Navigation

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 description

737-3/4/500 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.