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All of the information, photographs & schematics from this website and much more is now available in a 374 page printed book or in electronic format. *** Updated 20 Nov 2018 *** |
The IRS Mode Selector Unit has up to three amber landing system captions labelled: GPS, ILS, GLS, illumination of which could indicate either:
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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.
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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).
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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. |
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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 |
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![]() 737-NG/MAX EFIS Control Panel |
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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.
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![]() The -3/4/500 Electronic Horizontal Situation Indicator (Map mode) |
![]() 737-NG Navigation Display (Map mode) |
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Instrument TransferIf 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.
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1/200 |
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3/4/500 |
NG's |
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IRS Malfunction Codes (Classics)
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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.
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Navigation Mode Selectors
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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. |
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The ANS also performs computations related to lateral navigation which can provide LNAV commands to the AFDS in the event of an FMC failure. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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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. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
FutureIn 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). For more information about NPS and IAN see the section on Flight Instruments.
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![]() 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. |
ETOPSIn 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:
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