OPERATIONS MANUAL

Navigation Systems
Inertial Reference System: (continued)
Should an IRU, in NAV mode, lose its alignment during taxi or in flight, it
can be re-aligned for the NAV mode only when the aircraft is parked. An
attitude alignment, however, may be performed also in flight (ATT mode),
but in that mode the IRU will not provide navigational data; this means,
AFDS, FMS, standby navigation systems in the CDUs, ND track-up compass
roses, and other functions, will be disabled. The ATT mode will also disable
groundspeed dependent systems like autobrakes and antiskid.—During
an attitude alignment, the aircraft should be level and maintain a constant
airspeed. The alignment resets the pitch and bank angles to zero, regardless
of the current aircraft attitude. Thereafter, long-term gravity sensing will
gradually add the true vertical reference, so that, after a period of several
minutes, the correct aircraft attitude will be sensed. In ATT mode, the IRU
provides neither true nor magnetic heading. Instead, it provides the heading
the crew enters. The IRU just adds rotational motion to the entered heading.
The crew should enter magnetic headings, so that the VOR pointers on the
compass roses accurately indicate VOR radials with reference to magnetic
north as published. Also, when the heading reference switch is set to TRUE
while in ATT mode, the RMI and EFIS compass roses will be inoperative.
The crew may apply a fast 30-second alignment for the NAV mode when there
is not enough time for a 10-minute alignment. This requires the IRU being
aligned already (after a flight, for example) and showing no excessive position
errors: when the aircraft is parked, the crew sets the mode selectors from NAV
to ALIGN, and back to NAV. This fast alignment does not require a position
entry in the FMC; the entry is recommended though.
In the simulator, the IRS alignment time can be reduced with the
Time acceleration slider on Instructor > Situation > Time. The IRS can
also be aligned promptly by pushing the button Align IRS & standby gyro
on Instructor > Situation > Service. This works on the ground as well as
in flight, and has the effect of a 17-minute alignment. However, it will align
only those IRUs that are set to NAV. Once an IRU is in ATT mode, it will
remain in ATT mode until it is reset to OFF. Therefore, to assure an IRU is
actually set to NAV, first move the IRU mode selector to OFF, then to NAV,
then push the button on the Instructor.
SYSTEM
ANALYSIS
In the simulator, sine and cosine test results for each IRU are shown on
Instructor > Analysis > Miscellaneous under IRS latitude sine/cosine test.
Failed tests are marked in red.
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