR W
1 2 PULL-UP (SINK RATE) W
2 3 PULL-UP (TERRAIN CLOSURE) W
2A 4 PULL-UP (TERRAIN CLOSURE) W
V1 5 V1 CALLOUT I
TA 6 TERRAIN TERRAIN PULL-UP W
WXR 7 WINDSHEAR AHEAD W
2 8 TERRAIN TERRAIN C
6 9 MINIMUMS I
TA 10 CAUTION TERRAIN C
4 11 TOO LOW TERRAIN C
TCF 12 TOO LOW TERRAIN C
6 13 ALTITUDE CALLOUTS I
4 14 TOO LOW GEAR C
4 15 TOO LOW FLAPS C
1 16 SINK RATE C
3 17 DONT SINK C
5 18 GLIDESLOPE C
WXR 19 MONITOR RADAR DISPLAY C
6 20 APPROACHING MINIMUMS I
6 21 BANK ANGLE C
TCAS 22 RA (CLIMB, DESCEND, ETC.) W
TCAS 23 TA (TRAFFIC, TRAFFIC) C
TEST 24 BITE AND MAINTENANCE INFORMATION I

Radio Altimeter Callouts

Automatic rad-alt calls are a customer option on the 3-900 series. Calls can include any of the following:

2500 (“Twenty Five Hundred” or “Radio Altimeter”).
1000
500
400
300
200
100
50
40
30
20
10

“Minimums” or “Minimums, Minimums”
“Plus Hundred” when 100ft above DH
“Approaching Minimums” when 80ft above DH
“Approaching Decision Height”
“Decision Height”

Customers can also request special heights, such as 60ft.


 

Noise Levels

If is often commented how loud these callouts are. The volume level for these callouts and any other aural warnings is set so that they can still be audible at the highest ambient noise levels, this is considered to be when the aircraft is at Vmo (340kts) at 10,000ft.

The design sound pressure level at 35,000ft, M0.74, cruise thrust is 87dB at the Captains seat, compared to 90-93dB in the cabin.

Many pilots consider the 737 flightdeck to be generally loud. This is Boeings response to that charge:

“Using the flight deck noise levels measured by Boeing Noise Engineering during a typical flight profile (entire flight), a daily A-weighted sound exposure was calculated using ISO/DIS 1999 standards. This calculation indicates the time weight noise exposure is below 80 db(A) and should not cause hearing damage. Flight deck noise improvement continues to be a part of current Boeing product quality improvement activities.”

And when asked later about the particularly noisy NG:

“Boeing has conducted extensive flight tests to define the contributing noise sources for the 737 Flight Deck. Subsequently, various system and hardware modifications have been evaluated for possible improvements. Currently there are no proposed changes where the benefits are significant enough to warrant incorporation. Additional candidates are currently under study and if their merit is validated, they could be incorporated at a later date during production and retrofit.”

That said, in 2005 Boeing added 10 small vortex generators at the base of the windscreen which reduce flightdeck aerodynamic noise by 3dB. (See fuselage page for photo).

 

 

Stall Warning

Stall warning test requires AC power. Also, with no hydraulic pressure, the leading edge flaps may droop enough to cause an asymmetry signal, resulting in a failure of the stall warning system test. If this happens, switch the “B” system electric pump ON to fully retract all flaps and then repeat the test.

System test switches on the aft overhead panel

The 737-1/200’s had a different stall warning panel as shown right:

The OFF light may indicate either a failure of the heater of the angle of attack sensor a system signal failure or a power failure.

The test disc should rotate, indicating electrical continuity, when the switch is held to the test position.

737-200 Stall Warning Panel

 

TCAS

Various versions of TCAS have been fitted to the 737 since its introduction in the 1990’s. The early days of TCAS there were different methods of displaying the visuals. For the Honeywell system (Previously AlliedSignal, previous to that – Bendix/King), their most popular method for non-EFIS airplanes was to install an RA/VSI which was a mechanical VSI that had the “eyebrows” on the outer edge directing the pilot to climb (green) or stay away from (red) and use the separate Radar Indicator for the basic traffic display. Even early EFIS aircraft had the RA/VSI (see photos left & right)

TCAS is now integrated at production into the EFIS displays. The PFD/EADI will display advisories to climb, descend, or stay level since they give the vertical cue to the pilot. The ND/EHSI provides the map view looking down to show targets and their relative altitude and vertical movement relative to your aircraft.

TCAS display integrated onto the ND

 

TCAS control is from the transponder panel.

 

Weather Radar

The beamwidth of the 737 weather radar is 3.5 degrees.

To calculate the height of the cloud tops above your altitude use the following formula:

Cloud tops above a/c (ft) = range (nm) x (tilt – 1.5 deg) x 100

eg Wx at range 40nm stops painting at +2deg tilt. The tops would be 40 x 0.5 x 100 = 2000ft above your level.

Weather radar or terrain can be overlaid onto the EHSI with these switches on the classics. In the NG the overlay switches are part of the EFIS control panel. The colours may appear similar but their meanings are very different.

737 NG’s are fitted with predictive windshear system (PWS). This is available below 2300ft. You do not need weather radar to be switched on for PWS to work, since it switches on automatically when take-off thrust is set. However there is a 12 sec warm up period, so if you want PWS available for the take-off you should switch the weather radar on when you line up.

Windshear warning displayed on the ND. Notice the cone and range at which windshear is predicted.


EGPWS – Peaks Display

The Peaks display overlays EGPWS terrain information onto the EHSI. The colour coding is similar to wx radar but with several densities of each colour being used. The simplified key is:

 

Color Altitude Diff from Aircraft (ft)
Black No terrain
Cyan Zero ft MSL (Customer option)
Green -2000 to +250
Yellow -500 to +2000
Red +2000 or above
Magenta Terrain elevation unknown

The two overlaid numbers are the highest and lowest terrain elevations, in hundreds of ft amsl, currently being displayed. Here 5900ft and 800ft amsl. One of the main difference between Peaks display and others is that it will show terrain more than 2000ft below your level (eg a mountain range from cruise altitude). This can be very useful for situational awareness.

EGPWS Limitations

  • Do not use the terrain display for navigation.
  • Do not use within 15nm of an airfield not in the terrain database.

Honeywell EGPWS Pilots notes

 

PSEUProximity Switch Electronic Unit

The Proximity Switch Electronic Unit (PSEU) is a system that communicates the position or state of system components eg flaps, gear, doors, etc to other systems. The 737-NG’s are fitted with a PSEU which controls the following systems: Take-off and landing configuration warnings, landing gear transfer valve, landing gear position indicating and warning, air/ground relays, airstairs & door warnings and speedbrake deployed warning.

The PSEU light is inhibited from when the thrust levers are set for take-off power (thrust lever angle beyond 53 degrees) until 30 seconds after landing. If the PSEU light illuminates, you have a “non-dispatchable fault” and the QRH says do not take-off. In this condition the PSEU light can only be extinguished by fixing the fault. However if you only get the PSEU light on recall, you have a “dispatchable fault” which it is acceptable to go with. In this condition the PSEU light will extinguish when master caution is reset.

SFP aircraft (-800SFP / -900ER) also have an SPSEU which monitors the 2 position tailskid.

Warning Lights

     

Master Caution and System Annunciator lights, left and right.

The Master Caution system was developed for the 737 to ease pilot workload as it was the first Boeing airliner to be produced without a flight engineer. In simple terms it is an attention getter that also directs the pilot toward the problem area concerned. The system annunciators (shown above) are arranged such that the cautions are in the same orientation as the overhead panel e.g. FUEL bottom left, DOORS bottom of third column, etc.

On the ground, the master caution system will also tell you if the condition is dispatchable or if the QRH needs to be actioned. The FCOM gives the following guidance on master caution illuminations on the ground:

Before engine start, use individual system lights to verify the system status. If an individual system light indicates an improper condition:
• check the Dispatch Deviations Procedures Guide (DDPG) or the operator equivalent to decide if the condition has a dispatch effect
• decide if maintenance is needed

If, during or after engine start, a red warning or amber caution light illuminates:
• do the respective non-normal checklist (NNC)
• on the ground, check the DDPG or the operator equivalent

If, during recall, an amber caution illuminates and then extinguishes after a master caution reset:
• check the DDPG or the operator equivalent
• the respective non-normal checklist is not needed

Pressing the system annunciator will show any previously cancelled or single channel cautions. If a single channel caution is encountered, the QRH drill should not be actioned.

Master caution lights and the system annunciator are powered from the battery bus and will illuminate when an amber caution light illuminates. Exceptions to this include a single centre fuel tank LOW PRESSURE light (requires both), REVERSER lights (requires 12 seconds) and INSTR SWITCH (inside normal FoV).

When conducting a light test, during which the system will be inhibited, both bulbs of each caution light should be carefully checked. The caution lights are keyed to prevent them from being replaced incorrectly, but may be interchanged with others of the same caption.

Keying of warning lights

  • Red lights – Warning – indicate a critical condition and require immediate action.
  • Amber lights – Caution – require timely corrective action.
  • Blue lights – Advisory – eg valve positions and unless bright blue, ie a valve/switch disagreement, do not require crew action.
  • Green lights – Satisfactory – indicate a satisfactory or ON condition.

Aural Warnings

Cockpit aural warnings include the fire bell, take-off configuration warning, cabin altitude, landing gear configuration warning, mach/airspeed overspeed, stall warning, GPWS and TCAS. External aural warnings are: The fire bell in the wheel well and the ground call horn in the nose wheel-well for an E & E bay overheat or IRS’s on DC. Only certain warnings can be silenced whilst the condition exists.

To test the GPWS, ensure that the weather radar is on in TEST mode and displayed on the EHSI. Pressing SYS TEST quickly will give a short confidence test, pressing for 10 seconds will give a full vocabulary test.

The GPWS pane. Click photo to hear the GPWS vocabulary test. (175kb)

AURAL WARNING PRIORITY LOGIC
MODE PRIORITY DESCRIPTION ALERT LEVEL
7 1 WINDSHEAR WINDSHEAR WINDSHEAR W
1 2 PULL-UP (SINK RATE) W
2 3 PULL-UP (TERRAIN CLOSURE) W
2A 4 PULL-UP (TERRAIN CLOSURE) W
V1 5 V1 CALLOUT I
TA 6 TERRAIN TERRAIN PULL-UP W
WXR 7 WINDSHEAR AHEAD W
2 8 TERRAIN TERRAIN C
6 9 MINIMUMS I
TA 10 CAUTION TERRAIN C
4 11 TOO LOW TERRAIN C
TCF 12 TOO LOW TERRAIN C
6 13 ALTITUDE CALLOUTS I
4 14 TOO LOW GEAR C
4 15 TOO LOW FLAPS C
1 16 SINK RATE C
3 17 DONT SINK C
5 18 GLIDESLOPE C
WXR 19 MONITOR RADAR DISPLAY C
6 20 APPROACHING MINIMUMS I
6 21 BANK ANGLE C
TCAS 22 RA (CLIMB, DESCEND, ETC.) W
TCAS 23 TA (TRAFFIC, TRAFFIC) C
TEST 24 BITE AND MAINTENANCE INFORMATION I

Radio Altimeter Callouts

Automatic rad-alt calls are a customer option on the 3-900 series. Calls can include any of the following:

2500 (“Twenty Five Hundred” or “Radio Altimeter”).
1000
500
400
300
200
100
50
40
30
20
10

“Minimums” or “Minimums, Minimums”
“Plus Hundred” when 100ft above DH
“Approaching Minimums” when 80ft above DH
“Approaching Decision Height”
“Decision Height”

Customers can also request special heights, such as 60ft.


Noise Levels

If is often commented how loud these callouts are. The volume level for these callouts and any other aural warnings is set so that they can still be audible at the highest ambient noise levels, this is considered to be when the aircraft is at Vmo (340kts) at 10,000ft.

The design sound pressure level at 35,000ft, M0.74, cruise thrust is 87dB at the Captains seat, compared to 90-93dB in the cabin.

Many pilots consider the 737 flightdeck to be generally loud. This is Boeings response to that charge:

“Using the flight deck noise levels measured by Boeing Noise Engineering during a typical flight profile (entire flight), a daily A-weighted sound exposure was calculated using ISO/DIS 1999 standards. This calculation indicates the time weight noise exposure is below 80 db(A) and should not cause hearing damage. Flight deck noise improvement continues to be a part of current Boeing product quality improvement activities.”

And when asked later about the particularly noisy NG:

“Boeing has conducted extensive flight tests to define the contributing noise sources for the 737 Flight Deck. Subsequently, various system and hardware modifications have been evaluated for possible improvements. Currently there are no proposed changes where the benefits are significant enough to warrant incorporation. Additional candidates are currently under study and if their merit is validated, they could be incorporated at a later date during production and retrofit.”

That said, in 2005 Boeing added 10 small vortex generators at the base of the windscreen which reduce flightdeck aerodynamic noise by 3dB. (See fuselage page for photo).

Stall Warning

Stall warning test requires AC power. Also, with no hydraulic pressure, the leading edge flaps may droop enough to cause an asymmetry signal, resulting in a failure of the stall warning system test. If this happens, switch the “B” system electric pump ON to fully retract all flaps and then repeat the test.

System test switches on the aft overhead panel

The 737-1/200’s had a different stall warning panel as shown right:

The OFF light may indicate either a failure of the heater of the angle of attack sensor a system signal failure or a power failure.

The test disc should rotate, indicating electrical continuity, when the switch is held to the test position.

737-200 Stall Warning Panel

TCAS

Various versions of TCAS have been fitted to the 737 since its introduction in the 1990’s. The early days of TCAS there were different methods of displaying the visuals. For the Honeywell system (Previously AlliedSignal, previous to that – Bendix/King), their most popular method for non-EFIS airplanes was to install an RA/VSI which was a mechanical VSI that had the “eyebrows” on the outer edge directing the pilot to climb (green) or stay away from (red) and use the separate Radar Indicator for the basic traffic display. Even early EFIS aircraft had the RA/VSI (see photos left & right)

TCAS is now integrated at production into the EFIS displays. The PFD/EADI will display advisories to climb, descend, or stay level since they give the vertical cue to the pilot. The ND/EHSI provides the map view looking down to show targets and their relative altitude and vertical movement relative to your aircraft.

TCAS display integrated onto the ND

 

TCAS control is from the transponder panel.

Weather Radar

The beamwidth of the 737 weather radar is 3.5 degrees.

To calculate the height of the cloud tops above your altitude use the following formula:

Cloud tops above a/c (ft) = range (nm) x (tilt – 1.5 deg) x 100

eg Wx at range 40nm stops painting at +2deg tilt. The tops would be 40 x 0.5 x 100 = 2000ft above your level.

Weather radar or terrain can be overlaid onto the EHSI with these switches on the classics. In the NG the overlay switches are part of the EFIS control panel. The colours may appear similar but their meanings are very different.

737 NG’s are fitted with predictive windshear system (PWS). This is available below 2300ft. You do not need weather radar to be switched on for PWS to work, since it switches on automatically when take-off thrust is set. However there is a 12 sec warm up period, so if you want PWS available for the take-off you should switch the weather radar on when you line up.

Windshear warning displayed on the ND. Notice the cone and range at which windshear is predicted.


EGPWS – Peaks Display

The Peaks display overlays EGPWS terrain information onto the EHSI. The colour coding is similar to wx radar but with several densities of each colour being used. The simplified key is:

Color Altitude Diff from Aircraft (ft)
Black No terrain
Cyan Zero ft MSL (Customer option)
Green -2000 to +250
Yellow -500 to +2000
Red +2000 or above
Magenta Terrain elevation unknown

The two overlaid numbers are the highest and lowest terrain elevations, in hundreds of ft amsl, currently being displayed. Here 5900ft and 800ft amsl. One of the main difference between Peaks display and others is that it will show terrain more than 2000ft below your level (eg a mountain range from cruise altitude). This can be very useful for situational awareness.

EGPWS Limitations

  • Do not use the terrain display for navigation.
  • Do not use within 15nm of an airfield not in the terrain database.

Honeywell EGPWS Pilots notes

PSEUProximity Switch Electronic Unit

The Proximity Switch Electronic Unit (PSEU) is a system that communicates the position or state of system components eg flaps, gear, doors, etc to other systems. The 737-NG’s are fitted with a PSEU which controls the following systems: Take-off and landing configuration warnings, landing gear transfer valve, landing gear position indicating and warning, air/ground relays, airstairs & door warnings and speedbrake deployed warning.

The PSEU light is inhibited from when the thrust levers are set for take-off power (thrust lever angle beyond 53 degrees) until 30 seconds after landing. If the PSEU light illuminates, you have a “non-dispatchable fault” and the QRH says do not take-off. In this condition the PSEU light can only be extinguished by fixing the fault. However if you only get the PSEU light on recall, you have a “dispatchable fault” which it is acceptable to go with. In this condition the PSEU light will extinguish when master caution is reset.

SFP aircraft (-800SFP / -900ER) also have an SPSEU which monitors the 2 position tailskid.

Navigation – B737

Position

The aircraft has several nav positions, many of which are in use simultaneously! They can all be seen on the POS REF page of the FMC.

IRS L & IRS R Position: Each IRS computes its own position independently; consequently they will diverge slightly during the course of the flight. After the alignment process is complete, there is no updating of either IRS positions from any external sources. Therefore it is important to set the IRS position accurately in POS INIT.

GPS L & GPS R Position: (NG only) The FMC uses GPS position as first priority for FMC position updates. Note this allows the FMC to position update accurately on the ground, eg if no stand position is entered in POS INIT. This practically eliminates the need to enter a take-off shift in the TAKE-OFF REF page.

Radio Position: This is computed automatically by the FMC. Best results are achieved with both Nav boxes selected to AUTO (happens automatically on NG), thus allowing the FMC to select the optimum DME or VOR stations required for the position fix. Series 500 aircraft have an extra dedicated DME interogator (hidden) for this purpose and NG’s have two. Radio position is found from either a pair of DME stations that have the best range and geometry or from DME/VOR or even DME/LOC.The NAV STATUS page shows the current status of the navaids being tuned. Navaids being used for navigation (ie radio position) are highlighted (here WTM & OTR).

 

FMC Position: FMC navigational computations & LNAV are based upon this. The FMC uses GPS position (NG’s only) as first priority for FMC position updates, it will even position update on the ground. If GPS is not available, FMC position is biased approximately 80:20 toward radio position and IRS L. When radio updating is not available, an IRS NAV ONLY message appears. The FMC will then use a “most probable” position based on the IRS position error as found during previous monitoring when a radio position was available. The FMC position should be closely monitored if IRS NAV ONLY is in use for long periods.The POS SHIFT page shows the bearing & distance of other systems positions away from the FMC position. Use this page to force the FMC position to any of those offered.

 

RNP/ACTUAL

Actual Navigation Performance (ANP) is the FMC’s estimate of the quality of its position determination. The FMC is 95% certain the the aircraft’s actual position lies within a circle of radius ANP centred on the FMC position. Therefore the lower the ANP, the more confident the FMC is of its position estimate.

Required Navigation Performance (RNP) is the desired limit of navigational accuracy and is specified by the kind of airspace you are in. Eg for BRNAV above FL150, RNP=2.00nm. The RNP may be overwritten by crew.

ACTUAL should always be less than RNP.

If a navaid or GPS system is unreliable or giving invalid data then they can be inhibited using the NAV OPTIONS page.
There is an AFM limitation prohibiting use of LNAV when operating in QFE airspace. This is because several ARINC 424 leg types used in FMC nav databases terminate at MSL altitudes. If baro set is referenced to QFE, these legs will sequence at the wrong time and can lead to navigational errors.


EHSI & Navigation Display (ND)

 

 

EFIS Control Panel - Click to see description737-3/4/500 EFIS Control Panel

737-NG EFIS Control Panel
In the NG, if an EFIS control panel fails, you will get a DISPLAYS CONTROL PANEL annunciation on the ND. There is an additional, rather bizarre, attention getter because the altimeter will blank on the failed side, with an ALT flag, until the DISPLAYS – CONTROL PANEL switch is positioned to the good side. Note that this is not the same as the EFI switch on the -3/4/500’s which was used to switch symbol generators.

