737 MAX Flight Crew Operations Manual
Flight Management, Navigation -
Navigation Systems Description
11.20.8
MN-FLT-OH-201
IRS Alignment for the ADIRU
1B401, 1B801 - 1B806
After the MSU is rotated 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. An IRS must be
initialized with accurate airplane present position before it can enter the
navigation mode. The present position is normally entered through the left or right
FMC CDU. The present position can be entered through the ISDU keyboard. The
airplane must remain stationary during alignment. If alignment is interrupted due
to motion, the alignment process will stop, and the ADIRU sends "Excessive
Motion" message to the FMC/CDU and a status code 03 to the ISDU. Thirty (30)
seconds after ADIRU detects no motion, the IRS will automatically restart the
alignment process and reset the align time clock from the beginning.
Alignment time varies from 5 minutes to 17 minutes depending on airplane
latitude. For latitude between the equator and 60 degrees North or South, the
alignment time will vary between 5 minutes to 10 minutes as a function of the
latitude. Between 60 degrees and 70.2 degrees North and between 60 degrees and
70.2 degrees South, the alignment time is set at 10 minutes. For latitudes greater
than the 70.2 degrees limit (up to the maximum of 78.25 degrees North or South),
the alignment time is approximately 17 minutes.
If the 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.
The flashing ALIGN light alerts the crew that the position entered 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.
Note:
If IRS FAULT light illuminates during alignment, it is recommended that
present position is verified for accuracy against GPS position or known
local position, and alignment process is repeated.
March 1, 2021
