AIRPLANE
OPERATIONS
MANUAL
NAVIGATION AND
COMMUNICATION
JUNE 28, 2002
2-18-30
Page
5
Code
02
IRS OPERATING MODES
The IRS operating modes may be selected by setting the MSU mode
select switch to the desired mode. Under certain conditions the IRU
may automatically revert to specific modes and sub-modes.
ALIGNMENT MODES
MSU SET FROM OFF TO ALIGN OR NAV
In the alignment mode, the IRU aligns its reference axis to the local
vertical and true north, and estimates the horizontal earth rate
components to compute latitude. The latitude at which the IRU is
aligned affects the alignment time. The relationship between alignment
time and latitude is shown in the chart below.
0
5
10
15
20
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
ALIGNMENT TIME - minutes.....
ALIGNMENT LATITUDE - degrees Northern and Southern
The airplane must remain stationary during alignment, while the MSU
ALIGN annunciator is lit and IRS 1 (2) ALN advisory message
presented on the EICAS (only for airplanes equipped with EICAS 17
onwards). If the IRU detects excessive airplane motion (IRS 1 (2) EXC
MOTION advisory message is presented, on airplanes equipped with
EICAS 17 and on), it starts an automatic full realignment 30±1 seconds
after the motion stops. Normal passenger-loading or cargo-loading
activities should not cause an excessive airplane motion condition.
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