 

The -3/4/500 Electronic Horizontal Situation Indicator (Map mode) 737-NG Navigation Display (Map mode)
EHSI – Nav EHSI – Plan
EHSI – Full VOR/ILS EHSI – Expanded VOR/ILS
EHSI – Map

 

EHSI – Center Map

 

*** WARNING ***

The The ND DME readout below the VOR may not necessarily be that of the VOR which is displayed.

This photograph shows that Nav 1 has been manually tuned to 110.20 as shown in 1L of the FMC. DVL VOR identifier has been decoded by the auto-ident facility so “DVL” is displayed in large characters both on the FMC and the bottom left of the ND. Below this is displayed “DME 128” implying that this is the DME from DVL VOR.

However it can be seen on the ND that the DVL VOR is only about 70nm ahead. In fact DVL is only a VOR station and it has no DME facility, the DME was from another station on 110.20. The second station could be identified aurally by the higher pitched tone as “LRH” but was not displaying as such in line 2L of the FMC.

I only discovered this by chance as I happened to be following the aircraft progress by tuning beacons en-route (the way we used to do!). In my opinion, this illustrates the need to aurally identify any beacons, particularly DME, you may have to use, even if they are displayed as decoded.


 

Instrument Transfer

If either Nav receiver fails, the VHF NAV transfer switch may be used to display the functioning Nav information onto both EFIS and RDMI’s. With Nav transferred, the MCP course selector on the serviceable side becomes the master, but all other EFIS selections remain independent.

If an IRS fails, the IRS transfer switch is used to switch all associated systems to the functioning IRS.

1/200

3/4/500

NG’s


IRS Malfunction Codes (Classics)

Align Annunciator Malfunction Code Significance of Annunciator or Malfunction Code Recommended Action
Flashing (after 10 mins) None Failed align requirement Verify and re-enter present position
01 ISDU failed power up RAM test Replace ISDU
Steady 02 Entered latitude disagrees with latitude calculated by IRU Verify and re-enter present position. If fault persists do full align or replace IRU
02 IRU failure Replace IRU
Flashing 03 Excessive motion during align Restart a full align
Flashing (During full align) 04 Lat or Long entered is not within 1 degree of stored value Re-enter the identical position to the last position entered.
Flashing (During fast realign) 04 Lat is not within 1/2 degree or Long not within 1 deg of stored value Enter known accurate present position. If align light continues to flash, do full align.
05 Left DAA is transmitting a fault Replace left DAA.
06 Right DAA is transmitting a fault Replace right DAA.
07 Selected IRU has detected an invalid air data input. Replace DADC.
Flashing (after 10 mins) 08 Present position has not been entered Enter present position
Steady 09 Attitude mode has been selected Restart a full align. NB if ATT mode is desired, enter magnetic heading in POS INIT 1/2.
10 ISDU is not receiving power from both IRU’s. Ensure that both IRU’s are ON and receiving power.

Alternate Navigation System – ANS (If installed)

This is an option for the -3/4/500 series. ANS is an IRS based system which provides lateral navigation capability independent of the FMC. The ANS with the Control Display Units (AN/CDU) can be operated in parallel with the FMC for an independent cross-check of FMC/CDU operation.

 

Navigation Mode Selectors

The ANS is two separate systems, ANS-L & ANS-R. Each consists of its own AN/CDU and “on-side” IRS.

Each pilot has his own navigation mode selector to specify the source of navigation information to his EFIS symbol generator and flight director.

The ANS also performs computations related to lateral navigation which can provide LNAV commands to the AFDS in the event of an FMC failure.
  The IRS PROGRESS page is similar to the normal PROGRESS page except that all data is from the “on-side” IRS (L in this example).
AN/CDU Pages AN/CDU has no performance or navigation database. All waypoints must therefore be defined in terms of lat & long. The AN/CDU memory can only store 20 waypoints, these can be entered on the ground or in-flight and may be taken from FMC data using the CROSSLOAD function.

Future

In Jan 2003, the 737 became available with three new flight-deck technologies: Vertical Situation Display (VSD), Navigation Performance Scales (NPS) and Integrated Approach Navigation (IAN).

The Vertical Situation Display shows the current and predicted flight path of the airplane and indicates potential conflicts with terrain.

Navigation Performance Scales NPS use vertical and horizontal indicators to provide precise position awareness on the primary flight displays to will allow the aircraft to navigate through a narrower flight path with higher accuracy.

The Integrated Approach Navigation enhances current airplane landing approach capability by simplifying pilot procedures and potentially reducing the number of approach procedures pilots have learned in training.

For more information about NPS and IAN see the section on Flight Instruments.

 

Vertical Situation Display Vertical Situation Display

The VSD, now certified on NG’s, gives a graphical picture of the aircraft’s vertical flight path. The aim to is reduce the number of CFIT accidents; profile related incidents, particularly on non-precision approaches and earlier recognition of unstabilised approaches.

The VSD works with the Terrain Awareness and Warning System (TAWS) to display a vertical profile of the aircrafts predicted flight path (shown between the blue dashes) on the lower section of the ND. It is selected on with the DATA button on the EFIS control panel.

VSD can be retrofitted into any NG but it requires software changes to the displays and FMC and also some additional hardware displays.

Click here for presentation on VSD


ETOPS

In 1953, the United States developed regulations that prohibited two-engine airplanes from routes more than 60 min single-engine flying time from an adequate airport (FAR 121.161). These regulations were introduced based upon experience with the airliners of the time ie piston engined aircraft, which were much less reliable than modern jet aircraft. Nevertheless, the rule still stands.

ETOPS allows operators to deviate from this rule under certain conditions. By incorporating specific hardware improvements and establishing specific maintenance and operational procedures, operators can fly extended distances up to 180 min from the alternate airport. These hardware improvements were designed into Boeing 737-600/700/800/900.

The following table gives some FAA ETOPS approval times & dates:

Aircraft Series Engine ETOPS-120 approval date ETOPS-180 approval date
737-200 JT8D -9/9A Dec 1985
JT8D -15/15A Dec 1986
JT8D -17/17A Dec 1986
737-300/400/500 CFM56-3 Sept 1990
737-600/700/800/900 CFM56-7 Sept 1999
737-BBJ1/BBJ2 CFM56-7 Sept 1999

Position

The aircraft has several nav positions, many of which are in use simultaneously! They can all be seen on the POS REF page of the FMC.

IRS L & IRS R Position: Each IRS computes its own position independently; consequently they will diverge slightly during the course of the flight. After the alignment process is complete, there is no updating of either IRS positions from any external sources. Therefore it is important to set the IRS position accurately in POS INIT.

GPS L & GPS R Position: (NG only) The FMC uses GPS position as first priority for FMC position updates. Note this allows the FMC to position update accurately on the ground, eg if no stand position is entered in POS INIT. This practically eliminates the need to enter a take-off shift in the TAKE-OFF REF page.

Radio Position: This is computed automatically by the FMC. Best results are achieved with both Nav boxes selected to AUTO (happens automatically on NG), thus allowing the FMC to select the optimum DME or VOR stations required for the position fix. Series 500 aircraft have an extra dedicated DME interogator (hidden) for this purpose and NG’s have two. Radio position is found from either a pair of DME stations that have the best range and geometry or from DME/VOR or even DME/LOC.The NAV STATUS page shows the current status of the navaids being tuned. Navaids being used for navigation (ie radio position) are highlighted (here WTM & OTR).

FMC Position: FMC navigational computations & LNAV are based upon this. The FMC uses GPS position (NG’s only) as first priority for FMC position updates, it will even position update on the ground. If GPS is not available, FMC position is biased approximately 80:20 toward radio position and IRS L. When radio updating is not available, an IRS NAV ONLY message appears. The FMC will then use a “most probable” position based on the IRS position error as found during previous monitoring when a radio position was available. The FMC position should be closely monitored if IRS NAV ONLY is in use for long periods.The POS SHIFT page shows the bearing & distance of other systems positions away from the FMC position. Use this page to force the FMC position to any of those offered.

RNP/ACTUAL

Actual Navigation Performance (ANP) is the FMC’s estimate of the quality of its position determination. The FMC is 95% certain the the aircraft’s actual position lies within a circle of radius ANP centred on the FMC position. Therefore the lower the ANP, the more confident the FMC is of its position estimate.

Required Navigation Performance (RNP) is the desired limit of navigational accuracy and is specified by the kind of airspace you are in. Eg for BRNAV above FL150, RNP=2.00nm. The RNP may be overwritten by crew.

ACTUAL should always be less than RNP.

If a navaid or GPS system is unreliable or giving invalid data then they can be inhibited using the NAV OPTIONS page.
There is an AFM limitation prohibiting use of LNAV when operating in QFE airspace. This is because several ARINC 424 leg types used in FMC nav databases terminate at MSL altitudes. If baro set is referenced to QFE, these legs will sequence at the wrong time and can lead to navigational errors.


EHSI & Navigation Display (ND)

 

EFIS Control Panel - Click to see description737-3/4/500 EFIS Control Panel

737-NG EFIS Control Panel
In the NG, if an EFIS control panel fails, you will get a DISPLAYS CONTROL PANEL annunciation on the ND. There is an additional, rather bizarre, attention getter because the altimeter will blank on the failed side, with an ALT flag, until the DISPLAYS – CONTROL PANEL switch is positioned to the good side. Note that this is not the same as the EFI switch on the -3/4/500’s which was used to switch symbol generators.
The -3/4/500 Electronic Horizontal Situation Indicator (Map mode) 737-NG Navigation Display (Map mode)
EHSI – Nav EHSI – Plan
EHSI – Full VOR/ILS EHSI – Expanded VOR/ILS
EHSI – Map EHSI – Center Map

*** WARNING ***

The The ND DME readout below the VOR may not necessarily be that of the VOR which is displayed.

This photograph shows that Nav 1 has been manually tuned to 110.20 as shown in 1L of the FMC. DVL VOR identifier has been decoded by the auto-ident facility so “DVL” is displayed in large characters both on the FMC and the bottom left of the ND. Below this is displayed “DME 128” implying that this is the DME from DVL VOR.

However it can be seen on the ND that the DVL VOR is only about 70nm ahead. In fact DVL is only a VOR station and it has no DME facility, the DME was from another station on 110.20. The second station could be identified aurally by the higher pitched tone as “LRH” but was not displaying as such in line 2L of the FMC.

I only discovered this by chance as I happened to be following the aircraft progress by tuning beacons en-route (the way we used to do!). In my opinion, this illustrates the need to aurally identify any beacons, particularly DME, you may have to use, even if they are displayed as decoded.


Instrument Transfer

If either Nav receiver fails, the VHF NAV transfer switch may be used to display the functioning Nav information onto both EFIS and RDMI’s. With Nav transferred, the MCP course selector on the serviceable side becomes the master, but all other EFIS selections remain independent.

If an IRS fails, the IRS transfer switch is used to switch all associated systems to the functioning IRS.

1/200

3/4/500

NG’s


IRS Malfunction Codes (Classics)

Align Annunciator Malfunction Code Significance of Annunciator or Malfunction Code Recommended Action
Flashing (after 10 mins) None Failed align requirement Verify and re-enter present position
01 ISDU failed power up RAM test Replace ISDU
Steady 02 Entered latitude disagrees with latitude calculated by IRU Verify and re-enter present position. If fault persists do full align or replace IRU
02 IRU failure Replace IRU
Flashing 03 Excessive motion during align Restart a full align
Flashing (During full align) 04 Lat or Long entered is not within 1 degree of stored value Re-enter the identical position to the last position entered.
Flashing (During fast realign) 04 Lat is not within 1/2 degree or Long not within 1 deg of stored value Enter known accurate present position. If align light continues to flash, do full align.
05 Left DAA is transmitting a fault Replace left DAA.
06 Right DAA is transmitting a fault Replace right DAA.
07 Selected IRU has detected an invalid air data input. Replace DADC.
Flashing (after 10 mins) 08 Present position has not been entered Enter present position
Steady 09 Attitude mode has been selected Restart a full align. NB if ATT mode is desired, enter magnetic heading in POS INIT 1/2.
10 ISDU is not receiving power from both IRU’s. Ensure that both IRU’s are ON and receiving power.

Alternate Navigation System – ANS (If installed)

This is an option for the -3/4/500 series. ANS is an IRS based system which provides lateral navigation capability independent of the FMC. The ANS with the Control Display Units (AN/CDU) can be operated in parallel with the FMC for an independent cross-check of FMC/CDU operation.

Navigation Mode Selectors

The ANS is two separate systems, ANS-L & ANS-R. Each consists of its own AN/CDU and “on-side” IRS.

Each pilot has his own navigation mode selector to specify the source of navigation information to his EFIS symbol generator and flight director.

The ANS also performs computations related to lateral navigation which can provide LNAV commands to the AFDS in the event of an FMC failure.
The IRS PROGRESS page is similar to the normal PROGRESS page except that all data is from the “on-side” IRS (L in this example).
AN/CDU Pages AN/CDU has no performance or navigation database. All waypoints must therefore be defined in terms of lat & long. The AN/CDU memory can only store 20 waypoints, these can be entered on the ground or in-flight and may be taken from FMC data using the CROSSLOAD function.

Future

In Jan 2003, the 737 became available with three new flight-deck technologies: Vertical Situation Display (VSD), Navigation Performance Scales (NPS) and Integrated Approach Navigation (IAN).

The Vertical Situation Display shows the current and predicted flight path of the airplane and indicates potential conflicts with terrain.

Navigation Performance Scales NPS use vertical and horizontal indicators to provide precise position awareness on the primary flight displays to will allow the aircraft to navigate through a narrower flight path with higher accuracy.

The Integrated Approach Navigation enhances current airplane landing approach capability by simplifying pilot procedures and potentially reducing the number of approach procedures pilots have learned in training.

For more information about NPS and IAN see the section on Flight Instruments.

Vertical Situation Display Vertical Situation Display

The VSD, now certified on NG’s, gives a graphical picture of the aircraft’s vertical flight path. The aim to is reduce the number of CFIT accidents; profile related incidents, particularly on non-precision approaches and earlier recognition of unstabilised approaches.

The VSD works with the Terrain Awareness and Warning System (TAWS) to display a vertical profile of the aircrafts predicted flight path (shown between the blue dashes) on the lower section of the ND. It is selected on with the DATA button on the EFIS control panel.

VSD can be retrofitted into any NG but it requires software changes to the displays and FMC and also some additional hardware displays.

Click here for presentation on VSD


ETOPS

In 1953, the United States developed regulations that prohibited two-engine airplanes from routes more than 60 min single-engine flying time from an adequate airport (FAR 121.161). These regulations were introduced based upon experience with the airliners of the time ie piston engined aircraft, which were much less reliable than modern jet aircraft. Nevertheless, the rule still stands.

ETOPS allows operators to deviate from this rule under certain conditions. By incorporating specific hardware improvements and establishing specific maintenance and operational procedures, operators can fly extended distances up to 180 min from the alternate airport. These hardware improvements were designed into Boeing 737-600/700/800/900.

The following table gives some FAA ETOPS approval times & dates:

Aircraft Series Engine ETOPS-120 approval date ETOPS-180 approval date
737-200 JT8D -9/9A Dec 1985
JT8D -15/15A Dec 1986
JT8D -17/17A Dec 1986
737-300/400/500 CFM56-3 Sept 1990
737-600/700/800/900 CFM56-7 Sept 1999
737-BBJ1/BBJ2 CFM56-7 Sept 1999

Pneumatics – B737

737-3/500 Pneumatics Panel

See also Air Conditioning & Pressurisation

General

The pneumatic system can be supplied by engines, APU or a ground source. The manifold is normally split by the isolation valve. With the isolation valve switch in AUTO, the isolation valve will only open when an engine bleed air or pack switch is selected OFF.

Air for engine starting, air conditioning packs, wing anti-ice and the hydraulic reservoirs comes from their respective ducts. Air for pressurisation of the water tank and the aspirated TAT probe come from the left pneumatic duct. External air for engine starting feeds into the right pneumatic duct. Ground conditioned air feeds directly into the mix manifold.

The minimum pneumatic duct pressure (with anti-ice off) for normal operation is 18psi.

If engine bleed air temperature or pressure exceed limits, the BLEED TRIP OFF light will illuminate and the bleed valve will close. You may use the TRIP RESET switch after a short cooling period. If the BLEED TRIP OFF light does not extinguish, it may be due to an overpressure condition.

Bleed trip off’s are most common on full thrust, bleeds off, take-off’s. The reason is excessive leakage past the closed hi stage valve butterfly which leads to a pressure build up at the downstream port on the overpressure switch within the hi stage regulator. The simple in-flight fix is to reduce duct pressure by selecting CLB-2 and/or using engine and/or wing anti-ice.

WING-BODY OVERHEAT indicates a leak in the corresponding bleed air duct. This is particularly serious if the leak is in the left hand side, as this includes the ducting to the APU. The wing-body overheat circuits may be tested by pressing the OVHT TEST switch; both wing-body overheat lights should illuminate after a minimum of 5 seconds. This test is part of the daily inspection.

 

737-400 Pneumatics Panel

Differences

1/200’s – The PACK switches are simply OFF/ON, rather than OFF/AUTO/HIGH on all other series.

4/8/900’s – Have two recirc fans for pax comfort and PACK warning lights instead of PACK TRIP OFF. See Air conditioning for an explanation. There are also two sidewall risers either side instead of one on all other series, this is why there appear to be two missing windows forward of the engine inlet.

737-400 Sidewall risers

The hydraulic reservoirs are pressurised to ensure a positive flow of fluid reaches the pumps. A from the left manifold and B from the right. See wheel-well fwd.
 

Schematic

Schematic courtesy of Derek Watts

Click here to see a larger air conditioning & pneumatics schematic diagram for the 300/500 or 400.

Limitations

Max external air pressure: 60 psig
Max external air temp: 450°F / 232°C
One pack may be inoperative provided maximum altitude is: FL250

737-3/500 Pneumatics Panel

See also Air Conditioning & Pressurisation

General

The pneumatic system can be supplied by engines, APU or a ground source. The manifold is normally split by the isolation valve. With the isolation valve switch in AUTO, the isolation valve will only open when an engine bleed air or pack switch is selected OFF.

Air for engine starting, air conditioning packs, wing anti-ice and the hydraulic reservoirs comes from their respective ducts. Air for pressurisation of the water tank and the aspirated TAT probe come from the left pneumatic duct. External air for engine starting feeds into the right pneumatic duct. Ground conditioned air feeds directly into the mix manifold.

The minimum pneumatic duct pressure (with anti-ice off) for normal operation is 18psi.

If engine bleed air temperature or pressure exceed limits, the BLEED TRIP OFF light will illuminate and the bleed valve will close. You may use the TRIP RESET switch after a short cooling period. If the BLEED TRIP OFF light does not extinguish, it may be due to an overpressure condition.

Bleed trip off’s are most common on full thrust, bleeds off, take-off’s. The reason is excessive leakage past the closed hi stage valve butterfly which leads to a pressure build up at the downstream port on the overpressure switch within the hi stage regulator. The simple in-flight fix is to reduce duct pressure by selecting CLB-2 and/or using engine and/or wing anti-ice.

WING-BODY OVERHEAT indicates a leak in the corresponding bleed air duct. This is particularly serious if the leak is in the left hand side, as this includes the ducting to the APU. The wing-body overheat circuits may be tested by pressing the OVHT TEST switch; both wing-body overheat lights should illuminate after a minimum of 5 seconds. This test is part of the daily inspection.

737-400 Pneumatics Panel

Differences

1/200’s – The PACK switches are simply OFF/ON, rather than OFF/AUTO/HIGH on all other series.

4/8/900’s – Have two recirc fans for pax comfort and PACK warning lights instead of PACK TRIP OFF. See Air conditioning for an explanation. There are also two sidewall risers either side instead of one on all other series, this is why there appear to be two missing windows forward of the engine inlet.

737-400 Sidewall risers

The hydraulic reservoirs are pressurised to ensure a positive flow of fluid reaches the pumps. A from the left manifold and B from the right. See wheel-well fwd.

Schematic

Schematic courtesy of Derek Watts

Click here to see a larger air conditioning & pneumatics schematic diagram for the 300/500 or 400.

