Copyright . The Boeing Company. See title page for details.
Flight Management, Navigation -Flight Management System Operation 737 Flight Crew Operations Manual

Maintenance Index Page
The MAINT BITE INDEX page is available only on the ground and provides access to data for use by maintenance personnel.

INDEX
Push – displays the INIT/REF INDEX page.
Navigation Position
The FMC determines present position from the IRS, GPS, and navigation radios. The FMC uses its calculated present position to generate lateral steering commands along the active leg to the active waypoint.
YA686, YA687, YC153 - YK104 When the FMC Source Select Switch is positioned to NORMAL, the left FMC becomes primary, however, data from both FMCs is combined to determine a composite position and velocity for guidance and map displays.
FMC Position Update
On the ground, the FMC calculates present position based on GPS data. If GPS data is not available, the FMC calculates present position based on IRS data.
Copyright . The Boeing Company. See title page for details.
11.31.4  March 29, 2004

Flight Management, Navigation -Flight Management System Operation
737 Flight Crew Operations Manual
If GPS UPDATE is OFF, the FMC updates position to the takeoff runway threshold when a TO/GA switch is pushed. When making an intersection takeoff, the intersection data must be entered on the TAKEOFF REF page. If GPS UPDATE is ON, the TO/GA update is inhibited. GPS UPDATE is on the NAV OPTIONS page.
In flight, the FMC position is continually updated from the GPS, navigation radios, and IRS. Updating priority is based on the availability of valid data from the supporting systems.
FMC position updates from navigation sensor positions are used in the following priority order:
.
GPS

.
two or more DME stations
. one VOR with a collocated DME
. one localizer and collocated DME
. one localizer.

The station identifiers and frequencies of the selected radio navigation aids are displayed on the NAV STATUS page 1/2.
FMC logic selects the GPS position as the primary update to the FMC position. If
all GPS data becomes unavailable, the FMC reverts to radio or IRS updating. The dual frequency–scanning DME radios are automatically tuned by the FMC. The stations to be tuned are selected based upon the best available signals (in terms of geometry and strength) for updating the FMC position, unless a specific station is required by the flight plan. Radio position is determined by the intersection of two DME arcs.
If the DME radios fail, or if suitable DME stations are not available, FMC navigation is based on IRS position information only. The two VHF Nav radios are used by the FMC for localizer updating during an ILS approach and by the crew for navigation monitoring.
Note:  The FMC is designed to automatically reject unreliable navaid data during FMC position updating. However, in certain conditions, navaids which are in error may satisfy the reasonableness criteria and provide the FMC with an inaccurate radio position. One of the most vulnerable times is when a radio position update occurs just after takeoff. This is usually manifested in an abrupt heading correction after engaging LNAV. The position shift can be seen on the map which will shift the desired track and runway symbol to a position significantly different from that displayed during ground roll.
Copyright . The Boeing Company. See title page for details.
Flight Management, Navigation -Flight Management System Operation 737 Flight Crew Operations Manual

YA721, YA722
Note: If the flight crew observes either of these indications, and an extended
period of IRS NAV ONLY flight follows, the FMC should be carefully
monitored.
YA613 - YA687, YA801 - YK104 Note: If the flight crew observes either of these indications, the FMC should be
carefully monitored.
When adequate radio updating is not available, navigation display map mode may display a shift error. This error results in the displayed position of the airplane, route, waypoints, and navigation aids shifted from their actual positions.
An across track, undetected map shift may result in the airplane flying a ground track that is offset from the desired track. An along track, undetected map shift may result in the flight crew initiating altitude changes earlier or later than desired. In either case, an undetected map shift may compromise terrain or traffic separation.
Map shift errors can be detected by comparing the position of the airplane on the navigation display map mode with data from the ILS, VOR, DME, and ADF systems.
Navigation Performance
The FMC uses data from the navigation systems to accurately calculate the position of the airplane. The current FMC position is shown on line 1 of the POS REF page 2/3.
　
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