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Illustrated technical information covering Vol 2 Over 800 multi-choice systems questions Close up photos of internal and external components Illustrated history and description of all variants of 737 Databases and reports of all the major 737 accidents & incidents History and Development of the Boeing 737 - MAX General flightdeck views of each generation of 737's Technical presentations of 737 systems by Chris Brady Detailed tech specs of every series of 737 A collection of my favourite photographs that I have taken of or from the 737 Press reports of orders and deliveries Details about 737 production methods A compilation of links to other sites with useful 737 content Study notes and technical information A compilation of links to major 737 news stories with a downloadable archive A quick concise overview of the pages on this site



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*** Updated 23 Nov 2020 ***

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737NG IRS Panel

The IRS Mode Selector Unit has up to three amber landing system captions labelled: GPS, ILS, GLS, illumination of which could indicate either:

  • A failure of both GPS/ILS/GLS sensors.
  • A failure of a single GPS/ILS/GLS sensor when either system annunciator panel is pushed to initiate a recall.
  • With a single GPS/ILS/GLS sensor failure, light extinguishes when the system recall is set.


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.



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)

The NG/MAX EFIS control panel is glareshield mounted and has the additional features of a flight path vector, feet/meters display, Inches/mb and EGPWS terrain overlay. The MAX panel has a +/- range controller to increase or reduce map range rather than the numbered selections of previous generations

737-NG/MAX EFIS Control Panel

737-NG/MAX 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)


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.






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.



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


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
737-MAX LEAP-1B   21 Jun 2017
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