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737 MAX Flight Crew Operations Manual
Flight Management, Navigation -
Flight Management System
Description
11.30.2 MN-FLT-OH-201
The FMC and CDU are used for enroute and terminal area navigation, RNAV
approaches and to supplement primary navigation means when conducting all
types of instrument approaches.
The dual FMC installation is certified as a “sole source” navigation system.
Airplanes equipped with two FMCs are certified to operate outside radio navaid
coverage. The second FMC serves as a backup, providing complete navigational
functions if the other FMC fails.
With a dual FMC installation, one FMC is always designated as primary. This is
controlled by the position of the FMC Source Select switch. Refer to Chapter 11,
FMC Source Select Switch.
The primary FMC:
• allocates navaid tuning and updating functions between FMCs
• insures synchronization between FMCs
• controls CDU displays
• provides input to the autopilot
• provides input to the autothrottle system
• processes ACARS (data link) messages.
Positioning the FMC Source Select Switch to BOTH ON L or BOTH ON R
isolates FMC operation to use only the left or right FMC respectively. In the
NORMAL position, the left FMC is primary by default. Although the aircrew can
enter information into either CDU, the primary FMC is responsible for
synchronizing this information with the secondary FMC and updating both CDU
displays.
When external position updating is not available, the FMC uses the IRS position
as reference. When the IRS is the only position reference, the FMC applies an
automatic correction to the IRS position to determine the most probable FMC
position. This correction factor is developed by the FMC’s monitoring IRS
performance during periods of normal position updating to determine the typical
IRS error value. It is important to note that, when external position updating is not
available, navigation accuracy may be less than required. Flight crews should
closely monitor FMC navigation, especially when approaching the destination.
The accuracy of the FMC navigation should be determined during descent phase
by using radio navaids and radar information if available.
Note: Inaccurate position updating may cause the airplane to deviate from the
desired track.
March 1, 2021