NAVIGATION AND
COMMUNICATION
AIRPLANE
OPERATIONS
MANUAL
2-18-30
Page
10
Code
01
REVISION 25
AH-900 AHRS VERSION
The AH-900 AHRS version is basically an attitude and heading
reference system that senses linear motion and angular rates through
inertial sensors. Heading orientation is also obtained through the
inertial sensors, dispensing the magnetic flux detectors.
Each AHRS consists of one Attitude and Heading Reference Unit, the
AHRU 1 and AHRU 2, located in the forward electronics compartment.
There are no cockpit control panels.
ATTITUDE AND HEADING REFERENCE UNIT (AHRU)
The Attitude and Heading Reference Unit contains three laser gyros
and three accelerometers that are mounted on each of the three axis
inside of the AHRU, which it uses to measure inertial motion.
The AHRU requires initialization data from the Flight Management
System (FMS) and Air Data Computer (ADC). From inertial
measurements, initialization data, and air data inputs, the AHRU
performs the calculations necessary to provide heading and attitude
data to the airplane.
Each AHRU uses two 28 VDC power inputs, one for normal power
(primary source) and the other for backup power (airplane batteries).
The AHRU 1 primary power source is the Essential DC Bus 1 and its
backup power source is the Backup Essential Bus. The AHRU 2
primary power source is the DC Bus 2 and its backup power source is
the Backup Bus 2. If the AHRU loses primary power, it automatically
transfers to backup power.
When the AHRU operates solely on backup power, it will operate for
40 minutes and the AHRS 1 (2) ON BATT advisory message will be
presented on the EICAS.
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