737 MAX Flight Crew Operations Manual
Flight Management, Navigation -
Navigation Systems Description
MN-FLT-OH-201
11.20.7
The flashing ALIGN light alerts the crew that an entered position does not pass
one of the two internal comparison tests and should be checked for accuracy. If the
entered position does not agree with the last stored position, the first internal test
is failed, and the ALIGN light will flash. If the same position is reentered, the IRS
will accept the position and continue the alignment process. A second internal
position test compares the entered latitude with the system-computed latitude. If
this test is failed, the ALIGN light will again flash. If two consecutive entries of
the same position do not pass the second internal position test, the FAULT light
will illuminate. If the test is passed, the IRS will proceed to complete the
alignment process and enter NAV mode. When the alignment is complete, the IRS
changes to the navigate mode and the airplane can be moved.
Note:
When present position is entered through the CDU prompt boxes, the
prompt boxes change to dashes which allows additional entries of present
position if needed. When the IRS enters NAV mode, the dashes are cleared
from the CDU POS INIT page. When present position is entered through
the ISDU, the CDU prompt boxes remain on the POS INIT page. When the
IRS enters NAV mode, the prompt boxes are cleared.
During transit or through–flight stops with brief ground times, a thirty second fast
realignment and zeroing of ground speed error may be performed by selecting
ALIGN while the airplane is parked. Present position should be simultaneously
updated by manually entering latitude and longitude prior to selecting NAV.
When a fast realignment is initiated or the airplane parks after a flight and remains
motionless with the ADIRUs still in NAV mode, the ADIRUs will enter the ANR
(automatic navigation realignment) mode, which is an ALIGN mode that runs
while in NAV mode. After at least 5 minutes of no motion, the ANR mode will
realign the ADIRU by resetting the inertial states (i.e. attitude, heading and
horizontal velocities) to remove the drift errors accumulated on the flight. During
ANR mode the ADIRU will accept a manual position entry (i.e. during a fast
realignment) to correct the position drift error, this will result in the equivalent of
a complete re-alignment. If no position is entered the ANR mode will realign the
ADIRU by resetting all the inertial states except position and will continue in NAV
mode from its last present position. If the airplane continues to remain motionless
the ANR mode will apply small alignment resets once every minute to keep the
system from drifting until it moves again.
To prevent an ANR (automatic navigation realignment) pilots should perform an
initial full IRS alignment: (i.e. MSU to OFF, then to NAV).
Note:
If the airplane is moved or excessive motion is detected during alignment
or fast realignment, the IRS automatically begins the full alignment
process 30 seconds after the motion stopped.
March 1, 2021