Limitations

Max external air pressure: 60 psig
Max external air temp: 450°F / 232°C
One pack may be inoperative provided maximum altitude is: FL250

Power Plant – B737

History

The original choice of powerplant was the Pratt & Whitney JT8D-1, but before the first order had been finalised the JT8D-7 was used for commonality with the current 727. The -7 was flat rated to develop the same thrust (14,000lb.st) at higher ambient temperatures than the -1 and became the standard powerplant for the -100. By the end of the -200 production the JT8D-17R was up to 17,400lb.st. thrust.

Auxiliary inlet doors were fitted to early JT8D’s around the nose cowl. These were spring loaded and opened automatically whenever the pressure differential between inlet and external static pressures was high, ie slow speed, high thrust conditions (takeoff) to give additional engine air and closed again as airspeed increased causing inlet static pressure to rise.

JT8D Cutaway

The sole powerplant for all 737’s after the -200 is the CFM-56. The core is produced by General Electric and is virtually identical to the F101 as used in the Rockwell B-1. SNECMA produce the fan, IP compressor, LP turbine, thrust reversers and all external accessories. The name “CFM” comes from GE’s commercial engine designation “CF” and SNECMA’s “M” for Moteurs.CFM 56 - 3 Cutaway

One problem with such a high bypass engine was its physical size and ground clearance; this was overcome by mounting the accessories on the lower sides to flatten the nacelle bottom and intake lip to give the “hamster pouch” look. The engines were moved forward and raised, level with the upper surface of the wing and tilted 5 degrees up which not only helped the ground clearance but also directed the exhaust downwards which reduced the effects of pylon overheating and gave some vectored thrust to assist take-off performance. The CFM56-3 proved to be almost 20% more efficient than the JT8D.

The NG’s use the CFM56-7B which has a 61 inch diameter solid titanium wide-chord fan, new LP turbine turbomachinery, FADEC, and new single crystal material in the HP turbine. All of which give an 8% fuel reduction, 15% maintenance cost reduction and greater EGT margin compared to the CFM56-3.

One of the most significant improvements in the powerplant has been to the noise levels. The original JT8D-9 engines in 1967 produced 75 decibel levels, enough to disrupt normal conversation indoors, within a noise contour that extended 12 miles along the take-off flight path. Since 1997 with the introduction of the 737-700’s CFM56-7B engines, the 75-decibel noise contour is now only 3.5 miles long.

The core engine (N2) is governed by metering fuel (see below), whereas the fan (N1) is a free turbine. The advantages of this include: minimised inter-stage bleeding, fewer stalls or surges and an increased compression ratio without decreasing efficiency.

This quote from CFMI in 1997:

“Since entering service in 1984, the CFM56-3 has established itself as the standard against which all other engines are judged in terms of reliability, durability, and cost of ownership. The fleet of nearly 1,800 CFM56-3-powered 737s in service worldwide have logged more than 61 million hours and 44 million cycles while maintaining a 99.98 percent dispatch reliability rate (one flight delayed or cancelled for engine-caused reasons per 5,000 departures), a .070 shop visit rate (one unscheduled shop visit per 14,286 flight hours), and an in-flight shutdown rate of .003 (one incident per 333,333 hours).”

Tech Insertion

“Tech Insertion” is an upgrade to the CFM56-5B & 7B available from early 2007. The package includes improvements to the HP compressor, combustor and HP & LP turbines. The package give a longer time on wing, about 5% lower maintenance costs, 15-20% lower oxides of nitrogen (NOx) emissions, and 1% lower fuel burn.

Tech Insertion will become the new production configuration for both the CFM56-7B and CFM56-5B. CFM is also defining potential upgrade kits that could be made available to operators by late 2007.

CFM56-7BE “Evolution”

The new CFM56-7BE Product Improvement package announced in 2009 will have the following design changes & improvements:

  • HPC outlet guide vane diffuser area ratio improved and pressure losses reduced.
  • HPT blades numbers reduced, axial chord increased, tip geometry improved. Rotor redesigned.
  • LPT blade & vane numbers reduced and profiles based on optimized loading distribution. LEAP56 incorporated.
  • Primary nozzle, plug & strut faring all redesigned.

The -7BE will be able to be intermixed with regular SAC/DAC or Tech Insertion engines subject to updated FMC, MEDB and EEC. Entry into service is planned for mid-2011

From the press 2 Aug 2010:

CFM International has won certification for its upgraded CFM56-7BE engine from the FAA and the European Aviation Safety Agency (EASA), and is working with Boeing to prepare for flight tests on a Boeing 737 starting in the fourth quarter of this year.

Entry into service is planned for mid-2011 to coincide with 737 airframe improvements that, together with the engine upgrade, are designed to provide a 2% improvement in fuel consumption. CFM provisionally scheduled engine certification by the end of the third quarter, but says development, including recently completed flight tests, have progressed faster than expected. Improvements include a new high-pressure compressor outlet guide vane diffuser, high-pressure turbine blades, disks and forward outer seal. The package also includes a new design of low-pressure turbine blades, vanes and disk.

The first full CFM56-7BE type design engine completed ground testing in January 2010, and overall completed 390 hours of ground testing, says the Franco-U.S. engine maker. In addition, the upgraded CFM completed a 60-hour certification flight test program in May on GE’s modified 747 flying testbed in Victorville, Calif.

At the recent Farnborough International Airshow, company officials said discussions are continuing with Airbus about a possible upgrade for the CFM56-5B for the A320 family based on the same technology suite. A decision on whether or not an upgraded variant will be developed for Airbus will be finalized by year-end, adds the engine maker.

Fuel

Thrust (fuel flow) is controlled primarily by a hydro-mechanical MEC in response to thrust lever movement, as fitted to the original 737-1/200’s. In the –3/4/500 series, fuel flow is further refined electronically by the PMC, which acts without thrust lever movement. The 737-NG models go one stage further with FADEC (EEC).

The 3/4/500’s may be flown with PMC’s inoperative, but an RTOW penalty (ie N1 reduction) is imposed because the N1 section will increase by approximately 4% during take-off due to windmilling effects (FOTB 737-1, Jan 1985). This reduction should save reaching any engine limits. The thrust levers should not be re-adjusted during the take-off after thrust is set unless a red-line limit is likely to be exceeded, ie you should allow the N1’s to windmill up.

Fuel is heated to avoid icing by the returning oil in the MEC.

Oil

Oil pressure is measured before the bearings, where you need it; oil temperature on return, at its hottest; and oil quantity at the tank, which drops after engine start. Oil pressure is unregulated, therefore the yellow band (13-26psi) is only valid at take-off thrust whereas the lower red line (13psi) is valid at all times. If the oil pressure is ever at or below the red line, the LOW OIL PRESSURE light will illuminate and that engine should be shut down. NB on the 737-1/200 when the oil quantity gauge reads zero, there could still be up to 5 quarts present.

Ignition

There are two independent AC ignition systems, L & R. Starting with R selected on the first flight of the day provides a check of the AC standby bus, which would be your only electrical source with the loss of thrust on both engines and no APU. Normally, in-flight, no igniters are in use as the combustion is self-sustaining. During engine start or take-off & landing, GND & CONT use the selected igniters. In conditions of moderate or severe precipitation, turbulence or icing, or for an in-flight relight, FLT should be selected to use both igniters. NG aircraft: for in-flight engine starts, GRD arms both igniters.

The 737-NG’s allow the EEC to switch the ignition ON or OFF under certain conditions:

  • ON: For flameout protection. The EEC will automatically switch on both ignition systems if a flameout is detected.
  • OFF: For ground start protection. The EEC will automatically switch off both ignition systems if a hot or wet start is detected.

Note that older 737-200s have ignition switch positions named GRD, OFF, L IGN, R IGN and FLT while newer 737s use GRD, OFF, CONT and FLT. This is why QRH uses “ON” (eg in the One Engine Inop Landing checklist) to cover both LOW IGN & CONT for operators with mixed fleets consisting of old and new versions of the 737.

737-200 Ignition panel

Engine Starting

Min duct pressure for start (Classics only): 30psi at msl, -½psi per 1000ft pressure altitude. Max: 48psi.

Min 25% N2 (or 20% N2 at max motoring) to introduce fuel; any sooner could result in a hot start. Max motoring is when N2 does not increase by more than 1% in 5 seconds.

Aborted engine start criteria:

  • No N1 (before start lever is raised to idle).
  • No oil pressure (by the time the engine is stable).
  • No EGT (within 10 secs of start lever being raised to idle).
  • No increase, or very slow increase, in N1 or N2 (after EGT indication).
  • EGT rapidly approaching or exceeding 725˚C.

An abnormal start advisory does not by itself mean that you have to abort the engine start.

Starter cutout is approx 46% N2 -3/4/500; 56% N2 -NG’s.

Starter duty cycle is:

  • First attempt: 2mins on, 20sec off.
  • Second and subsequent attempts: 2mins on, 3mins off.

Do not re-engage engine start switch until N2 is below 20%.

During cold weather starts, oil pressure may temporary exceed the green band or may not show any increase until oil temperature rises. No indication of oil pressure by the time idle RPM is achieved requires an immediate engine shutdown. At low ambient temperatures, a temporary high oil pressure above the green band may be tolerated.

When starting the engines in tailwind conditions, Boeing recommends making a normal start. Expect a longer cranking time to ensure N1 is rotating in the correct direction before moving the start lever. A higher than normal EGT should be expected, yet the same limits and procedures should apply.

Upper DU

Lower DU

Upper DU in Compact Display mode

The Compact Display mode can only be shown when the MFD ENG button is pressed for the first time after the aircraft has been completely shut down. The photo shows this display with one engine started and nicely illustrates the blank parameters which are controlled by the EEC and hence are only displayed when the EEC powers up when the associated start switch is selected to GND. During start-up the EEC’s receive electrical power from the AC transfer busses, but their normal source of power are their own alternators which cut-in when N2 is above 15%.

-200Adv Engine Instruments

Round Dial -3/400 Engine Instruments

3/4/500 EIS
NG EIS

EIS Display

The introduction of Engine Instrument System (EIS) in late 1988 gave many advantages over the electromechanical instruments present since 1967. ie a 10lb weight reduction, improved reliability, reduction in power consumption, detection of impending abnormal starts, storage of exceedances and a Built In Test Equipment (BITE) check facility.

The BITE check is accessed by pressing a small recessed button at the bottom of each eis panel, this is only possible when both engines N1 are below 10%. Pressing these buttons will show an LED check during which the various checks are conducted. If any of the checks fail, the appropriate code will be shown in place of the affected parameters readout. The following codes are used:

Primary EIS BITE Codes
Code Fault
ROM Read Only Memory check
RAM Random Access Memory check
FDC Frequency to Digital Converter check
ENG Engine Identity Inputs (not fuel flow)
PWR Power Monitor
MMF Maint Module Fault (fuel flow only)
RTC Real Time Clock (fuel flow only)
ERF Exceedance RAM Full (fuel flow only)
A/D Analogue to Digital Converter (fuel flow only)
ARF ARINC Receiver Fault (fuel flow only)
uP Microprocessor

Any exceedance of either N1, N2 or EGT is recorded at 1 sec intervals in a non-volatile memory along with the fuel flow at the time, this data can be downloaded by connecting an ARINC 429 bus reader. Up to 10 minutes of data can be stored. The last exceedance is also put into volatile memory and can be read straight from the EIS before aircraft electrical power is removed. This is done by pressing the primary EIS BITE button twice within 2 seconds, this will then alternately display the highest reading and the duration of the exceedance in seconds.

Secondary EIS BITE Codes
Code Fault
0- Microprocessor
1- Program Memory
2- Random Access Memory check
3- Analogue to Digital Converter
4- Power Monitor
5- 400Hz Reference Voltage
6- ARINC Receiver Fault

 

Airborne Vibration Monitors (AVM)

All series of 737 have the facility for AVM although not all 737-200’s have them fitted. The early 737-1/200’s had two vibration pickup points; One at the turbine section and one at the engine inlet there was a selector switch so that the crew could choose which to monitor. Some even had a high and low frequency filter selection switch.

From Boeing Flt Ops Review, Feb 2003: “On airplanes with AVM procedures, flight crews should also be made aware that AVM indications are not valid while at takeoff power settings, during power changes, or until after engine thermal stabilization. High AVM indications can also be observed during operations in icing conditions.”

 

High Pressure Turbine Clearance Control

The HPTCC system uses HP compressor bleed air to obtain maximum steady state HPT performance and to minimise EGT transient overshoot during rapid change of engine speed.

Variable Stator Vanes

The VSV actuation system controls primary airflow through the HP compressor by varying the angle of the inlet guide vanes and three stages of variable stator vanes.

Variable Bleed Valves

Control airflow quantity to the HP compressor. They are fully open during rapid accelerations and reverse thrust operation.

 

Dual Annular Combustors (DAC)

The CFM56-7B is available with an optional DAC system, known as the CFM56-7B/2, which considerably reduces NOx emissions. DAC have 20 double tip fuel nozzles instead of the single tip and a dual annular shaped combustion chamber. The number of nozzles in use: 20/0, 20/10 or 20/20, varies depending upon thrust required. The precise N1 ranges of the different modes varies with ambient conditions.

  • 20/20 mode – High power (cruise N1 and above)
  • 20/10 mode – Medium power
  • 20/00 mode – Low power (Idle N1)

This gives a lean fuel/air mixture, which reduces flame temperatures, and also gives higher throughput velocities which reduce the residence time available to form NOx. The net result is up to 40% less NOx emissions than a standard CFM56-7.

The first were installed on the 737-600 fleet of SAS but unfortunately were subject to resonance in the LPT-1 blades during operation in the 20/10 mode, which occurred in an N1 range usually used during descent and approach. Although there were no in-flight shutdowns, boroscope inspections revealed that the LPT blades were starting to separate. CFM quickly replaced all blades on all DAC engines with reinforced blades and have since replaced them again with a new redesigned blade.

 

Reverse Thrust

The original 737-1/200 thrust reversers were pneumatically powered clamshell doors taken straight from the 727 (shown left). When reverse was selected, 13th stage bleed air was ported to a pneumatic actuator that rotated the deflector doors and clamshell doors into position. Unfortunately they were relatively ineffective and apparently tended to push the aircraft up off the runway when deployed. This reduced the downforce on the main wheels thereby reducing the effectiveness of the wheel brakes.

By 1969 these had been changed by Boeing and Rohr to the much more successful hydraulically powered target type thrust reversers (shown right). This required a 48 inch extension to the tailpipe to accommodate the two cylindrical deflector doors which were mounted on a four bar linkage system and associated hydraulics. The doors are set 35 degrees away from the vertical to allow the exhaust to be deflected inboard and over the wings and outboard and under the wings. This ensures that exhaust and debris is not blown into the wheel-well, nor is it blown directly downwards which would lift the weight off the wheels or be re-ingested. Fortunately the new longer nacelle improved cruise performance by improving internal airflow within the engine and also reduced cruise drag. These thrust reversers are locked against inadvertent deployment by both deflector door locks and the four bar linkage being overcenter. To illustrate how poor the original clamshell system was, Boeings own data says target type thrust reversers at 1.5 EPR are twice as effective as clamshells at full thrust!

The CFM56 uses blocker doors and cascade vanes to direct fan air forwards. Net reverse thrust is defined as: fan reverser air, minus forward thrust from engine core, plus form drag from the blocker door. As this is significantly greater at higher thrust, reverse thrust should be used immediately after landing or RTO and, if conditions allow, should be reduced to idle by 60kts to avoid debris ingestion damage. Caution: It is possible to deploy reverse thrust when either Rad Alt is below 10ft – this is not recommended.

The REVERSER light shows either control valve or sleeve position disagreement or that the auto-restow circuit is activated. This light will illuminate every time the reverser is commanded to stow, but extinguishes after the stow has completed, and will only bring up master caution ENG if a malfunction has occurred. Recycling the reverse thrust will often clear the fault. If this occurs in-flight, reverse thrust will be available after landing.

The REVERSER UNLOCKED light (EIS panel) is potentially much more serious and will illuminate in-flight if a sleeve has mechanically unlocked. Follow the QRH drill, but only multiple failures will allow the engine to go into reverse thrust.

The 737-1/200 thrust reverser panel has a LOW PRESSURE light which refers to the reverser accumulator pressure when insufficient pressure is available to deploy the reversers. The blue caption between the switches is ISOLATION VALVE and illuminates when the three conditions for reverse thrust are satisfied: Engine running, Aircraft on ground & Fire switches in normal position. The guarded NORMAL / OVERRIDE switches to enable the reverse thrust to be selected on the ground with the engines stopped (for maintenance purposes).

 

 

HushKits

The first “hushkit” was not visible externally, in 1982 exhaust mixers were made available for the JT8D-15, -17 or -17R. These were fitted behind the LP turbine to mix the hot gas core airflow with the cooler bypassed fan air. This increased mixing reduced noise levels by up to 3.6 EPNdB.

Several different Stage III hushkits have been available from manufacturers Nordam (shown right) and AvAero since 1992. The Nordam comes in HGW and LGW versions.

As hushkits use more fuel, the EU tried to ban all hushkitted aircraft flying into the EU from April 2002. This was strongly opposed and the directive has been changed to allow hushkitted aircraft to use airports which will accept them.

737 classics may be fitted with hardwall forward acoustic panels which reduce noise by 1 EPNdB

 

Additional References

 

Limitations

Series 1/200 3/4/500 6/7/8/900/BBJ
Engine JT8-17A CFM56-3 CFM56-7
 
Max time limit for take-off or go-around thrust: 5 mins 5 mins * 5 mins *
Max N1 102.4% 106% 104%
Max N2 100% 105% 105%
 

Max EGT’s:

Take-off (5 min limit) 650°C 930°C 950°C
Continuous 610°C 895°C 925°C
Start 575°C 725°C 725°C
 

Oil T’s & P’s

Max temperature 165°C 165°C 155°C
15 minute limit (45 minute limit on NG) 130-165°C 160-165°C 140-155°C
Max continuous 130°C 160°C 140°C
Min oil press 40psi 13psi (warning light), 26psi (gauge) 13psi (warning light), 26psi (gauge)
Min oil quantity (at dispatch) 2.25 USG 60% full (12 US Quarts) ** 60% full (12 US Quarts) **
Starting pressures prior to starter engagement

30psi -1/2psi per 1000′ amsl

N/A
Starter duty cycle 1st attempt: 2min on, 20sec off

2nd & subsequent attempt: 2min on, 3min off

2mins on, 10secs off.
* May be increased to 10 mins if certified** See AMM Task 12-13-11-600-801

Engine Ratings

Maximum Certified Thrust – This is the maximum thrust certified during testing for each series of 737. This is also the thrust that you get when you firewall the thrust levers, regardless of the maximum rated thrust.

On the 737NG, the EEC limits the maximum certified thrust gained from data in the engine strut according to the airplane model as follows:

Aircraft Series Maximum Certified Thrust
737-600 CFM56-7B22 = 22,700lb.st
737-700 CFM56-7B24 = 24,200lb.st
737-800 CFM56-7B27 = 27,300lb.st
737-900 CFM56-7B27 = 27,300lb.st

 

Maximum Rated Thrust – This is the maximum thrust for the installed engine that the autothrottle will command. This is specified by the operator from the options in the table below.

Engine Aircraft Series Max Static Thrust (lb.st.) Bypass Ratio EGT Margin (C)
JT8D-7/7A/7B 1/200 14,000 1.10
JT8D-9/9A 1/200 14,500 1.04
JT8D-15/15A 200Adv 15,500 0.99
JT8D-17/17A 200Adv 16,000 1.02
JT8D-17R 200Adv 17,400 1.00
CFM56-3B4 500 18,500 5.0 90
CFM56-3B1 3/500 20,000 5.0 70
CFM56-3B2 3/400 22,000 5.0 50
CFM56-3C1 400 23,500 4.9 45
CFM56-7B18 600 19,500 5.5 145
CFM56-7B20 6/700 20,600 5.4 148
CFM56-7B22 6/700 22,700 5.3 150
CFM56-7B24 7/8/900 24,200 5.3 125
CFM56-7B26 7/8/900/BBJ 26,400 5.1 85
CFM56-7B27 8/900/BBJ 27,300 5.0 ?

Misc Photos

The left hand side of the CFM56-3. The large silver coloured pipe is the start air manifold with the starter located at its base. The black unit below that is the CSD. The green unit forward (left) of the CSD is the generator cooling air collector shroud, the silver-gold thing forward of that (with the wire bundle visible) is the generator, and the green cap most forward is the generator cooling air inlet.
The view into the JT8D jetpipe.The corrugated ring is the mixer unit, this is designed to thoroughly mix the bypass air with the turbine exhaust.

