AIRPLANE
OPERATIONS
MANUAL
NAVIGATION AND
COMMUNICATION
MARCH 28, 2002
2-18-30
Page
13
Code
01
ATTITUDE MODE
The attitude mode is the AHRU’s reversionary mode. It is automatically
entered by the AHRU if power is lost in flight, and it provides a quick
attitude restart: during the first 20 seconds in the attitude transitional
mode, the AHRU enters the erect attitude transitional mode. In this
transitional mode the AHRS 1 (2) ALN advisory message is displayed
on the EICAS and the AHRU computes a new level axis set. The
aircraft must be held steady, straight and level until the AHRS 1 (2)
ALN message extinguishes.
When operating in the attitude mode the AHRS 1 (2) ATT MODE
advisory message is presented on the EICAS. In this mode, attitude
outputs are not as accurate as when operating in the NAV mode, and
magnetic heading is not available.
For airplanes equipped with EICAS 18 and on, the AH-900 must be
initialized with magnetic heading. In this case the operator needs to
know the airplane’s magnetic heading. Whenever the airplane is in the
air and the AH-900 is in attitude mode, a menu bezel button
annunciates MHDG INIT on the pilot’s MFD. The AHRS 1 (2) NO MAG
HDG or AHRS 1-2 NO MAG HDG advisory messages will be displayed
on the EICAS. By selecting MHDG INIT, the operator will access the
Magnetic Heading Initialization menu, and will be able to set the
magnetic heading with the data set knob. The magnetic heading data
is sent to the AH-900 computer when the ENT bezel button is pressed.
The associated EICAS messages are cleared.
POWER-DOWN MODE
The AHRU enters the power-down mode automatically when the
system detects an “end-of-flight” event. In this mode, the AHRU will
transfer the last calculated position and other AHRS parameters to its
non-volatile memory.
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