737 MAX Flight Crew Operations Manual
Flight Management, Navigation -
Navigation Systems Description
11.20.6
MN-FLT-OH-201
When a fast realignment is initiated or the airplane parks after a flight and remains
motionless for at least 7.5 to 15 minutes with the ADIRUs still in the NAV mode
and valid GPS data available, the ANR (automatic navigation realignment) mode
will perform a complete realignment automatically. The ADIRU's position
(latitude/longitude) will reset and inertial states (i.e. attitude, heading and
horizontal velocities) will reset to remove the drift errors accumulated on the
flight. The outputs will reset in a step like manner. The airplane must be stationary
long enough for ANR to complete (7.5 to 15 minutes). If the airplane is not
stationary long enough for ANR to complete, the ADIRU will not apply any
position corrections using GPS and will start over from the beginning when the
airplane is stationary again. 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 a full
alignment, the alignment is paused. The IRS will automatically resume the
full alignment 30 seconds after the motion stops. If motion is detected
during a fast realignment, the IRS will discontinue the quick alignment,
and 30 seconds after the motion stops, will automatically perform a full
alignment.
Manual Present Position Entry for IRS Alignment of ADIRU with Hybrid
GPS-Inertial
If a GPS position from an MMR is not available during alignment, a manually
entered present position alignment process can be used.
Normal alignment between 78 degrees 15 minutes North or South is initiated by
rotating the MSU switch from OFF to NAV. The IRS performs a short power test,
during which the ON DC light illuminates. When the ON DC light extinguishes
and the ALIGN light illuminates, the alignment process begins. Airplane present
position should be entered at this time. The present position is normally entered
through the FMC CDU POS INIT page. If the present position cannot be entered
through the FMC CDU, it may be entered through the ISDU keyboard. Alignment
time varies from five minutes to seventeen minutes depending on airplane
latitude. The airplane must remain stationary during alignment.
If an entered latitude/longitude position is not within 4 NM of the origin airport,
the CDU scratchpad message VERIFY POSITION is displayed. If the entered
latitude/longitude position does not pass the IRS internal comparison tests, the
scratchpad message ENTER IRS POSITION is displayed.
March 1, 2021