The exhaust cone makes a divergent flow which slows down the exhaust and also protects the rear face of the last turbine stage.

The view into the CFM56-3 jetpipe.This is the turbine exhaust area, no mixing is required as the bypass air is exhausted coaxially.
There are two fan inlet temperature sensors in the CFM56-3 engine intake. The one at the 2 o’clock position is used by the PMC and the one at the 11 o’clock position is used by the MEC. The MEC uses the signal to establish parameters to control low and high idle power schedules.The temperature data is used for thrust management and variable bleed valves, variable stator vanes & high / low pressure turbine clearance control systems.
The CFM56-7 inlet has just one fan inlet temperature probe, which is for the EEC (because there is no PMC on the NG’s).A subtle difference between the NG & classic temp probes is that the NG’s only use inlet temp data on the ground and for 5 minutes after take-off. In-flight after 5 minutes temp data is taken from the ADIRU’s.

The temperature data is used for thrust management and variable bleed valves, variable stator vanes & high / low pressure turbine clearance control systems.

The CFM56-7 spinner has a unique conelliptical profile. The first 737-3/400’s had a conical (sharp pointed) spinner but these tended to shed ice into the core. This was one of the reasons for the early limitation of minimum 45% N1 in icing conditions which made descent management quite difficult. They were later replaced with elliptical (round nosed) spinners which succeeded in deflecting the ice away from the core, but because of their larger stagnation point, were more prone to picking up ice in the first place. The conelliptical spinner of the NG’s neatly solves both problems.
The CFM56-7 tailpipe is slightly longer then the CFM56-3 and has a small tube protruding from the faring. This is the Aft Fairing Drain Tube for any hydraulic fluid, oil or fuel that may collect in there. There is also a second drain tube that does not protrude located on the inside of the fairing.
The JT8D tailpipe fitted as standard from l/n 135 onwards.The original thrust reversers were totally redesigned by Boeing and Rohr since the aircraft had inherited the same internal pneumatically powered clamshell thrust reversers as the 727 which were relatively ineffective and apparently tended to lift the aircraft off the runway when deployed! The redesign to external hydraulically powered target reversers cost Boeing $24 million but dramatically improved its short field performance which boosted sales to carriers proposing to use the aircraft as a regional jet from short runways. Also the engine nacelles were extended by 1.14m as a drag reduction measure.
The outboard side of the JT8D-9A with the cowling open.

istory

The original choice of powerplant was the Pratt & Whitney JT8D-1, but before the first order had been finalised the JT8D-7 was used for commonality with the current 727. The -7 was flat rated to develop the same thrust (14,000lb.st) at higher ambient temperatures than the -1 and became the standard powerplant for the -100. By the end of the -200 production the JT8D-17R was up to 17,400lb.st. thrust.

Auxiliary inlet doors were fitted to early JT8D’s around the nose cowl. These were spring loaded and opened automatically whenever the pressure differential between inlet and external static pressures was high, ie slow speed, high thrust conditions (takeoff) to give additional engine air and closed again as airspeed increased causing inlet static pressure to rise.

JT8D Cutaway

The sole powerplant for all 737’s after the -200 is the CFM-56. The core is produced by General Electric and is virtually identical to the F101 as used in the Rockwell B-1. SNECMA produce the fan, IP compressor, LP turbine, thrust reversers and all external accessories. The name “CFM” comes from GE’s commercial engine designation “CF” and SNECMA’s “M” for Moteurs.CFM 56 - 3 Cutaway

One problem with such a high bypass engine was its physical size and ground clearance; this was overcome by mounting the accessories on the lower sides to flatten the nacelle bottom and intake lip to give the “hamster pouch” look. The engines were moved forward and raised, level with the upper surface of the wing and tilted 5 degrees up which not only helped the ground clearance but also directed the exhaust downwards which reduced the effects of pylon overheating and gave some vectored thrust to assist take-off performance. The CFM56-3 proved to be almost 20% more efficient than the JT8D.

The NG’s use the CFM56-7B which has a 61 inch diameter solid titanium wide-chord fan, new LP turbine turbomachinery, FADEC, and new single crystal material in the HP turbine. All of which give an 8% fuel reduction, 15% maintenance cost reduction and greater EGT margin compared to the CFM56-3.

One of the most significant improvements in the powerplant has been to the noise levels. The original JT8D-9 engines in 1967 produced 75 decibel levels, enough to disrupt normal conversation indoors, within a noise contour that extended 12 miles along the take-off flight path. Since 1997 with the introduction of the 737-700’s CFM56-7B engines, the 75-decibel noise contour is now only 3.5 miles long.

The core engine (N2) is governed by metering fuel (see below), whereas the fan (N1) is a free turbine. The advantages of this include: minimised inter-stage bleeding, fewer stalls or surges and an increased compression ratio without decreasing efficiency.

This quote from CFMI in 1997:

“Since entering service in 1984, the CFM56-3 has established itself as the standard against which all other engines are judged in terms of reliability, durability, and cost of ownership. The fleet of nearly 1,800 CFM56-3-powered 737s in service worldwide have logged more than 61 million hours and 44 million cycles while maintaining a 99.98 percent dispatch reliability rate (one flight delayed or cancelled for engine-caused reasons per 5,000 departures), a .070 shop visit rate (one unscheduled shop visit per 14,286 flight hours), and an in-flight shutdown rate of .003 (one incident per 333,333 hours).”

Tech Insertion

“Tech Insertion” is an upgrade to the CFM56-5B & 7B available from early 2007. The package includes improvements to the HP compressor, combustor and HP & LP turbines. The package give a longer time on wing, about 5% lower maintenance costs, 15-20% lower oxides of nitrogen (NOx) emissions, and 1% lower fuel burn.

Tech Insertion will become the new production configuration for both the CFM56-7B and CFM56-5B. CFM is also defining potential upgrade kits that could be made available to operators by late 2007.

CFM56-7BE “Evolution”

The new CFM56-7BE Product Improvement package announced in 2009 will have the following design changes & improvements:

  • HPC outlet guide vane diffuser area ratio improved and pressure losses reduced.
  • HPT blades numbers reduced, axial chord increased, tip geometry improved. Rotor redesigned.
  • LPT blade & vane numbers reduced and profiles based on optimized loading distribution. LEAP56 incorporated.
  • Primary nozzle, plug & strut faring all redesigned.

The -7BE will be able to be intermixed with regular SAC/DAC or Tech Insertion engines subject to updated FMC, MEDB and EEC. Entry into service is planned for mid-2011

From the press 2 Aug 2010:

CFM International has won certification for its upgraded CFM56-7BE engine from the FAA and the European Aviation Safety Agency (EASA), and is working with Boeing to prepare for flight tests on a Boeing 737 starting in the fourth quarter of this year.

Entry into service is planned for mid-2011 to coincide with 737 airframe improvements that, together with the engine upgrade, are designed to provide a 2% improvement in fuel consumption. CFM provisionally scheduled engine certification by the end of the third quarter, but says development, including recently completed flight tests, have progressed faster than expected. Improvements include a new high-pressure compressor outlet guide vane diffuser, high-pressure turbine blades, disks and forward outer seal. The package also includes a new design of low-pressure turbine blades, vanes and disk.

The first full CFM56-7BE type design engine completed ground testing in January 2010, and overall completed 390 hours of ground testing, says the Franco-U.S. engine maker. In addition, the upgraded CFM completed a 60-hour certification flight test program in May on GE’s modified 747 flying testbed in Victorville, Calif.

At the recent Farnborough International Airshow, company officials said discussions are continuing with Airbus about a possible upgrade for the CFM56-5B for the A320 family based on the same technology suite. A decision on whether or not an upgraded variant will be developed for Airbus will be finalized by year-end, adds the engine maker.

Fuel

Thrust (fuel flow) is controlled primarily by a hydro-mechanical MEC in response to thrust lever movement, as fitted to the original 737-1/200’s. In the –3/4/500 series, fuel flow is further refined electronically by the PMC, which acts without thrust lever movement. The 737-NG models go one stage further with FADEC (EEC).

The 3/4/500’s may be flown with PMC’s inoperative, but an RTOW penalty (ie N1 reduction) is imposed because the N1 section will increase by approximately 4% during take-off due to windmilling effects (FOTB 737-1, Jan 1985). This reduction should save reaching any engine limits. The thrust levers should not be re-adjusted during the take-off after thrust is set unless a red-line limit is likely to be exceeded, ie you should allow the N1’s to windmill up.

Fuel is heated to avoid icing by the returning oil in the MEC.

Oil

Oil pressure is measured before the bearings, where you need it; oil temperature on return, at its hottest; and oil quantity at the tank, which drops after engine start. Oil pressure is unregulated, therefore the yellow band (13-26psi) is only valid at take-off thrust whereas the lower red line (13psi) is valid at all times. If the oil pressure is ever at or below the red line, the LOW OIL PRESSURE light will illuminate and that engine should be shut down. NB on the 737-1/200 when the oil quantity gauge reads zero, there could still be up to 5 quarts present.

Ignition

There are two independent AC ignition systems, L & R. Starting with R selected on the first flight of the day provides a check of the AC standby bus, which would be your only electrical source with the loss of thrust on both engines and no APU. Normally, in-flight, no igniters are in use as the combustion is self-sustaining. During engine start or take-off & landing, GND & CONT use the selected igniters. In conditions of moderate or severe precipitation, turbulence or icing, or for an in-flight relight, FLT should be selected to use both igniters. NG aircraft: for in-flight engine starts, GRD arms both igniters.

The 737-NG’s allow the EEC to switch the ignition ON or OFF under certain conditions:

  • ON: For flameout protection. The EEC will automatically switch on both ignition systems if a flameout is detected.
  • OFF: For ground start protection. The EEC will automatically switch off both ignition systems if a hot or wet start is detected.

Note that older 737-200s have ignition switch positions named GRD, OFF, L IGN, R IGN and FLT while newer 737s use GRD, OFF, CONT and FLT. This is why QRH uses “ON” (eg in the One Engine Inop Landing checklist) to cover both LOW IGN & CONT for operators with mixed fleets consisting of old and new versions of the 737.

737-200 Ignition panel

Engine Starting

Min duct pressure for start (Classics only): 30psi at msl, -½psi per 1000ft pressure altitude. Max: 48psi.

Min 25% N2 (or 20% N2 at max motoring) to introduce fuel; any sooner could result in a hot start. Max motoring is when N2 does not increase by more than 1% in 5 seconds.

Aborted engine start criteria:

  • No N1 (before start lever is raised to idle).
  • No oil pressure (by the time the engine is stable).
  • No EGT (within 10 secs of start lever being raised to idle).
  • No increase, or very slow increase, in N1 or N2 (after EGT indication).
  • EGT rapidly approaching or exceeding 725˚C.

An abnormal start advisory does not by itself mean that you have to abort the engine start.

Starter cutout is approx 46% N2 -3/4/500; 56% N2 -NG’s.

Starter duty cycle is:

  • First attempt: 2mins on, 20sec off.
  • Second and subsequent attempts: 2mins on, 3mins off.

Do not re-engage engine start switch until N2 is below 20%.

During cold weather starts, oil pressure may temporary exceed the green band or may not show any increase until oil temperature rises. No indication of oil pressure by the time idle RPM is achieved requires an immediate engine shutdown. At low ambient temperatures, a temporary high oil pressure above the green band may be tolerated.

When starting the engines in tailwind conditions, Boeing recommends making a normal start. Expect a longer cranking time to ensure N1 is rotating in the correct direction before moving the start lever. A higher than normal EGT should be expected, yet the same limits and procedures should apply.

Upper DU

Lower DU

Upper DU in Compact Display mode

The Compact Display mode can only be shown when the MFD ENG button is pressed for the first time after the aircraft has been completely shut down. The photo shows this display with one engine started and nicely illustrates the blank parameters which are controlled by the EEC and hence are only displayed when the EEC powers up when the associated start switch is selected to GND. During start-up the EEC’s receive electrical power from the AC transfer busses, but their normal source of power are their own alternators which cut-in when N2 is above 15%.

-200Adv Engine Instruments

Round Dial -3/400 Engine Instruments

3/4/500 EIS
NG EIS

EIS Display

The introduction of Engine Instrument System (EIS) in late 1988 gave many advantages over the electromechanical instruments present since 1967. ie a 10lb weight reduction, improved reliability, reduction in power consumption, detection of impending abnormal starts, storage of exceedances and a Built In Test Equipment (BITE) check facility.

The BITE check is accessed by pressing a small recessed button at the bottom of each eis panel, this is only possible when both engines N1 are below 10%. Pressing these buttons will show an LED check during which the various checks are conducted. If any of the checks fail, the appropriate code will be shown in place of the affected parameters readout. The following codes are used:

Primary EIS BITE Codes
Code Fault
ROM Read Only Memory check
RAM Random Access Memory check
FDC Frequency to Digital Converter check
ENG Engine Identity Inputs (not fuel flow)
PWR Power Monitor
MMF Maint Module Fault (fuel flow only)
RTC Real Time Clock (fuel flow only)
ERF Exceedance RAM Full (fuel flow only)
A/D Analogue to Digital Converter (fuel flow only)
ARF ARINC Receiver Fault (fuel flow only)
uP Microprocessor

Any exceedance of either N1, N2 or EGT is recorded at 1 sec intervals in a non-volatile memory along with the fuel flow at the time, this data can be downloaded by connecting an ARINC 429 bus reader. Up to 10 minutes of data can be stored. The last exceedance is also put into volatile memory and can be read straight from the EIS before aircraft electrical power is removed. This is done by pressing the primary EIS BITE button twice within 2 seconds, this will then alternately display the highest reading and the duration of the exceedance in seconds.

Secondary EIS BITE Codes
Code Fault
0- Microprocessor
1- Program Memory
2- Random Access Memory check
3- Analogue to Digital Converter
4- Power Monitor
5- 400Hz Reference Voltage
6- ARINC Receiver Fault

Airborne Vibration Monitors (AVM)

All series of 737 have the facility for AVM although not all 737-200’s have them fitted. The early 737-1/200’s had two vibration pickup points; One at the turbine section and one at the engine inlet there was a selector switch so that the crew could choose which to monitor. Some even had a high and low frequency filter selection switch.

From Boeing Flt Ops Review, Feb 2003: “On airplanes with AVM procedures, flight crews should also be made aware that AVM indications are not valid while at takeoff power settings, during power changes, or until after engine thermal stabilization. High AVM indications can also be observed during operations in icing conditions.”

High Pressure Turbine Clearance Control

The HPTCC system uses HP compressor bleed air to obtain maximum steady state HPT performance and to minimise EGT transient overshoot during rapid change of engine speed.

Variable Stator Vanes

The VSV actuation system controls primary airflow through the HP compressor by varying the angle of the inlet guide vanes and three stages of variable stator vanes.

Variable Bleed Valves

Control airflow quantity to the HP compressor. They are fully open during rapid accelerations and reverse thrust operation.

Dual Annular Combustors (DAC)

The CFM56-7B is available with an optional DAC system, known as the CFM56-7B/2, which considerably reduces NOx emissions. DAC have 20 double tip fuel nozzles instead of the single tip and a dual annular shaped combustion chamber. The number of nozzles in use: 20/0, 20/10 or 20/20, varies depending upon thrust required. The precise N1 ranges of the different modes varies with ambient conditions.

  • 20/20 mode – High power (cruise N1 and above)
  • 20/10 mode – Medium power
  • 20/00 mode – Low power (Idle N1)

This gives a lean fuel/air mixture, which reduces flame temperatures, and also gives higher throughput velocities which reduce the residence time available to form NOx. The net result is up to 40% less NOx emissions than a standard CFM56-7.

The first were installed on the 737-600 fleet of SAS but unfortunately were subject to resonance in the LPT-1 blades during operation in the 20/10 mode, which occurred in an N1 range usually used during descent and approach. Although there were no in-flight shutdowns, boroscope inspections revealed that the LPT blades were starting to separate. CFM quickly replaced all blades on all DAC engines with reinforced blades and have since replaced them again with a new redesigned blade.

Reverse Thrust

The original 737-1/200 thrust reversers were pneumatically powered clamshell doors taken straight from the 727 (shown left). When reverse was selected, 13th stage bleed air was ported to a pneumatic actuator that rotated the deflector doors and clamshell doors into position. Unfortunately they were relatively ineffective and apparently tended to push the aircraft up off the runway when deployed. This reduced the downforce on the main wheels thereby reducing the effectiveness of the wheel brakes.

By 1969 these had been changed by Boeing and Rohr to the much more successful hydraulically powered target type thrust reversers (shown right). This required a 48 inch extension to the tailpipe to accommodate the two cylindrical deflector doors which were mounted on a four bar linkage system and associated hydraulics. The doors are set 35 degrees away from the vertical to allow the exhaust to be deflected inboard and over the wings and outboard and under the wings. This ensures that exhaust and debris is not blown into the wheel-well, nor is it blown directly downwards which would lift the weight off the wheels or be re-ingested. Fortunately the new longer nacelle improved cruise performance by improving internal airflow within the engine and also reduced cruise drag. These thrust reversers are locked against inadvertent deployment by both deflector door locks and the four bar linkage being overcenter. To illustrate how poor the original clamshell system was, Boeings own data says target type thrust reversers at 1.5 EPR are twice as effective as clamshells at full thrust!

The CFM56 uses blocker doors and cascade vanes to direct fan air forwards. Net reverse thrust is defined as: fan reverser air, minus forward thrust from engine core, plus form drag from the blocker door. As this is significantly greater at higher thrust, reverse thrust should be used immediately after landing or RTO and, if conditions allow, should be reduced to idle by 60kts to avoid debris ingestion damage. Caution: It is possible to deploy reverse thrust when either Rad Alt is below 10ft – this is not recommended.

The REVERSER light shows either control valve or sleeve position disagreement or that the auto-restow circuit is activated. This light will illuminate every time the reverser is commanded to stow, but extinguishes after the stow has completed, and will only bring up master caution ENG if a malfunction has occurred. Recycling the reverse thrust will often clear the fault. If this occurs in-flight, reverse thrust will be available after landing.

The REVERSER UNLOCKED light (EIS panel) is potentially much more serious and will illuminate in-flight if a sleeve has mechanically unlocked. Follow the QRH drill, but only multiple failures will allow the engine to go into reverse thrust.

The 737-1/200 thrust reverser panel has a LOW PRESSURE light which refers to the reverser accumulator pressure when insufficient pressure is available to deploy the reversers. The blue caption between the switches is ISOLATION VALVE and illuminates when the three conditions for reverse thrust are satisfied: Engine running, Aircraft on ground & Fire switches in normal position. The guarded NORMAL / OVERRIDE switches to enable the reverse thrust to be selected on the ground with the engines stopped (for maintenance purposes).

HushKits

The first “hushkit” was not visible externally, in 1982 exhaust mixers were made available for the JT8D-15, -17 or -17R. These were fitted behind the LP turbine to mix the hot gas core airflow with the cooler bypassed fan air. This increased mixing reduced noise levels by up to 3.6 EPNdB.

Several different Stage III hushkits have been available from manufacturers Nordam (shown right) and AvAero since 1992. The Nordam comes in HGW and LGW versions.

As hushkits use more fuel, the EU tried to ban all hushkitted aircraft flying into the EU from April 2002. This was strongly opposed and the directive has been changed to allow hushkitted aircraft to use airports which will accept them.

737 classics may be fitted with hardwall forward acoustic panels which reduce noise by 1 EPNdB

Additional References

Limitations

Series 1/200 3/4/500 6/7/8/900/BBJ
Engine JT8-17A CFM56-3 CFM56-7
 
Max time limit for take-off or go-around thrust: 5 mins 5 mins * 5 mins *
Max N1 102.4% 106% 104%
Max N2 100% 105% 105%
Max EGT’s:
Take-off (5 min limit) 650°C 930°C 950°C
Continuous 610°C 895°C 925°C
Start 575°C 725°C 725°C
Oil T’s & P’s
Max temperature 165°C 165°C 155°C
15 minute limit (45 minute limit on NG) 130-165°C 160-165°C 140-155°C
Max continuous 130°C 160°C 140°C
Min oil press 40psi 13psi (warning light), 26psi (gauge) 13psi (warning light), 26psi (gauge)
Min oil quantity (at dispatch) 2.25 USG 60% full (12 US Quarts) ** 60% full (12 US Quarts) **
Starting pressures prior to starter engagement

30psi -1/2psi per 1000′ amsl

N/A
Starter duty cycle 1st attempt: 2min on, 20sec off

2nd & subsequent attempt: 2min on, 3min off

2mins on, 10secs off.
* May be increased to 10 mins if certified** See AMM Task 12-13-11-600-801

Engine Ratings

Maximum Certified Thrust – This is the maximum thrust certified during testing for each series of 737. This is also the thrust that you get when you firewall the thrust levers, regardless of the maximum rated thrust.

