AIRCRAFT NAVIGATION SYSTEM

NAVIGATION INTRODUCTION

Finding the way from one place to another is called NAVIGATION. Moving of an aircraft from one point to another is the most important part for any kind of mission. Plotting on the paper or on the map a course towards a specific area of the earth , in the passed, used to be a task assigned to a specialised member of the aircraft’s crew such a navigator. Such a task was quite complicated and not always accurate. Since it depended on the observation , using simple maps and geometrical instruments for calculations. Today, aerial navigation has become an art which nears to perfection. Both external Navaids (Navigational Aids) and on-board systems help navigate any aircraft over thousand of miles with such accuracy that could only be imagined a few decades ago.

The Method of Navigation
There are three main methods of air navigation. There are:
1. Pilotage , 2. Dead Reckoning , 3. Radio.

  • Pilotage or Piloting is the most common method of air navigation. This method, the pilot keeps on course by following a series of landmarks on the ground. Usually before take-off, pilot will making pre-flight planning , the pilot will draws a line on the aeronautical map to indicate the desired course. Pilot will nots various landmarks , such as highways , railroad tracks, rivers , bridges . As the pilot flies over each of landmark , pilot will checks it off on the chart or map. If the plane does not pass directly over thelandmark , the pilot will know that he has to correct the course.
  • Dead Reckoning is the primary navigation method used in the early days of flying. It is the method on which Lindberg relied on his first trans-Atlantic flight. A pilot used this method when flying over large bodies of water, forest, deserts. It demands more skill and experience than pilotage does. It is based on time, distance, and direction only. The pilot must know the distance from one point to the next, the magnetic heading to be flown. Pilot works on the pre-flight plan chart , pilot plan a route in advance. Pilot calculate the time to know exactly to reach the distination while flying at constant speed. In the air, the pilot uses compass to keep the plane heading in the right direction. Dead reckoning is not always a successful method of navigation because of changing wind direction. It is the fundamental of VFR flight.
  • Radio Navigation is used by almost all pilots. Pilots can find out from an aeronautical chart what radio station they should tune to in a particular area. They can then tune their radio navigation equipment to a signal from this station. A needle on the navigation equipment tells the pilot where they are flying to or from station, on course or not .
    see sample of aeronautical chart , preflight plan chart : click here

Pilots have various navigation aids that help them takeoff,fly, and land safely. One of the most important aids is a series of air route traffic control , operated throughout the world. Most of the traffic control uses a radar screen to make sure all the planes in its vicinity are flying in their assigned airways. Airliners carry a special type of radar receiver and transmitter called a transponder. It receives a radar signal from control center and immediately bounces it back. When the signal got to the ground, it makes the plane show up on the radar screen.
Pilots have special methods for navigating across oceans. Three commonly used methods are:
1. Inertial GuidanceThis system has computer and other special devices that tell pilots where are the plane located.
2.LORAN Long Range Navigation The plane has equipment for receiving special radio signals sent out continuous from transmitter stations. The signals will indicate the plane location
3.GPS Global Positioning System. is the only system today able to show your exact position on the earth any time, anywhere, and any weather. The system receiver on the aircraft will receives the signals from sattelites around the globe.

TERMINOLOGY

ADF Automatic Direction Finder. An aircraft radio navigation which senses and indicates the direction to a Low/Medium Frequency non-directional radio beacon (NDB) ground transmitter.

DME Distance Measuring Equipment. Ground and aircraft equipment which provide distance information and primary serve operational needs of en-route or terminal area navigation.

EAT Estimated Approach Time

EFIS Electronic Flight Instrument System , in which multi-function CRT displays replace traditional instruments for providing flight, navigation and aircraft system information, forming a so-called ” glass cockpit “.

ETA Estimated Time of Arrival

GPS Global Positioning System . A navigation system based on the transmission of signals from satellites provided and maintained by the United States of America and available to civil aviation users.

HDG Heading. The direction in which an aircraft’s nose points in flight in the horizontal plane, expressed in compass degrees (eg. 000 or 360 is North, 090 is East)

HSI Horizontal Situation Indicator. A cockpit navigation display, usually part of a flight-director system, which combines navigation and heading.

IFR Instrument Flight Rule . prescribed for the operation of aircraft in instrument meteorological condition.

ILS Instrument Landing System . consists of the localizer, the glideslope and marker radio beacons (outer, middle, inner). It provides horizontal and vertical guidance for the approach.

INS Inertial Navigation System. It uses gyroscopes and other electronic tracking systems to detect acceleration and deceleration, and computes an aircraft’s position in latitude and longitude. Its accuracy, however, declines on long flights. Also called IRS, or Inertial Reference System.

KNOT (kt) Standard Unit of speed in aviation and marine transportation, equivalent to one nautical mile per hour. One knot is equal to 1.1515 mph., and one nautical mile equals to 6,080 feet or 1.1515 miles. One knot is equal to one nautical mile per one hour.

LORAN C Long Range Navigation is a Long-Range low frequency Radio Navigation. Its range is about 1,200 nm by day to 2,300 nm. by night.

MAGNETIC COURSE Horizontal direction, measured in degrees clockwise from the magnetic north.

MACH NUMBER Ratio of true airspeed to the speed of sound. Mach 1 is the speed of sound at sea level. Its values is approximately 760 mph.

NDB Non-Directional Beacon. A medium frequency navigational aid which transmits non-directional signals , superimposed with a Morse code identifier and received by an aircraft’s ADF.

RMI Radio Magnetic Indicator. A navigation aid which combines DI ,VOR and /or ADF display and will indicate bearings to stations, together with aircraft heading.

RNAV Area Navigation. A system of radio navigation which permits direct point-to-point off-airways navigation by means of an on-board computer creating phantom VOR/DME transmitters termed waypoints.

TACAN TACtical Air Navigation. Combines VOR and DME and used by military aircraft only.System which uses UHF frequencies , providing information about the bearing and distance from the ground station we have tuned into.

TCAS Traffic Alert and Collision Avoidance System. Radar based airborne collision avoidance system operating independently of ground-based equipment. TCAS-I generates traffic advisories only. TCAS-II provides advisories and collision avoidance instructions in the vertical plane.

TRANSPONDER Airborne receiver / transmitter which receives the interrogation signal from the ground and automatically replies according to mode and code selected. Mode A and B wre used for identification, using a four digit number allocated by air traffic control. Mode C gives automatic altitude readout from an encoding altimeter.

VFR Visual Flight Rules. Rules applicable to flights in visual meteorological conditions.

VHF Very High Frequency. Radio frequency in the 30-300 Mhz band, used for most civil air to ground communication.

VOR Very High Frequency Omnidirectional Range. A radio navigation aid operating in the 108-118 Mhz band. A VOR groun station transmits a two- phase directional signal through 360 degrees. The aircraft’s VOR receiver enables a pilot to identify his radial or bearing From/To the ground station . VOR is the most commonly used radio navigation aid in private flying.

VORTAC A special VOR which combines VOR and DME for civil and military used . System provides information about the bearing and distance from the ground station we have tuned into.

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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.