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.

aeronautical chart

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