On the 737NG, the EEC limits the maximum certified thrust gained from data in the engine strut according to the airplane model as follows:

Aircraft Series Maximum Certified Thrust
737-600 CFM56-7B22 = 22,700lb.st
737-700 CFM56-7B24 = 24,200lb.st
737-800 CFM56-7B27 = 27,300lb.st
737-900 CFM56-7B27 = 27,300lb.st

Maximum Rated Thrust – This is the maximum thrust for the installed engine that the autothrottle will command. This is specified by the operator from the options in the table below.

Engine Aircraft Series Max Static Thrust (lb.st.) Bypass Ratio EGT Margin (C)
JT8D-7/7A/7B 1/200 14,000 1.10
JT8D-9/9A 1/200 14,500 1.04
JT8D-15/15A 200Adv 15,500 0.99
JT8D-17/17A 200Adv 16,000 1.02
JT8D-17R 200Adv 17,400 1.00
CFM56-3B4 500 18,500 5.0 90
CFM56-3B1 3/500 20,000 5.0 70
CFM56-3B2 3/400 22,000 5.0 50
CFM56-3C1 400 23,500 4.9 45
CFM56-7B18 600 19,500 5.5 145
CFM56-7B20 6/700 20,600 5.4 148
CFM56-7B22 6/700 22,700 5.3 150
CFM56-7B24 7/8/900 24,200 5.3 125
CFM56-7B26 7/8/900/BBJ 26,400 5.1 85
CFM56-7B27 8/900/BBJ 27,300 5.0 ?

Misc Photos

The left hand side of the CFM56-3. The large silver coloured pipe is the start air manifold with the starter located at its base. The black unit below that is the CSD. The green unit forward (left) of the CSD is the generator cooling air collector shroud, the silver-gold thing forward of that (with the wire bundle visible) is the generator, and the green cap most forward is the generator cooling air inlet.
The view into the JT8D jetpipe.The corrugated ring is the mixer unit, this is designed to thoroughly mix the bypass air with the turbine exhaust.

The exhaust cone makes a divergent flow which slows down the exhaust and also protects the rear face of the last turbine stage.

The view into the CFM56-3 jetpipe.This is the turbine exhaust area, no mixing is required as the bypass air is exhausted coaxially.
There are two fan inlet temperature sensors in the CFM56-3 engine intake. The one at the 2 o’clock position is used by the PMC and the one at the 11 o’clock position is used by the MEC. The MEC uses the signal to establish parameters to control low and high idle power schedules.The temperature data is used for thrust management and variable bleed valves, variable stator vanes & high / low pressure turbine clearance control systems.
The CFM56-7 inlet has just one fan inlet temperature probe, which is for the EEC (because there is no PMC on the NG’s).A subtle difference between the NG & classic temp probes is that the NG’s only use inlet temp data on the ground and for 5 minutes after take-off. In-flight after 5 minutes temp data is taken from the ADIRU’s.

The temperature data is used for thrust management and variable bleed valves, variable stator vanes & high / low pressure turbine clearance control systems.

The CFM56-7 spinner has a unique conelliptical profile. The first 737-3/400’s had a conical (sharp pointed) spinner but these tended to shed ice into the core. This was one of the reasons for the early limitation of minimum 45% N1 in icing conditions which made descent management quite difficult. They were later replaced with elliptical (round nosed) spinners which succeeded in deflecting the ice away from the core, but because of their larger stagnation point, were more prone to picking up ice in the first place. The conelliptical spinner of the NG’s neatly solves both problems.
The CFM56-7 tailpipe is slightly longer then the CFM56-3 and has a small tube protruding from the faring. This is the Aft Fairing Drain Tube for any hydraulic fluid, oil or fuel that may collect in there. There is also a second drain tube that does not protrude located on the inside of the fairing.
The JT8D tailpipe fitted as standard from l/n 135 onwards.The original thrust reversers were totally redesigned by Boeing and Rohr since the aircraft had inherited the same internal pneumatically powered clamshell thrust reversers as the 727 which were relatively ineffective and apparently tended to lift the aircraft off the runway when deployed! The redesign to external hydraulically powered target reversers cost Boeing $24 million but dramatically improved its short field performance which boosted sales to carriers proposing to use the aircraft as a regional jet from short runways. Also the engine nacelles were extended by 1.14m as a drag reduction measure.
The outboard side of the JT8D-9A with the cowling open.

Ice and Rain Protection

Panels

737-1/200 Ice & Rain Panel

737-3/4/500 Ice & Rain Panel

737-NG Ice & Rain Panel

Differences:

  1. No alpha vanes
  2. WAI has ground test position
  3. Engine anti-ice captions are: COWL VALVE OPEN, R VALVE OPEN, L VALVE OPEN.
  1. Alpha vanes added
  2. Now only one temp probe (many 1/200’s had two)
  3. TAT TEST button (ie aspirated probe). NB if there is no TAT TEST button you have an unaspirated probe.
  1. Static ports not heated
  2. Aux pitot added.

Window Heat

If window heat is switched ON but the ON light is extinguished, this means that heat is not being applied to the associated window. This could be because the heat controller has detected that the window is becoming overheated (normal on hot days in direct sunlight) and can be verified by touching the window. The heat will automatically be restored when the window has cooled down. To verify that window heat is still available a PWR TEST should illuminate all ON lights if the window heat switches are ON. The PWR TEST forces the temperature controller to full power but overheat protection is still available.

If an OVERHEAT light illuminates, either a window has overheated or electrical power to the window has been interrupted. The affected window heat must be switched OFF and allowed 2-5mins to cool before switching ON again. The OVHT TEST simulates an overheat condition.

 

Pitot Heat

See Instrument Probes page for explanation.

 

Wing Anti Ice

Wing anti-ice (WAI) is very effective and is normally used as a de-icing system in-flight, in applications of 1 minute. On the ground it should be used continuously in icing conditions.

The WAI switch logic is interesting, on the ground, bleed air for WAI will cut-off if either thrust lever is above the take-off warning setting, but will be restored after the thrust is reduced. This allows you to perform engine run-ups etc without having to check that the WAI is still on afterwards. The switch is solenoid held and will trip off at lift-off, this is for performance considerations as the bleed air penalty is considerable.

Note that on early systems, ie those with a GND TEST position, with the WAI switch ON on the ground, the WAI is inhibited until lift-off ie “armed”, This is opposite to the present system.

WAI, unlike engine AI, uses bleed air from the main pneumatic manifold, this is to ensure a source of bleed air during engine out operations. Only the leading edge slats have WAI (ie not leading edge flaps). The NG series outboard slat has no wing anti-ice facility (see photo) believed to be due to excessive bleed requirements. However in June 2005 it was announced that the 737-MMA will have raked wingtips with anti-ice along the full span. This is because the MMA will be spending long periods of time on patrol at low level where it will be exposed to icing conditions.

NB Where QRH ENGINE FAILURE/SHUTDOWN drills ask “If wing anti-ice is required:”, if icing conditions are anticipated, these actions should be completed in preparation for WAI use to prevent asymmetric application. There is no bleed penalty for this reconfiguration until WAI is actually used.

On the NG, if WAI is used for more than 5 secs in-flight, the SMYD will adjust the stick shaker speeds and manouvre speed bars to allow for airframe ice.

Photo: Wing ice on the outboard slat of a 737-700

Engine Anti Ice

Engine anti-ice (EAI) heats the engine cowl to prevent ice build-up, which could break off and enter the engine. The 3/4/500 spinner was originally conical to prevent ice buildup but was changed to an elliptical shape to deflect ice away from the engine core. The NG’s have the best of both worlds with a coneliptical shaped spinner (see photo left) that does both jobs. EAI should be used continuously on the ground and in the air in icing conditions. It uses 5th stage bleed air, augmented by 9th stage as required, from the associated engine. COWL ANTI-ICE lights will illuminate if an overtemp (825F) or overpressure (65psig) condition exists in either duct. In this situation thrust on the associated engine should be reduced until the light extinguishes.

Wing and engine VALVE OPEN lights use the bright blue/dim blue – valve position in disagreement / agreement logic. The wing L and R VALVE OPEN lights in particular may remain bright blue after start and during taxy. This is because they are pneumatically operated, they can be made to open with a modest amount of engine thrust.

 

Airframe Visual Icing Cues

An ice detection system is an option that is rarely taken up on the 737 so it is up to the crew to spot ice formation and take the necessary action. The following photos show some of the places where ice accretion is visible from the flight deck. Note engine anti-ice should be used whenever the temperature and visible moisture criteria are met and not left until ice is seen, to avoid inlet ice build up which may shed into the engine.

 

Under the windscreen wiper blades.

This is one of the first places that ice will form, precipitation falls on the bottom of the windscreen and runs up to the wipers.

This is not an accurate indication of the amount of icing on the airframe because of the stagnation point where the blade and windscreen meet and also because the windscreen is heated.

I would describe conditions where ice forms here as LIGHT ICING.

On the wiper nut

This is my preferred indication of airframe ice accretion. If ice is seen here it is surely also on other parts of the airframe.

The weight and aerodynamic effect of all this ice on the the airframe and control surfaces is why there is the “residual ice” penalty of several tons on the landing performance graphs “If operating in icing conditions during any part of the flight when the forecast landing temperature is below 8C, reduce the normal climb limited landing weight by xxxxkg.” (FPPM 1.3.3).

I would describe conditions where ice forms here as MODERATE ICING.

On the central windscreen pillar

For ice to form on a flat heated windscreen, conditions must be bad. You can see how the shape of the formation follows the airflow lines. You can imagine how much ice is on the rest of the aircraft, especially when you consider that most of it is unheated, particularly on the fin and stabiliser.

Vol 1 SP.16.8 states “Avoid prolonged operation in moderate to severe icing conditions.” This photo was taken at about 20,000ft climbing through the tops of rain bearing frontal cloud. The ice shown here formed in under a minute.

I would describe conditions where ice forms here as SEVERE ICING.

Non-environmental Icing

The NG’s have a problem with frost forming after landing on the wing above the tanks where fuel has been cold soaked. This is officially known as “Wing upper surface non-environmental icing”. The reason is the increased surface area of the fuel that comes into contact with the upper surface of the wing. This is because the shape of the wing fuel tanks was changed (moved outboard) to accommodate the longer landing gear that was in turn required for the increased fuselage lengths of the NG family to reduce the risk of tailstrikes! The only solution until recently has been to limit your arrival fuel to less than approx 4,000kg. Now Boeing have issued guidelines on the acceptable location and amount of upper wing frost.

The Boeing advice is as follows: “Flight crews should visually inspect the lower wing surface. If there is frost or ice on the lower surface, outboard of measuring stick 4, there may also be frost or ice on the upper surface. The distance the frost extends outboard of measuring stick 4 can be used as an indication of the extent of frost on the upper surface. It should be noted that if the thickness of the frost on the lower surface of the wing is 1/16 inch (1.5 mm) thick or less, the thickness of the frost on the upper surface will be less than 1/16 inch (1.5 mm) thick. If the thickness of the frost on the lower surface is greater than 1/16 inch (1.5 mm), then a physical inspection of the upper surface frost is required.”

737-1/200

737-1/2/3/4/500

737-NG

Wiper Controls

One of the most welcome features of the 737-NG is the improvement to the windscreen wipers. The wipers are now independent, have an intermittent position and best of all – are almost silent.

Rain Repellent

The rain repellent has been removed due to worries about the environmental effects of the “RainBoe” fluid used as it contains CFC’s. It is also poisonous and in 1991 Boeing added D-limonine which has a strong smell of orange peel into RainBoe so that leakage could be detected. There are no plans to replace the rain repellent with another liquid product even though there are safe alternatives eg “Le Bozec”.

On 25 May 1982, a 737-200Adv (PP-SMY) was written off by a heavy landing in a rainstorm. One report stated that “The pilots misuse of rain repellent caused an optical illusion”.

Since early 1994 all Boeing aircraft have been built with Surface Seal coated glass from PPG Industries which has a hydrophobic coating. The coating does deteriorate with time depending upon wiper use and windscreen cleaning methods etc, but can be re-applied.

Check out this video of a 737-900 DV window opening during the take-off roll during flight testing. Notice that a high speed abort is not necessary if the DV window opens.

Limitations

Engine anti-ice must be on when icing conditions exist or are anticipated, except during climb and cruise below -40°C SAT.

Use of wing anti-ice above FL350 may cause bleed trip off and possible loss of cabin pressure. (SP.16.8)

 

See also maintenance notes by Ferreira

 

News

13 Aug 2009 – PPG Aerospace to redesign windshields for 737NG

Boeing requests windshield liner to keep glass from flight deck in bird-strike event

HUNTSVILLE, Ala., Aug. 13, 2009 – PPG Industries’ (NYSE:PPG) aerospace transparencies business has been awarded a contract by Spirit AeroSystems to redesign the laminated glass windshields for Boeing’s Next-Generation 737 airplanes. The windshields are being redesigned at Boeing’s request to accommodate airframe improvements. To meet Boeing specifications, the redesigned windshields will be slightly smaller than the current versions and include an inboard plastic antispall liner to prevent broken glass from entering the flight deck during a bird-strike event, according to Art Scott, PPG Aerospace global sales director for commercial original-equipment transparencies. “Boeing has asked for an alternate approach to bird-strike performance for the windshields that works structurally with the 737 airframe,” Scott said. “Adding an antispall liner to the windshields for Next-Generation 737 airplanes enables Boeing to keep the structural airframe design while incorporating newer technology.” PPG will be the sole source of the redesigned windshields for production and aftermarket applications. Scott said PPG expects certification of the new-design windshields in the second quarter 2010. The windshields will be designed and manufactured at PPG’s Huntsville, Ala., facility for delivery to Wichita, Kan., where Spirit makes the fuselage for Boeing.

Panels

737-1/200 Ice & Rain Panel

737-3/4/500 Ice & Rain Panel

737-NG Ice & Rain Panel

Differences:

  1. No alpha vanes
  2. WAI has ground test position
  3. Engine anti-ice captions are: COWL VALVE OPEN, R VALVE OPEN, L VALVE OPEN.
  1. Alpha vanes added
  2. Now only one temp probe (many 1/200’s had two)
  3. TAT TEST button (ie aspirated probe). NB if there is no TAT TEST button you have an unaspirated probe.
  1. Static ports not heated
  2. Aux pitot added.

Window Heat

If window heat is switched ON but the ON light is extinguished, this means that heat is not being applied to the associated window. This could be because the heat controller has detected that the window is becoming overheated (normal on hot days in direct sunlight) and can be verified by touching the window. The heat will automatically be restored when the window has cooled down. To verify that window heat is still available a PWR TEST should illuminate all ON lights if the window heat switches are ON. The PWR TEST forces the temperature controller to full power but overheat protection is still available.

If an OVERHEAT light illuminates, either a window has overheated or electrical power to the window has been interrupted. The affected window heat must be switched OFF and allowed 2-5mins to cool before switching ON again. The OVHT TEST simulates an overheat condition.

Pitot Heat

See Instrument Probes page for explanation.

Wing Anti Ice

Wing anti-ice (WAI) is very effective and is normally used as a de-icing system in-flight, in applications of 1 minute. On the ground it should be used continuously in icing conditions.

The WAI switch logic is interesting, on the ground, bleed air for WAI will cut-off if either thrust lever is above the take-off warning setting, but will be restored after the thrust is reduced. This allows you to perform engine run-ups etc without having to check that the WAI is still on afterwards. The switch is solenoid held and will trip off at lift-off, this is for performance considerations as the bleed air penalty is considerable.

Note that on early systems, ie those with a GND TEST position, with the WAI switch ON on the ground, the WAI is inhibited until lift-off ie “armed”, This is opposite to the present system.

WAI, unlike engine AI, uses bleed air from the main pneumatic manifold, this is to ensure a source of bleed air during engine out operations. Only the leading edge slats have WAI (ie not leading edge flaps). The NG series outboard slat has no wing anti-ice facility (see photo) believed to be due to excessive bleed requirements. However in June 2005 it was announced that the 737-MMA will have raked wingtips with anti-ice along the full span. This is because the MMA will be spending long periods of time on patrol at low level where it will be exposed to icing conditions.

NB Where QRH ENGINE FAILURE/SHUTDOWN drills ask “If wing anti-ice is required:”, if icing conditions are anticipated, these actions should be completed in preparation for WAI use to prevent asymmetric application. There is no bleed penalty for this reconfiguration until WAI is actually used.

On the NG, if WAI is used for more than 5 secs in-flight, the SMYD will adjust the stick shaker speeds and manouvre speed bars to allow for airframe ice.

Photo: Wing ice on the outboard slat of a 737-700

Engine Anti Ice

Engine anti-ice (EAI) heats the engine cowl to prevent ice build-up, which could break off and enter the engine. The 3/4/500 spinner was originally conical to prevent ice buildup but was changed to an elliptical shape to deflect ice away from the engine core. The NG’s have the best of both worlds with a coneliptical shaped spinner (see photo left) that does both jobs. EAI should be used continuously on the ground and in the air in icing conditions. It uses 5th stage bleed air, augmented by 9th stage as required, from the associated engine. COWL ANTI-ICE lights will illuminate if an overtemp (825F) or overpressure (65psig) condition exists in either duct. In this situation thrust on the associated engine should be reduced until the light extinguishes.

Wing and engine VALVE OPEN lights use the bright blue/dim blue – valve position in disagreement / agreement logic. The wing L and R VALVE OPEN lights in particular may remain bright blue after start and during taxy. This is because they are pneumatically operated, they can be made to open with a modest amount of engine thrust.

Airframe Visual Icing Cues

An ice detection system is an option that is rarely taken up on the 737 so it is up to the crew to spot ice formation and take the necessary action. The following photos show some of the places where ice accretion is visible from the flight deck. Note engine anti-ice should be used whenever the temperature and visible moisture criteria are met and not left until ice is seen, to avoid inlet ice build up which may shed into the engine.

Under the windscreen wiper blades.

This is one of the first places that ice will form, precipitation falls on the bottom of the windscreen and runs up to the wipers.

This is not an accurate indication of the amount of icing on the airframe because of the stagnation point where the blade and windscreen meet and also because the windscreen is heated.

I would describe conditions where ice forms here as LIGHT ICING.

On the wiper nut

This is my preferred indication of airframe ice accretion. If ice is seen here it is surely also on other parts of the airframe.

The weight and aerodynamic effect of all this ice on the the airframe and control surfaces is why there is the “residual ice” penalty of several tons on the landing performance graphs “If operating in icing conditions during any part of the flight when the forecast landing temperature is below 8C, reduce the normal climb limited landing weight by xxxxkg.” (FPPM 1.3.3).

I would describe conditions where ice forms here as MODERATE ICING.

On the central windscreen pillar

For ice to form on a flat heated windscreen, conditions must be bad. You can see how the shape of the formation follows the airflow lines. You can imagine how much ice is on the rest of the aircraft, especially when you consider that most of it is unheated, particularly on the fin and stabiliser.

Vol 1 SP.16.8 states “Avoid prolonged operation in moderate to severe icing conditions.” This photo was taken at about 20,000ft climbing through the tops of rain bearing frontal cloud. The ice shown here formed in under a minute.

I would describe conditions where ice forms here as SEVERE ICING.

Non-environmental Icing

The NG’s have a problem with frost forming after landing on the wing above the tanks where fuel has been cold soaked. This is officially known as “Wing upper surface non-environmental icing”. The reason is the increased surface area of the fuel that comes into contact with the upper surface of the wing. This is because the shape of the wing fuel tanks was changed (moved outboard) to accommodate the longer landing gear that was in turn required for the increased fuselage lengths of the NG family to reduce the risk of tailstrikes! The only solution until recently has been to limit your arrival fuel to less than approx 4,000kg. Now Boeing have issued guidelines on the acceptable location and amount of upper wing frost.

