737 Flight Crew Operations Manual
Flight Management, Navigation -
Flight Management System
Operation
Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.
D6-27370-866-EGP 11.31.5
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.
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.
September 15, 2016
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