737 Flight Crew Operations Manual
Flight Management, Navigation -
Navigation Systems Description
Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.
D6-27370-866-EGP 11.20.5
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.
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.
Note: If the airplane is moved during alignment or fast realignment, the IRS
automatically begins the full alignment process.
Loss of Alignment
If an IRS loses both AC and DC power, the alignment is lost. Alignment can be
lost if the MSU switch is moved out of the NAV position.
If alignment is lost in–flight, the navigation mode (including present position and
ground speed outputs) is inoperative for the remainder of the flight. However,
selecting ATT allows the attitude mode to be used to relevel the system and
provide an attitude reference. The attitude mode requires approximately thirty
seconds of straight and level unaccelerated flight to complete releveling. Some
attitude errors may occur during acceleration, but will be slowly removed after
acceleration stops.
The attitude mode can also provide heading information, but to establish compass
synchronization the crew must manually enter the initial magnetic heading. Drift
of up to 15 degrees per hour can occur in the IRS heading. Therefore, when in
attitude mode, an operating compass system must be periodically cross–checked
and an updated magnetic heading entered in the IRS, as required.
IRS Entries
Manual IRS entries of present position or magnetic heading are normally
accomplished on the POS INIT page of the FMC/CDU. The ISDU may also be
used.
September 28, 2006
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