The Boeing advice is as follows: “Flight crews should visually inspect the lower wing surface. If there is frost or ice on the lower surface, outboard of measuring stick 4, there may also be frost or ice on the upper surface. The distance the frost extends outboard of measuring stick 4 can be used as an indication of the extent of frost on the upper surface. It should be noted that if the thickness of the frost on the lower surface of the wing is 1/16 inch (1.5 mm) thick or less, the thickness of the frost on the upper surface will be less than 1/16 inch (1.5 mm) thick. If the thickness of the frost on the lower surface is greater than 1/16 inch (1.5 mm), then a physical inspection of the upper surface frost is required.”

737-1/200

737-1/2/3/4/500

737-NG

Wiper Controls

One of the most welcome features of the 737-NG is the improvement to the windscreen wipers. The wipers are now independent, have an intermittent position and best of all – are almost silent.

Rain Repellent

The rain repellent has been removed due to worries about the environmental effects of the “RainBoe” fluid used as it contains CFC’s. It is also poisonous and in 1991 Boeing added D-limonine which has a strong smell of orange peel into RainBoe so that leakage could be detected. There are no plans to replace the rain repellent with another liquid product even though there are safe alternatives eg “Le Bozec”.

On 25 May 1982, a 737-200Adv (PP-SMY) was written off by a heavy landing in a rainstorm. One report stated that “The pilots misuse of rain repellent caused an optical illusion”.

Since early 1994 all Boeing aircraft have been built with Surface Seal coated glass from PPG Industries which has a hydrophobic coating. The coating does deteriorate with time depending upon wiper use and windscreen cleaning methods etc, but can be re-applied.

Check out this video of a 737-900 DV window opening during the take-off roll during flight testing. Notice that a high speed abort is not necessary if the DV window opens.

Limitations

Engine anti-ice must be on when icing conditions exist or are anticipated, except during climb and cruise below -40°C SAT.

Use of wing anti-ice above FL350 may cause bleed trip off and possible loss of cabin pressure. (SP.16.8)

See also maintenance notes by Ferreira

News

13 Aug 2009 – PPG Aerospace to redesign windshields for 737NG

Boeing requests windshield liner to keep glass from flight deck in bird-strike event

HUNTSVILLE, Ala., Aug. 13, 2009 – PPG Industries’ (NYSE:PPG) aerospace transparencies business has been awarded a contract by Spirit AeroSystems to redesign the laminated glass windshields for Boeing’s Next-Generation 737 airplanes. The windshields are being redesigned at Boeing’s request to accommodate airframe improvements. To meet Boeing specifications, the redesigned windshields will be slightly smaller than the current versions and include an inboard plastic antispall liner to prevent broken glass from entering the flight deck during a bird-strike event, according to Art Scott, PPG Aerospace global sales director for commercial original-equipment transparencies. “Boeing has asked for an alternate approach to bird-strike performance for the windshields that works structurally with the 737 airframe,” Scott said. “Adding an antispall liner to the windshields for Next-Generation 737 airplanes enables Boeing to keep the structural airframe design while incorporating newer technology.” PPG will be the sole source of the redesigned windshields for production and aftermarket applications. Scott said PPG expects certification of the new-design windshields in the second quarter 2010. The windshields will be designed and manufactured at PPG’s Huntsville, Ala., facility for delivery to Wichita, Kan., where Spirit makes the fuselage for Boeing.

Landing Gear – B737

General

The landing gear on the NG has been extensively redesigned. The nose gear is 3.5″ longer to relieve higher dynamic loads and the nose-wheelwell has been extended 3″ forward. The main gear is also longer to cater for the increased fuselage lengths of the -8/900 series and is constructed from a one piece titanium gear beam based on 757/767 designs. There is an externally mounted trunnion bearing on the gear, a re-located gas charging valve, and the uplock link is separate from the reaction link. It is fitted with 43.5″ tyres and digital antiskid.

Unfortunately the 737-700 was particularly prone to a dramatic shudder from the main landing gear if you tried to land smoothly. Fortunately Boeing started fitting shimmy dampers to this series from L/N 406 (Nov 1999) and a retrofit was made available.

One of the peculiarities of the 737 is that it invariably appears to crab when taxying. Theories for this include: A slightly castoring main gear to increase the crosswind capability; Play in the scissor link pins; Weather-cocking into any crosswind impinging on the fin; Torque reaction from the anti-collision light !!! Engineers will tell you that is due to the main gear having a couple of degrees of play due to the shimmy dampers.

 

Tyres

Tyres are tubeless and inflated with nitrogen. Pressures vary with series, maximum taxi weight, temperature and size of tyres. Unfortunately this large variation in tyre pressures makes it difficult to know your aquaplaning speed. The table below should prove helpful, notice how the aquaplaning speeds are all just below the typical landing speeds. Note: Once aquaplaning has started, it will continue to a much lower speed.

Series Main Gear Aquaplaning Speed Nose Gear Aquaplaning Speed
Originals 96 – 183psi 84 – 116Kts 125 – 145psi 96 – 104Kts
Classics 185 – 217psi 118 – 128Kts 163 – 194psi 111 – 121Kts
NG’s 117 – 205psi 93 – 123Kts 123 – 208psi 95 – 124Kts

Another oddity of the 737 is the resonant vibration during taxying that occurs at approx 17kts in classics and 24kts in NG’s. This is due to tyre “cold set”. This is a temporary flat spot that occurs in tyres with nylon chord (ie all Boeing tyres) when hot tyres are parked and they cool to ambient temperature. Hence the reason why the flat spot is most pronounced in cold weather and tends to disappear during taxying as the tyres warm up again.

The speed rating of all tyres is 225mph (195kts).

Gear Seals

Notice that none of the 737 series have ever had full main gear doors. Instead the outer wall of the tyres meet with aerodynamic seals in the wheel well to make a smooth surface along the underside of the aircraft. The first few 737’s had inflatable seals which were inflated by bleed air when the gear was either up or down and deflated during transit. The landing gear panel had a NOT SEALED caption which would illuminate during transit (normal), if it illuminated at any other time you could have a puncture and the seal could be depressurised with the GEAR SEAL SHUTOFF switch to save bleed requirements.

These were soon dropped as being too complicated and a similar drag and noise advantage was achieved with the present fixed rubber seals.

 

Brakes

The standard 737 brakes are a steel alloy called Cerametalix(R) with versions made by either Goodrich or Honeywell. Since 2008 the 737NG has had a carbon brake option from either Goodrich with Duracarb(R) or Messier-Bugatti with SepCarb® III-OR. They are both about 300kgs lighter than steel and last twice as long.

The brake pressure gauge merely shows the pressure of the air side of the accumulator and should normally indicate 3000psi. The normal brake system and autobrakes are powered by hydraulic system B. If brake pressure drops below 1500psi, hydraulic system A automatically provides alternate brakes which are manual only (ie no autobrake) and the brake pressure returns to 3000psi. Antiskid is available with alternate brakes, but not touchdown or locked wheel protection on series before the NG’s.

If both system A and B lose pressure, the accumulator isolation valve closes at 1900psi and you are just left with residual hydraulic pressure and the pre-charge. The gauge will indicate approx 3000psi and should provide 6 full applications of brake power through the normal brake lines (so full antiskid is available) As the brakes are applied the residual pressure reduces until it reaches 1000psi at which point you will have no more braking available.

If the brake pressure gauge ever shows zero, this merely indicates that the pre-charge has leaked out, normal and alternate braking are unaffected if you still have the hydraulic systems (see QRH). The accumulator also provides pressure for the parking brake.

Note that on the 737-1/200, hydraulic system A operates the inboard brakes and system B operates the outboard brakes. Both brake pressures are indicated on the single hydraulic brake pressure gauge.

There are four thermal fuse plugs in the inner wheel half which prevent tyre explosion caused by hot brakes. The plugs melt to release tyre pressure at approx 177C (351F).

Brake Pressure Indication (psi) Condition
3000 Normal.
3000 No hydraulics, minimum 6 applications of brakes available with accumulator.
1000 No hydraulics, accumulator used up.
Zero No pre-charge, normal braking available with hydraulics.

 

Brake Accumulator

Brake Wear Pin

Autobrakes

Autobrake Selector

Max Pressure at Brakes (PSI)

Deceleration Rate (ft/sec²)

1

1250

4

2

1500

5

3

2000

7.2

Max

3000

12 (below 80kts)

14 (above 80kts)

RTO

Full

Not Controlled

There is an “on ramp” period where autobrake pressure is applied over a period of time. Approximately 750psi is applied in 1.75 sec, then the pressures above are reached in another 1.25sec for autobrakes 1, 2, or 3 and approx. 1.0 sec for autobrake MAX.

Notice from the table above that autobrake Max does not give full brake pressure. For absolute maximum braking on landing, select autobrake Max to assure immediate application after touch down then override with full toe brake pressure.

Using high autobrake settings with idle reverse is particularly hard on the brakes as they will be working for the given deceleration rate without the assistance of full reverse thrust.

To cancel the autobrake on the landing roll with toe brakes you must apply a brake pressure in excess of 800psi (ie less than that required for autobrake 1). This is more difficult on the NG’s because the feedback springs on the brake pedals are stiffer. Autobrake can also be cancelled by putting the speedbrake lever down or by switching the autobrake off. I would advise against the latter in case you accidentally select RTO and get the full 3000psi of braking!

Occasionally you may see the brakes (rather than the cabin crew!) smoking during a turnaround. This may be due to hard braking at high landing weights. But the most common reason is that too much grease is put on the axle at wheel change so that when the wheel is pushed on, the grease is deposited inside the torque tube; when this gets hot, it smokes. It could also be contamination from hydraulic fluid either from bleeding operation or a leak either from the brakes or another source.

Photos

The landing gear panel is located between  the engine instruments and F/O’s instrument panel.The Green lights tell you that the gear is down and locked and the red lights warn you if the landing gear is in disagreement with the gear lever position.

With the gear UP and locked and the lever UP or OFF, all lights should be extinguished.

On a couple of occasions I have seen 3 reds and 3 greens after the gear has been selected down. This was because the telescopic gear handle had not fully compressed back toward the panel. If this happens to you, give it a tap back in and the red lights will extinguish.

737’s used for cargo operations have an extra set of green “GEAR DOWN” lights on the aft overhead panel. This is because with the cabin filled with freight, the main gear downlock viewer could not be guaranteed to be accessible in-flight.

The NG’s also have these lights because they do not have gear downlock viewers installed.

Main gear viewer (not NG’s)

If any green gear lights do not illuminate after the gear is lowered, you might consider a visual inspection through the gear viewers. The main gear viewer is in the cabin and the nose gear viewer is on the flight deck. The main gear viewers are not installed on NG series aircraft.This is the main gear viewer and it is located in the isle, just behind the emergency exit row.

The first time you look through a viewer it will probably take you several minutes to find what you are looking for, hardly ideal if you are in the situation for real so it is worth acquainting yourself with its use.

 

Main gear viewer prisms (not NG’s)

There are two prisms, one for each main gear leg. Don’t forget to switch on the wheel-well light if at night.

 

Gear locked marks (not NG’s)

Eventually, you should be able to see three red marks on the undercarriage, if they line up then your gear is certainly down and probably locked.

Main wheel-well and downlock viewer (classics)

The location of the main gear downlock viewer in the wheel well can be clearly seen in this photograph.

 

Nosegear viewer (not NG’s)

The nosegear viewer is located under a panel toward the aft of the flightdeck. There is no prism, just a long tube. This viewer directs your eye exactly toward the correct place for viewing but is usually more dirty.

The nosegear down marks are two red arrows pointing at each other.

Manual gear extension access hatch If the gear fails to extend properly or hydraulic system A is lost, the gear can be manually extended by pulling the manual gear extension handles, located in the flight deck. This should be done in accordance with the QRH procedure.On NG aircraft opening this hatch affects the operation of landing gear extension & retraction.
Tyre damage fitting – NG only This pin is designed to detect any loose tyre tread during gear retraction. If any object impacts on it during retraction, then the gear will automatically extend. The affected gear cannot be retracted until this fitting is replaced. There is one pin at the aft outside of each main wheel well.

Other landing gear system photographs

Nosewheel door (classic)

 

Gravel Deflector (-200)

Nose wheel

Tail skid (-400)

General

The landing gear on the NG has been extensively redesigned. The nose gear is 3.5″ longer to relieve higher dynamic loads and the nose-wheelwell has been extended 3″ forward. The main gear is also longer to cater for the increased fuselage lengths of the -8/900 series and is constructed from a one piece titanium gear beam based on 757/767 designs. There is an externally mounted trunnion bearing on the gear, a re-located gas charging valve, and the uplock link is separate from the reaction link. It is fitted with 43.5″ tyres and digital antiskid.

Unfortunately the 737-700 was particularly prone to a dramatic shudder from the main landing gear if you tried to land smoothly. Fortunately Boeing started fitting shimmy dampers to this series from L/N 406 (Nov 1999) and a retrofit was made available.

One of the peculiarities of the 737 is that it invariably appears to crab when taxying. Theories for this include: A slightly castoring main gear to increase the crosswind capability; Play in the scissor link pins; Weather-cocking into any crosswind impinging on the fin; Torque reaction from the anti-collision light !!! Engineers will tell you that is due to the main gear having a couple of degrees of play due to the shimmy dampers.

Tyres

Tyres are tubeless and inflated with nitrogen. Pressures vary with series, maximum taxi weight, temperature and size of tyres. Unfortunately this large variation in tyre pressures makes it difficult to know your aquaplaning speed. The table below should prove helpful, notice how the aquaplaning speeds are all just below the typical landing speeds. Note: Once aquaplaning has started, it will continue to a much lower speed.

Series Main Gear Aquaplaning Speed Nose Gear Aquaplaning Speed
Originals 96 – 183psi 84 – 116Kts 125 – 145psi 96 – 104Kts
Classics 185 – 217psi 118 – 128Kts 163 – 194psi 111 – 121Kts
NG’s 117 – 205psi 93 – 123Kts 123 – 208psi 95 – 124Kts

Another oddity of the 737 is the resonant vibration during taxying that occurs at approx 17kts in classics and 24kts in NG’s. This is due to tyre “cold set”. This is a temporary flat spot that occurs in tyres with nylon chord (ie all Boeing tyres) when hot tyres are parked and they cool to ambient temperature. Hence the reason why the flat spot is most pronounced in cold weather and tends to disappear during taxying as the tyres warm up again.

The speed rating of all tyres is 225mph (195kts).

Gear Seals

Notice that none of the 737 series have ever had full main gear doors. Instead the outer wall of the tyres meet with aerodynamic seals in the wheel well to make a smooth surface along the underside of the aircraft. The first few 737’s had inflatable seals which were inflated by bleed air when the gear was either up or down and deflated during transit. The landing gear panel had a NOT SEALED caption which would illuminate during transit (normal), if it illuminated at any other time you could have a puncture and the seal could be depressurised with the GEAR SEAL SHUTOFF switch to save bleed requirements.

These were soon dropped as being too complicated and a similar drag and noise advantage was achieved with the present fixed rubber seals.

Brakes

The standard 737 brakes are a steel alloy called Cerametalix(R) with versions made by either Goodrich or Honeywell. Since 2008 the 737NG has had a carbon brake option from either Goodrich with Duracarb(R) or Messier-Bugatti with SepCarb® III-OR. They are both about 300kgs lighter than steel and last twice as long.

The brake pressure gauge merely shows the pressure of the air side of the accumulator and should normally indicate 3000psi. The normal brake system and autobrakes are powered by hydraulic system B. If brake pressure drops below 1500psi, hydraulic system A automatically provides alternate brakes which are manual only (ie no autobrake) and the brake pressure returns to 3000psi. Antiskid is available with alternate brakes, but not touchdown or locked wheel protection on series before the NG’s.

If both system A and B lose pressure, the accumulator isolation valve closes at 1900psi and you are just left with residual hydraulic pressure and the pre-charge. The gauge will indicate approx 3000psi and should provide 6 full applications of brake power through the normal brake lines (so full antiskid is available) As the brakes are applied the residual pressure reduces until it reaches 1000psi at which point you will have no more braking available.

If the brake pressure gauge ever shows zero, this merely indicates that the pre-charge has leaked out, normal and alternate braking are unaffected if you still have the hydraulic systems (see QRH). The accumulator also provides pressure for the parking brake.

Note that on the 737-1/200, hydraulic system A operates the inboard brakes and system B operates the outboard brakes. Both brake pressures are indicated on the single hydraulic brake pressure gauge.

There are four thermal fuse plugs in the inner wheel half which prevent tyre explosion caused by hot brakes. The plugs melt to release tyre pressure at approx 177C (351F).

Brake Pressure Indication (psi) Condition
3000 Normal.
3000 No hydraulics, minimum 6 applications of brakes available with accumulator.
1000 No hydraulics, accumulator used up.
Zero No pre-charge, normal braking available with hydraulics.

Brake Accumulator

Brake Wear Pin

Autobrakes

Autobrake Selector

Max Pressure at Brakes (PSI)

Deceleration Rate (ft/sec²)

1

1250

4

2

1500

5

3

2000

7.2

Max

3000

12 (below 80kts)

14 (above 80kts)

RTO

Full

Not Controlled

There is an “on ramp” period where autobrake pressure is applied over a period of time. Approximately 750psi is applied in 1.75 sec, then the pressures above are reached in another 1.25sec for autobrakes 1, 2, or 3 and approx. 1.0 sec for autobrake MAX.

Notice from the table above that autobrake Max does not give full brake pressure. For absolute maximum braking on landing, select autobrake Max to assure immediate application after touch down then override with full toe brake pressure.

Using high autobrake settings with idle reverse is particularly hard on the brakes as they will be working for the given deceleration rate without the assistance of full reverse thrust.

To cancel the autobrake on the landing roll with toe brakes you must apply a brake pressure in excess of 800psi (ie less than that required for autobrake 1). This is more difficult on the NG’s because the feedback springs on the brake pedals are stiffer. Autobrake can also be cancelled by putting the speedbrake lever down or by switching the autobrake off. I would advise against the latter in case you accidentally select RTO and get the full 3000psi of braking!

Occasionally you may see the brakes (rather than the cabin crew!) smoking during a turnaround. This may be due to hard braking at high landing weights. But the most common reason is that too much grease is put on the axle at wheel change so that when the wheel is pushed on, the grease is deposited inside the torque tube; when this gets hot, it smokes. It could also be contamination from hydraulic fluid either from bleeding operation or a leak either from the brakes or another source.

Photos

The landing gear panel is located between  the engine instruments and F/O’s instrument panel.The Green lights tell you that the gear is down and locked and the red lights warn you if the landing gear is in disagreement with the gear lever position.

With the gear UP and locked and the lever UP or OFF, all lights should be extinguished.

On a couple of occasions I have seen 3 reds and 3 greens after the gear has been selected down. This was because the telescopic gear handle had not fully compressed back toward the panel. If this happens to you, give it a tap back in and the red lights will extinguish.

737’s used for cargo operations have an extra set of green “GEAR DOWN” lights on the aft overhead panel. This is because with the cabin filled with freight, the main gear downlock viewer could not be guaranteed to be accessible in-flight.

The NG’s also have these lights because they do not have gear downlock viewers installed.

Main gear viewer (not NG’s)

If any green gear lights do not illuminate after the gear is lowered, you might consider a visual inspection through the gear viewers. The main gear viewer is in the cabin and the nose gear viewer is on the flight deck. The main gear viewers are not installed on NG series aircraft.This is the main gear viewer and it is located in the isle, just behind the emergency exit row.

The first time you look through a viewer it will probably take you several minutes to find what you are looking for, hardly ideal if you are in the situation for real so it is worth acquainting yourself with its use.

 

Main gear viewer prisms (not NG’s)

There are two prisms, one for each main gear leg. Don’t forget to switch on the wheel-well light if at night.

 

Gear locked marks (not NG’s)

Eventually, you should be able to see three red marks on the undercarriage, if they line up then your gear is certainly down and probably locked.

Main wheel-well and downlock viewer (classics)

The location of the main gear downlock viewer in the wheel well can be clearly seen in this photograph.

