737 MAX Flight Crew Operations Manual
Flight Management, Navigation -
Flight Management System
Operation
11.31.6
MN-FLT-OH-201
If the DME radios fail, or if suitable DME stations are not available and all GPS
data is unavailable, 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.
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.
The FMC position is derived from a mathematical combination of the positions
determined by the IRS, radio, and GPS systems. It represents the FMC’s estimate
of the actual position of the airplane. Its accuracy varies according to the accuracy
of the other position determining systems.
Note:
If the GPS position update is excessive, GPS updating is suspended until
the GPS position can be determined to be reasonable.
March 1, 2021