 

Nosegear viewer (not NG’s)

The nosegear viewer is located under a panel toward the aft of the flightdeck. There is no prism, just a long tube. This viewer directs your eye exactly toward the correct place for viewing but is usually more dirty.

The nosegear down marks are two red arrows pointing at each other.

Manual gear extension access hatch If the gear fails to extend properly or hydraulic system A is lost, the gear can be manually extended by pulling the manual gear extension handles, located in the flight deck. This should be done in accordance with the QRH procedure.On NG aircraft opening this hatch affects the operation of landing gear extension & retraction.
Tyre damage fitting – NG only This pin is designed to detect any loose tyre tread during gear retraction. If any object impacts on it during retraction, then the gear will automatically extend. The affected gear cannot be retracted until this fitting is replaced. There is one pin at the aft outside of each main wheel well.

Other landing gear system photographs

Nosewheel door (classic)

Gravel Deflector (-200)

Nose wheel

Tail skid (-400)

Hydraulics -B737

Pumps

The hydraulic pump panel -1/200

The 737-1/200 had system A powered by the two Engine Driven Pumps (EDP’s) and system B powered by the two Electric Motor Driven Pumps (EMDP’s). There is also a ground interconnect switch to allow system A to be powered when the engines are shut down.

 

The hydraulic pump panel -300 onwards

From the 737-300 onwards each hydraulic system had both an EDP and an EMDP for greater redundancy in the event of an engine or generator failure.

To see the hydraulic systems (pumps, reservoirs, gauges etc) see wheel-well fwd

Services Supplied

Services Supplied

System A

System B

Standby

A/P “A” A/P “B”  
Ailerons Ailerons  
Rudder Rudder Rudder
  Yaw damper Standby yaw damper (as installed)
Elev & Elev feel Elev & Elev feel  
Inboard flight spoiler Outboard flight spoiler  
Ground spoilers    
  L/E flaps & slats L/E flaps & slats (for extension only)
  T/E flaps  
PTU for autoslats Autoslats  
No1 thrust reverser No2 thrust reverser Nos 1 & 2 thrust reversers (slow)
Nose wheel steering Alt nose wheel steering  
Alternate brakes (man only) Normal (auto & man) brakes  
Landing gear Landing gear transfer unit (retraction only)  

Reservoirs

Hydraulic System B Reservoir Pressure Gauge

The hydraulic reservoirs are pressurised from the pneumatic manifold to ensure a positive flow of fluid reaches the pumps. A from the left manifold and B from the right (see wheel-well fwd). The latest 737’s (mid 2003 onwards) have had their hydraulic reservoir pressurisation system extensively modified to fix two in-service problems 1) hydraulic vapours in the flight deck caused by hydraulic fluid leaking up the reservoir pressurisation line back to the pneumatic manifold giving hydraulic fumes in the air-conditioning and 2) pump low pressure during a very long flight in a cold soaked aircraft. The latter is due to water trapped in the reservoir pressurisation system freezing blocking reservoir bleed air supply. Aircraft which have been modified (SB 737-29-1106) are recognised by only having one reservoir pressure gauge in the wheel well.

 

Fuses

Hydraulic Fuses

Also in the wheel well can be seen the hydraulic fuses. These are essentially spring-loaded shuttle valves which close the hydraulic line if they detect a sudden increase in flow such as a burst downstream, thereby preserving hydraulic fluid for the rest of the services. Hydraulic fuses are fitted to the brake system, L/E flap/slat extend/retract lines, nose gear extend/retract lines and the thrust reverser pressure and return lines.

 

Above schematic courtesy of Leon Van Der Linde. For a more detailed hydraulic schematic diagram, click here.

737-3/400 Hydraulic Gauges

On pre-EIS aircraft (before 1988) the hydraulic gauges were similar to the 737-200. There are now separate quantity gauges since the reservoirs are not interconnected and the markings have been simplified. There is now just a single brake pressure gauge showing the normal brake pressure from system B.

 

 

737-200 Hydraulic Gauges.

Notice that there is only a system A quantity gauge, this is because on the 737-1/200 system B is filled from system A reservoir. System B quantity is monitored by the amber “B LOW QUANTITY” light above. The hydraulic brake pressure gauge has two needles because system A operates the inboard brakes and system B the outboard brakes, each has an accumulator.


Quantities

This table shows the nominal quantities at different levels in the reservoirs

Aircraft Series Originals Classics NG’s
System Gauges EIS Upper CDU
A Full level 3.6 USG 100% 100% (5.7Gal / 21.6Ltrs)
Refill 2.35 USG 88% 76%
EDP Standpipe ? 22% 20%
EMDP Standpipe N/A 0% 0%
B Full level Full 100% 100% (8.2Gal / 31.1Ltrs)
Refill 3/4 88% 76%
Fill & balance line (to standby reservoir) ? 64% 72%
EDP Standpipe N/A 40% 0%
EMDP Standpipe ? 11% 0%

Eg. If you are in say a 737-300 and you notice to System B hydraulic quantity drop to 64%, then from the table above, you may suspect a leak in the balance line or standby reservoir.

Note: Refill figure valid only when airplane is on ground with both engines shutdown or after landing with flaps up during taxi-in.

The hydraulic reservoirs can be filled from the ground service connection point on the forward wall of the stbd wheel well.

Hydraulic ground service connection

Normal hydraulic pressure is 3000 psi

Minimum hydraulic pressure is 2800 psi

Maximum hydraulic pressure is 3500 psi

Normal brake accumulator precharge is 1000 psi

NB The alternate flap system will extend (but not retract) LE devices with standby hydraulic power. It will also extend or retract TE flaps with an electric drive motor but there is no asymmetry protection for this.

LGTU makes Hyd B pressure available for gear retraction when Engine No1 falls below 50% N2

 

Methods for Transfer of Hydraulic Fluid

It should go without saying that if a hydraulic system is low on quantity then you should top up that system with fresh fluid (and find out why it was low!) to avoid cross contamination. However if you really want to move fluid from one system to another here is how to do it.

A to B (1% transfer per cycle)

  1. Chock the aircraft & ensure area around stabiliser is clear.
  2. Switch both EMDP’s OFF.
  3. Release parking brakes and deplete accumulator to below 1800psi by pumping toe brakes.
  4. Switch Sys A EMDP ON and apply parking brakes.
  5. Switch Sys A EMDP OFF and depressurise through control column. (Use stabiliser rather than ailerons to prevent damage to equipment or personnel)
  6. Switch Sys B EMDP ON and release parking brakes. (Sends the fluid back to system B because the shuttle/priority valves send the fluid back to the normal brake system.)

A to B – An alternative method

  1. Chock the aircraft & ensure area around stabiliser is clear.
  2. Switch both EMDP’s ON.
  3. Switch Sys B EMDP OFF and depressurise through control column. (Use stabiliser rather than ailerons to prevent damage to equipment or personnel)
  4. Switch Sys A EMDP ON and apply parking brakes. (Uses fluid from system A)
  5. Switch Sys B EMDP ON and release parking brakes. (Sends the fluid back to system B because the shuttle/priority valves send the fluid back to the normal brake system.)

B to A (4% transfer per cycle)

  1. Ensure area around No1 thrust reverser is clear.
  2. Switch both EMDP’s OFF
  3. Switch either FLT CONTROL to SBY RUD.
  4. Select No1 thrust reverser OUT (uses standby hyd sys)
  5. Switch FLT CONTROL to ON.
  6. Switch Hyd Sys A EMDP ON.
  7. Stow No 1 thrust reverser (using sys A)

Fuel -B737

Fuel Panels

737-1/200 Fuel Panel

 

737-Classic 4-Tank Fuel Panel

 

NG Fuel Panel

 

The maximum declarable fuel capacity for tech log, nav log, etc is 16,200kgs for 3-Tank Classics, 20,800kgs for NG’s and up to 37,712kgs for BBJ’s depending upon how many tanks the customer has specified (max 12). The AFM limits are higher, but not normally achievable with standard SG’s.

The fuel panels for the various series have not changed much over the years. The NG’s have separate ENG VALVE CLOSED & SPAR VALVE CLOSED lights in place of FUEL VALVE CLOSED. The -1/200 panel also has blue VALVE OPEN lights similar to that on the crossfeed valve. The FILTER BYPASS lights were FILTER ICING on the 1/200.

The 1/200’s had heater switches; these used bleed air to heat the fuel and de-ice the fuel filter. They were solenoid held and automatically moved back to OFF after one minute.

NG: The engine spar valves and APU are normally powered by the hot battery bus but have a dedicated battery to ensure that there is always power to shut off the fuel in an emergency.

Fuel Gauges

Analogue Fuel Gauges

-1/200’s and some older -300’s

 

 

Digital Sunburst Fuel Gauges – Simmonds 4 Tank

– 3/4/500’s

Digital Sunburst Fuel Gauges – Smiths

– 3/4/500’s

Fuel Gauge Accuracy

The 737 fuel quantity indication system has the following accuracy tolerances:

737-100/-200:
FQIS accuracy: +/- 3.0%

737-300/-400/-500,
FQIS accuracy with digital indicators: +/- 2.5 %
FQIS accuracy with analog indicators: +/- 3.0%

The total tolerance for the FQIS system is based on a full tank. For example, if the fuel tank maximum capacity is 10,000 KG, then the tolerance of the gauging is 0.03 (airplane with analog indicators) * 10000 = 300 KG. The system tolerance is then +/- 300 KG at any fuel level within the tank.

The accuracy of the fuel flow transmitter is a function of the fuel flow. At engine idle, the system tolerance can be 12%. During cruise, the tolerance is less than 1.5%. The fuel flow indication is integrated over time to calculate the fuel used for each engine.

737-600/-700/-800/-900 with densitometer:
FQIS accuracy: +/- 1.0% overall
Main tanks > 50%, -1 to 5 deg pitch, +/- 1 deg roll: +/- 1.5%
Main tanks < 50%, -1 to 5 deg pitch, +/- 1 deg roll: +/- 1.0%

737-600/-700/-800/-900 without densitometer:
FQIS accuracy: +/- 2.0% overall
Main tanks > 50%, -1 to 5 deg pitch, +/- 1 deg roll: +/- 2.5%
Main tanks < 50%, -1 to 5 deg pitch, +/- 1 deg roll: +/- 2.0

The total tolerance for the FQIS system is based on a full tank. For example, if the fuel tank maximum capacity is 10,000 KG, then the tolerance of the gauging is 0.02 (airplane without a densitometer) * 10000 = 200 KG. The system tolerance is then +/- 200 KG at any fuel level within the tank.

The accuracy tolerance of the fuel flow transmitter is a function of the fuel flow. At engine idle, the system tolerance can be 12%. During cruise, the tolerance is less than 0.5%. The fuel flow indication is integrated over time to calculate the fuel used for each engine.

On the Digital Sunburst fuel gauges, pressing the “Qty test” button will start a self test of the display and the fuel quantity indicating system. After the test, each gauge will display any error codes that they may have.

Note: The gauges are still considered to be operating normally with error codes 1, 3, 5 or 7 on the Simmonds gauges or error codes 1,3 and 6 on the Smiths gauges. ie If the gauge is indicating (rather than zero) the gauge may be used.

 DU Fuel Gauges

-NG’s

 

Low fuel quantity indication illuminates below either 907 or 453kg

– NG’s

NG fuel gauges can give messages such as LOW, CONFIG or IMBAL

  Digital Fuel Quantity Indicator Error Codes – Simmonds

Error Code Fuel Quantity Indicator Reading Probable Cause Gauges considered to be operating normally?
0 Zero Missing or disconnected tank unit
1 Normal Tank contamination Yes
2 Zero Bad HI-Z lead
3 Normal Bad compensator unit wiring Yes
4 Zero Bad tank unit wiring
5 Normal Bad compensator unit Yes
6 Zero Bad tank unit
7 Normal Contamination/water in compensator Yes
8 Zero Bad fuel quantity indicator
9 Normal or zero Improperly calibrated indicator
Blank Bad fuel quantity indicator

Digital Fuel Quantity Indicator Error Codes – Smiths

Error Code Fuel Quantity Indicator Reading Probable Cause Gauges considered to be operating normally?
1 Normal Open or short in compensator LO-Z wiring Yes
2 Zero Short circuit in compensator unit
3 Normal Too much leakage in compensator unit Yes
4 Zero Open or short circuit in a LO-Z to a tank unit
5 Zero Short circuit in a tank unit
6 Normal Too much leakage in tank unit Yes
7 Zero (or ERR in flight) Calibration unit does not operate correctly
8 Blank An error in the DCTU data
9 Zero (or ERR in flight) A problem with the indicator memory
  10 Zero Open or short circuit in the HI-Z line
Dripsticks

If a fuel gauge is u/s the quantity must be determined by using the dripsticks (floatsticks in later aircraft). The classics have 5 dripsticks in each wing tank and none in the centre tank. The NG has 6 dripsticks in each wing tank and 4 in the centre tank. Because of cumulative errors it is recommended that the wings are filled once every few sectors to ensure an even fuel balance. In-flight, the GW must be periodically updated to ensure the accuracy of VNAV speeds, buffet margin and max altitude.

Floatstick

Fuel quantity is measured by using a series of capacitors in the tanks with fuel acting as the dielectric. Calibration of the fuel gauges is done by capacitance trimmers, these are adjusted to standardise the total tank capacitance and allows for the replacement of gauges. On older aircraft the trimmers were accessible from the flightdeck (below the F/O’s FMC) but they have since been removed to a safer place! Capacitance trimmers

Pumps

There are two AC powered fuel pumps in each tank; there are also EDP’s at each engine. Both fuel pump low pressure lights in any tank are required to illuminate the master caution to avoid spurious warnings at high AoA’s or accelerations. Centre tank LP lights are armed only when their pumps are ON.

Leaving a fuel pump on with a low pressure light illuminated is not only an explosion risk (see Thai and Philippine write offs) but also if a pump is left running dry for over approx 10 minutes it will lose all the fuel required for priming which will render it inoperative even when the tank is refuelled. If you switch on the centre tank pumps and the LP lights remain illuminated for more than 19 seconds then this is probably what has happened. The pumps should be switched off and considered inop until they can be re-primed.

On the 1-500’s, the centre tank pumps are located in a dry area of the wing root but on the NG’s the pumps are actually inside the fuel tank (see photo below). This is why only the NG’s are affected by AD 2002-19-52 which requires the crew to maintain certain minimum fuel levels in the center fuel tanks. You can see the location of the centre tank pumps on the forward wall of the wheel well on the NG’s, since the forward wall is actually the back of the centre fuel tank.

Note: for aircraft delivered after May 2004, centre tank fuel pumps will automatically shut off when they detect a low output pressure.

Right centre tank fuel pump on the forward wall of the wheel well – NG’s only

Centre Tank Scavange Pumps

These transfer fuel from the centre tank into tank 1 at a minimum rate of 100kg/hr, although usually nearer 200kg/hr. The trigger for the scavenge pump is different for the series as follows:

  • Originals: Only fitted after l/n 990 (Dec 1983). Operates the same as the classics.
  • Classics: Switching both centre tank pumps OFF will cause the centre tank scavenge pump to transfer centre tank fuel into tank 1 for 20 minutes.
  • NG’s: The centre tank scavenge pump starts automatically when main tank 1 is half full and its FWD pump is operating. Once started, it will continue for the remainder of the flight.

NB On the classics, when departing with less than 1,000kg of fuel in the centre tank, an imbalance may occur during the climb. This is because the RH centre tank pump will stop feeding due to the body angle so number 2 engine fuel is drawn from main tank 2, while engine 1 is still drawing fuel from the centre tank. When this “runs dry” the scavenge pump will also transfer any remaining centre tank fuel into main tank 1, thereby exacerbating the imbalance.

The APU uses fuel from the number 1 tank. If AC power is available, select the No 1 tank pumps ON for APU operation to assist the fuel control unit, especially during start. Newer –500 series aircraft have an extra, DC operated APU fuel pump in the No 1 tank which operates automatically during the start sequence. The APU burns about 160Kgs/hr with electrics and an air-conditioning pack on and this should be considered in the fuel calculations if expecting a long turnaround or waiting with pax on board for a late slot.


Fuel Temperature

Limitations: Max fuel temp +49ºC, Min fuel temp -45ºC or freezing point +3ºC, whichever is higher. Typical freezing point of Jet A1 is -47ºC. If the fuel temp is approaching the lower limits you could descend into warmer air or accelerate to increase the kinetic heating. Fuel temp is taken from main tank 1 because this will be the coldest as it has less heating from the smaller hydraulic system A.

A fuel sampling and testing kit is kept on the flight deck of all aircraft to test for water.

The NG series are prone to “Upper Wing Surface Non-environmental Icing” or “Cold Soaked Fuel Frost” CSFF. This is due to cold soaked fuel causing frost to form on the wings during the turnarounds – even in warm conditions! From July 2004 NGs have been delivered with markings on the upper surface of the wings where this frost is allowable for despatch under the following conditions:

Takeoff with CSFF on the upper wing surfaces is permissible, provided the following are met:

  • the frost on the upper surface is less than 1/16 inch (1.5 mm) thick
  • the extent of the frost is similar on both wings
  • the frost is on or between the black lines defining the permissible CSFF area
  • the outside air temperature is above freezing(0 C, 32 F)
  • there is no precipitation or visible moisture at the wing surface (rain, drizzle, snow, fog)


 

Auxiliary Fuel System

The standard number of fuel tanks is three. Classics could be fitted with an auxiliary fourth tank which was controlled from the main panel as shown at the top of the page. The 737-200Adv could also be fitted with an auxiliary tank at the forward end of the aft hold; these were available in either 3,065 or 1,421 litre capacities.

BBJ Aux fuel panel located on Capt’s & F/O’s main panels

The BBJ can have up to 9 aux fuel tanks giving it a maximum fuel quantity of 37,712kgs (83,000lbs) although in practice this would probably take you over MTOW if any payload was carried. This fuel would give a theoretical range in excess of 6200nm. The aux tanks are located at the rear of the fwd hold and the front of the aft hold, this reduces the C of G movement as fuel is loaded and used.

Aux fuel tank in aft hold of a BBJ2 (-800 fuselage)

Refuelling of the aux tanks is done by moving the guarded switch in the refuelling panel to AUX TANKS. The controls for main tanks 1 and 2 change to aft aux (AA) and fwd aux (FA) respectively.

The aux fuel system is essentially automatic. It works by transferring fuel from the aux tanks into the centre tank where it is then fed to the engines in the normal way. Flight crew can select fwd or aft tanks but normal practice is to use both to maintain C of G balance. The fwd and aft tanks are switched off when the ALERT light illuminates on the main panel.

Aux fuel control panel (Overhead panel)

There are no pumps in the aux fuel system. Cabin differential pressure (and bleed air as a backup) is used to maintain a head of pressure in the aux tanks to push the aux fuel into the centre tank.

Aux fuel control panel (Aft overhead panel)

(All Aux fuel photos: Capt D Britchford)


Ferry Tank

The 737-200 had provision for a ferry kit. This comprised a 2,000 US Gal (7,570 litre) bladder cell which attached to the seat tracks of the passenger cabin. The fuel was fed to the centre tank through a manual valve by cabin pressure.


Centre Fuel Tank Inerting

To date, two 737’s, 737-400 HS-TDC of Thai Airways on 3 Mar 2001 and 737-300 EI-BZG operated by Philippine Airlines on 5 Nov 1990 have been destroyed on the ground due to explosions in the empty centre fuel tank. The common factor in both accidents was that the centre tank fuel pumps were running in high ambient temperatures with empty or almost empty centre fuel tanks.

Even an empty tank has some unusable fuel which in hot conditions will evaporate and create an explosive mixture with the oxygen in the air. These incidents, and 15 more on other types since 1959, caused the FAA to issue SFAR88 in June 2001 which mandates improvements to the design and maintenance of fuel tanks to reduce the chances of such explosions in the future. These improvements include the redesign of fuel pumps, FQIS, any wiring in tanks, proximity to hot air-conditioning or pneumatic systems, etc.

737s delivered since May 2004 have had centre tank fuel pumps which automatically shut off when they detect a low output pressure and there have been many other improvements to wiring and FQIS. But the biggest improvement will be centre fuel tank inerting. This is universally considered to be the safest way forward, but is very expensive and possibly impractical. The NTSB recommended many years ago to the FAA that a fuel tank inerting system be made mandatory, but the FAA have repeatedly rejected it on cost grounds.

Boeing has developed a Nitrogen Generating System (NGS) which decreases the flammability exposure of the center wing tank to a level equivalent to or less than the main wing tanks. The NGS is an onboard inert gas system that uses an air separation module (ASM) to separate oxygen and nitrogen from the air. After the two components of the air are separated, the nitrogenenriched air (NEA) is supplied to the center wing tank and the oxygenenriched air (OEA) is vented overboard. NEA is produced in sufficient quantities, during most conditions, to decrease the oxygen content to a level where the air volume (ullage) will not support combustion. The FAA Technical Center has determined that an oxygen level of 12% is sufficient to prevent ignition, this is achievable with one module on the 737 but will require up to six on the 747.

On 21 Feb 2006 the Honeywell NGS was certified by the FAA after over 1000hrs flight testing on two 737-NGs. Aircraft from l/n 1935 (Aug 2006) to 2006 were delivered with basic provisions for NGS and more comprehensive provisioning up to l/n 2019. Full production cutover is scheduled for l/n 2620 onwards. The NGS requires no flight or ground crew action for normal system operation and is not dispatch critical.

NGS Panel in the wheel-well

Photo: Lonnie Ganz

This from the FAA Systems Fire Group website:

B-737 Ground / Flight Testing

A series of aircraft flight and ground tests were performed by the Federal Aviation Administration and the Boeing Company to evaluate the effectiveness of ground-based inerting (GBI) as a means of reducing the flammability of fuel tanks in the commercial transport fleet. Boeing made available a Boeing 737 for modification and testing. A nitrogen-enriched air (NEA) distribution manifold, designed, built, and installed by Boeing, allowed for deposit of the ground-based NEA into the center wing tank (CWT). The fuel tank was instrumented with gas sample tubing and thermocouples to allow for a measurement of fuel tank inerting and heating during the testing. The FAA developed an in-flight gas sampling system, integrated with eight oxygen analyzers, to continuously monitor the ullage oxygen concentration at eight different locations. Other data such as fuel load, air speed, altitude, and similar flight parameters were made available from the aircraft data bus. A series of ten tests were performed (five flight, five ground) under different ground and flight conditions to demonstrate the ability of GBI to reduce fuel tank flammability. It was demonstrated under the most hazardous condition-an empty center wing tank-that GBI would remain effective for a large portion of the flight, or until aircraft descent. However, it was also shown that the dual venting configuration of some Boeing airplanes would have to be modified to prevent loss of inerting at certain ground and flight cross flow conditions.

Download the Final Report (DOT/FAA/AR-01/63)  (4.8Mb)

 


Limitations

Max temp +49°C
Min temp -43°C or freeze pt +3°C
Max quantity 1/200: 4300 + 4100 + 4300 = 12,700kg  (2 bag ctr bays)
200Adv: 4300 + 5400 + 4300 = 14,000kg  (3 bag ctr bays)
200Adv: 4300 + 7000 + 4300 = 15,600kg  (3 bag integral)
3/4/500: 4600 + 7000 + 4600 = 16,200kg
NG’s:    3900 + 13000 + 3900 = 20,800kg
Max lateral imbalance 1/200: 680kg; All other series: 453kg
Main tanks to be full if centre contains over 453kg
For ground operation, centre tank pumps must be not be positioned to ON, unless defuelling or transferring fuel, if quantity is below 453kgs.
Centre tank pumps must be switched OFF when both LP lights illuminate.
Centre tank pumps must not be left ON unless personnel are available in the flight deck to monitor LP lights.
Centre tank pumps should not be allowed to run dry or be left running unsupervised. Crew reset of fuel pump circuit breakers in-flight is prohibited (QRH CI.2.2)

Fuel Panels

737-1/200 Fuel Panel 737-Classic 4-Tank Fuel Panel NG Fuel Panel
The maximum declarable fuel capacity for tech log, nav log, etc is 16,200kgs for 3-Tank Classics, 20,800kgs for NG’s and up to 37,712kgs for BBJ’s depending upon how many tanks the customer has specified (max 12). The AFM limits are higher, but not normally achievable with standard SG’s.

The fuel panels for the various series have not changed much over the years. The NG’s have separate ENG VALVE CLOSED & SPAR VALVE CLOSED lights in place of FUEL VALVE CLOSED. The -1/200 panel also has blue VALVE OPEN lights similar to that on the crossfeed valve. The FILTER BYPASS lights were FILTER ICING on the 1/200.

The 1/200’s had heater switches; these used bleed air to heat the fuel and de-ice the fuel filter. They were solenoid held and automatically moved back to OFF after one minute.

NG: The engine spar valves and APU are normally powered by the hot battery bus but have a dedicated battery to ensure that there is always power to shut off the fuel in an emergency.

Fuel Gauges

Analogue Fuel Gauges

-1/200’s and some older -300’s

 

Digital Sunburst Fuel Gauges – Simmonds 4 Tank

– 3/4/500’s

Digital Sunburst Fuel Gauges – Smiths

– 3/4/500’s

Fuel Gauge Accuracy

The 737 fuel quantity indication system has the following accuracy tolerances:

737-100/-200:
FQIS accuracy: +/- 3.0%

737-300/-400/-500,
FQIS accuracy with digital indicators: +/- 2.5 %
FQIS accuracy with analog indicators: +/- 3.0%

The total tolerance for the FQIS system is based on a full tank. For example, if the fuel tank maximum capacity is 10,000 KG, then the tolerance of the gauging is 0.03 (airplane with analog indicators) * 10000 = 300 KG. The system tolerance is then +/- 300 KG at any fuel level within the tank.

The accuracy of the fuel flow transmitter is a function of the fuel flow. At engine idle, the system tolerance can be 12%. During cruise, the tolerance is less than 1.5%. The fuel flow indication is integrated over time to calculate the fuel used for each engine.

737-600/-700/-800/-900 with densitometer:
FQIS accuracy: +/- 1.0% overall
Main tanks > 50%, -1 to 5 deg pitch, +/- 1 deg roll: +/- 1.5%
Main tanks < 50%, -1 to 5 deg pitch, +/- 1 deg roll: +/- 1.0%

737-600/-700/-800/-900 without densitometer:
FQIS accuracy: +/- 2.0% overall
Main tanks > 50%, -1 to 5 deg pitch, +/- 1 deg roll: +/- 2.5%
Main tanks < 50%, -1 to 5 deg pitch, +/- 1 deg roll: +/- 2.0

The total tolerance for the FQIS system is based on a full tank. For example, if the fuel tank maximum capacity is 10,000 KG, then the tolerance of the gauging is 0.02 (airplane without a densitometer) * 10000 = 200 KG. The system tolerance is then +/- 200 KG at any fuel level within the tank.

The accuracy tolerance of the fuel flow transmitter is a function of the fuel flow. At engine idle, the system tolerance can be 12%. During cruise, the tolerance is less than 0.5%. The fuel flow indication is integrated over time to calculate the fuel used for each engine.

On the Digital Sunburst fuel gauges, pressing the “Qty test” button will start a self test of the display and the fuel quantity indicating system. After the test, each gauge will display any error codes that they may have.

Note: The gauges are still considered to be operating normally with error codes 1, 3, 5 or 7 on the Simmonds gauges or error codes 1,3 and 6 on the Smiths gauges. ie If the gauge is indicating (rather than zero) the gauge may be used.

 DU Fuel Gauges

-NG’s

 

Low fuel quantity indication illuminates below either 907 or 453kg

– NG’s

NG fuel gauges can give messages such as LOW, CONFIG or IMBAL

  Digital Fuel Quantity Indicator Error Codes – Simmonds

Error Code Fuel Quantity Indicator Reading Probable Cause Gauges considered to be operating normally?
0 Zero Missing or disconnected tank unit
1 Normal Tank contamination Yes
2 Zero Bad HI-Z lead
3 Normal Bad compensator unit wiring Yes
4 Zero Bad tank unit wiring
5 Normal Bad compensator unit Yes
6 Zero Bad tank unit
7 Normal Contamination/water in compensator Yes
8 Zero Bad fuel quantity indicator
9 Normal or zero Improperly calibrated indicator
Blank Bad fuel quantity indicator

Digital Fuel Quantity Indicator Error Codes – Smiths

Error Code Fuel Quantity Indicator Reading Probable Cause Gauges considered to be operating normally?
1 Normal Open or short in compensator LO-Z wiring Yes
2 Zero Short circuit in compensator unit
3 Normal Too much leakage in compensator unit Yes
4 Zero Open or short circuit in a LO-Z to a tank unit
5 Zero Short circuit in a tank unit
6 Normal Too much leakage in tank unit Yes
7 Zero (or ERR in flight) Calibration unit does not operate correctly
8 Blank An error in the DCTU data
9 Zero (or ERR in flight) A problem with the indicator memory
  10 Zero Open or short circuit in the HI-Z line
Dripsticks

If a fuel gauge is u/s the quantity must be determined by using the dripsticks (floatsticks in later aircraft). The classics have 5 dripsticks in each wing tank and none in the centre tank. The NG has 6 dripsticks in each wing tank and 4 in the centre tank. Because of cumulative errors it is recommended that the wings are filled once every few sectors to ensure an even fuel balance. In-flight, the GW must be periodically updated to ensure the accuracy of VNAV speeds, buffet margin and max altitude.

Floatstick

Fuel quantity is measured by using a series of capacitors in the tanks with fuel acting as the dielectric. Calibration of the fuel gauges is done by capacitance trimmers, these are adjusted to standardise the total tank capacitance and allows for the replacement of gauges. On older aircraft the trimmers were accessible from the flightdeck (below the F/O’s FMC) but they have since been removed to a safer place! Capacitance trimmers

Pumps

There are two AC powered fuel pumps in each tank; there are also EDP’s at each engine. Both fuel pump low pressure lights in any tank are required to illuminate the master caution to avoid spurious warnings at high AoA’s or accelerations. Centre tank LP lights are armed only when their pumps are ON.

Leaving a fuel pump on with a low pressure light illuminated is not only an explosion risk (see Thai and Philippine write offs) but also if a pump is left running dry for over approx 10 minutes it will lose all the fuel required for priming which will render it inoperative even when the tank is refuelled. If you switch on the centre tank pumps and the LP lights remain illuminated for more than 19 seconds then this is probably what has happened. The pumps should be switched off and considered inop until they can be re-primed.

On the 1-500’s, the centre tank pumps are located in a dry area of the wing root but on the NG’s the pumps are actually inside the fuel tank (see photo below). This is why only the NG’s are affected by AD 2002-19-52 which requires the crew to maintain certain minimum fuel levels in the center fuel tanks. You can see the location of the centre tank pumps on the forward wall of the wheel well on the NG’s, since the forward wall is actually the back of the centre fuel tank.

Note: for aircraft delivered after May 2004, centre tank fuel pumps will automatically shut off when they detect a low output pressure.

Right centre tank fuel pump on the forward wall of the wheel well – NG’s only

Centre Tank Scavange Pumps

These transfer fuel from the centre tank into tank 1 at a minimum rate of 100kg/hr, although usually nearer 200kg/hr. The trigger for the scavenge pump is different for the series as follows:

  • Originals: Only fitted after l/n 990 (Dec 1983). Operates the same as the classics.
  • Classics: Switching both centre tank pumps OFF will cause the centre tank scavenge pump to transfer centre tank fuel into tank 1 for 20 minutes.
  • NG’s: The centre tank scavenge pump starts automatically when main tank 1 is half full and its FWD pump is operating. Once started, it will continue for the remainder of the flight.

NB On the classics, when departing with less than 1,000kg of fuel in the centre tank, an imbalance may occur during the climb. This is because the RH centre tank pump will stop feeding due to the body angle so number 2 engine fuel is drawn from main tank 2, while engine 1 is still drawing fuel from the centre tank. When this “runs dry” the scavenge pump will also transfer any remaining centre tank fuel into main tank 1, thereby exacerbating the imbalance.

The APU uses fuel from the number 1 tank. If AC power is available, select the No 1 tank pumps ON for APU operation to assist the fuel control unit, especially during start. Newer –500 series aircraft have an extra, DC operated APU fuel pump in the No 1 tank which operates automatically during the start sequence. The APU burns about 160Kgs/hr with electrics and an air-conditioning pack on and this should be considered in the fuel calculations if expecting a long turnaround or waiting with pax on board for a late slot.


Fuel Temperature

Limitations: Max fuel temp +49ºC, Min fuel temp -45ºC or freezing point +3ºC, whichever is higher. Typical freezing point of Jet A1 is -47ºC. If the fuel temp is approaching the lower limits you could descend into warmer air or accelerate to increase the kinetic heating. Fuel temp is taken from main tank 1 because this will be the coldest as it has less heating from the smaller hydraulic system A.

A fuel sampling and testing kit is kept on the flight deck of all aircraft to test for water.

The NG series are prone to “Upper Wing Surface Non-environmental Icing” or “Cold Soaked Fuel Frost” CSFF. This is due to cold soaked fuel causing frost to form on the wings during the turnarounds – even in warm conditions! From July 2004 NGs have been delivered with markings on the upper surface of the wings where this frost is allowable for despatch under the following conditions:

Takeoff with CSFF on the upper wing surfaces is permissible, provided the following are met:

  • the frost on the upper surface is less than 1/16 inch (1.5 mm) thick
  • the extent of the frost is similar on both wings
  • the frost is on or between the black lines defining the permissible CSFF area
  • the outside air temperature is above freezing(0 C, 32 F)
  • there is no precipitation or visible moisture at the wing surface (rain, drizzle, snow, fog)


Auxiliary Fuel System

The standard number of fuel tanks is three. Classics could be fitted with an auxiliary fourth tank which was controlled from the main panel as shown at the top of the page. The 737-200Adv could also be fitted with an auxiliary tank at the forward end of the aft hold; these were available in either 3,065 or 1,421 litre capacities.

BBJ Aux fuel panel located on Capt’s & F/O’s main panels

The BBJ can have up to 9 aux fuel tanks giving it a maximum fuel quantity of 37,712kgs (83,000lbs) although in practice this would probably take you over MTOW if any payload was carried. This fuel would give a theoretical range in excess of 6200nm. The aux tanks are located at the rear of the fwd hold and the front of the aft hold, this reduces the C of G movement as fuel is loaded and used.

Aux fuel tank in aft hold of a BBJ2 (-800 fuselage)

Refuelling of the aux tanks is done by moving the guarded switch in the refuelling panel to AUX TANKS. The controls for main tanks 1 and 2 change to aft aux (AA) and fwd aux (FA) respectively.

The aux fuel system is essentially automatic. It works by transferring fuel from the aux tanks into the centre tank where it is then fed to the engines in the normal way. Flight crew can select fwd or aft tanks but normal practice is to use both to maintain C of G balance. The fwd and aft tanks are switched off when the ALERT light illuminates on the main panel.

Aux fuel control panel (Overhead panel)

There are no pumps in the aux fuel system. Cabin differential pressure (and bleed air as a backup) is used to maintain a head of pressure in the aux tanks to push the aux fuel into the centre tank.

Aux fuel control panel (Aft overhead panel)

(All Aux fuel photos: Capt D Britchford)


Ferry Tank

The 737-200 had provision for a ferry kit. This comprised a 2,000 US Gal (7,570 litre) bladder cell which attached to the seat tracks of the passenger cabin. The fuel was fed to the centre tank through a manual valve by cabin pressure.


Centre Fuel Tank Inerting

To date, two 737’s, 737-400 HS-TDC of Thai Airways on 3 Mar 2001 and 737-300 EI-BZG operated by Philippine Airlines on 5 Nov 1990 have been destroyed on the ground due to explosions in the empty centre fuel tank. The common factor in both accidents was that the centre tank fuel pumps were running in high ambient temperatures with empty or almost empty centre fuel tanks.

Even an empty tank has some unusable fuel which in hot conditions will evaporate and create an explosive mixture with the oxygen in the air. These incidents, and 15 more on other types since 1959, caused the FAA to issue SFAR88 in June 2001 which mandates improvements to the design and maintenance of fuel tanks to reduce the chances of such explosions in the future. These improvements include the redesign of fuel pumps, FQIS, any wiring in tanks, proximity to hot air-conditioning or pneumatic systems, etc.

737s delivered since May 2004 have had centre tank fuel pumps which automatically shut off when they detect a low output pressure and there have been many other improvements to wiring and FQIS. But the biggest improvement will be centre fuel tank inerting. This is universally considered to be the safest way forward, but is very expensive and possibly impractical. The NTSB recommended many years ago to the FAA that a fuel tank inerting system be made mandatory, but the FAA have repeatedly rejected it on cost grounds.

Boeing has developed a Nitrogen Generating System (NGS) which decreases the flammability exposure of the center wing tank to a level equivalent to or less than the main wing tanks. The NGS is an onboard inert gas system that uses an air separation module (ASM) to separate oxygen and nitrogen from the air. After the two components of the air are separated, the nitrogenenriched air (NEA) is supplied to the center wing tank and the oxygenenriched air (OEA) is vented overboard. NEA is produced in sufficient quantities, during most conditions, to decrease the oxygen content to a level where the air volume (ullage) will not support combustion. The FAA Technical Center has determined that an oxygen level of 12% is sufficient to prevent ignition, this is achievable with one module on the 737 but will require up to six on the 747.

On 21 Feb 2006 the Honeywell NGS was certified by the FAA after over 1000hrs flight testing on two 737-NGs. Aircraft from l/n 1935 (Aug 2006) to 2006 were delivered with basic provisions for NGS and more comprehensive provisioning up to l/n 2019. Full production cutover is scheduled for l/n 2620 onwards. The NGS requires no flight or ground crew action for normal system operation and is not dispatch critical.

NGS Panel in the wheel-well

Photo: Lonnie Ganz

This from the FAA Systems Fire Group website:

B-737 Ground / Flight Testing

A series of aircraft flight and ground tests were performed by the Federal Aviation Administration and the Boeing Company to evaluate the effectiveness of ground-based inerting (GBI) as a means of reducing the flammability of fuel tanks in the commercial transport fleet. Boeing made available a Boeing 737 for modification and testing. A nitrogen-enriched air (NEA) distribution manifold, designed, built, and installed by Boeing, allowed for deposit of the ground-based NEA into the center wing tank (CWT). The fuel tank was instrumented with gas sample tubing and thermocouples to allow for a measurement of fuel tank inerting and heating during the testing. The FAA developed an in-flight gas sampling system, integrated with eight oxygen analyzers, to continuously monitor the ullage oxygen concentration at eight different locations. Other data such as fuel load, air speed, altitude, and similar flight parameters were made available from the aircraft data bus. A series of ten tests were performed (five flight, five ground) under different ground and flight conditions to demonstrate the ability of GBI to reduce fuel tank flammability. It was demonstrated under the most hazardous condition-an empty center wing tank-that GBI would remain effective for a large portion of the flight, or until aircraft descent. However, it was also shown that the dual venting configuration of some Boeing airplanes would have to be modified to prevent loss of inerting at certain ground and flight cross flow conditions.

Download the Final Report (DOT/FAA/AR-01/63)  (4.8Mb)


Limitations

Max temp +49°C
Min temp -43°C or freeze pt +3°C
Max quantity 1/200: 4300 + 4100 + 4300 = 12,700kg  (2 bag ctr bays)
200Adv: 4300 + 5400 + 4300 = 14,000kg  (3 bag ctr bays)
200Adv: 4300 + 7000 + 4300 = 15,600kg  (3 bag integral)
3/4/500: 4600 + 7000 + 4600 = 16,200kg
NG’s:    3900 + 13000 + 3900 = 20,800kg
Max lateral imbalance 1/200: 680kg; All other series: 453kg
Main tanks to be full if centre contains over 453kg
For ground operation, centre tank pumps must be not be positioned to ON, unless defuelling or transferring fuel, if quantity is below 453kgs.
Centre tank pumps must be switched OFF when both LP lights illuminate.
Centre tank pumps must not be left ON unless personnel are available in the flight deck to monitor LP lights.
Centre tank pumps should not be allowed to run dry or be left running unsupervised. Crew reset of fuel pump circuit breakers in-flight is prohibited (QRH CI.2.2)