Attitude Heading Reference System (Ahrs) - BOMBARDIER LEARJET 40 Pilot's Manual

Pilot's Manual

ATTITUDE HEADING REFERENCE SYSTEM (AHRS)

Due to the design of the Honeywell Primus 1000 Avionics System, the
various avionics systems are very integrated. The AHRS uses the PFDs
as its primary display and the MFD in the event of PFD failure. The
display units, display controllers and appropriate reversion switches
are considered part of the electronic display system and are covered lat-
er in this section.
The Learjet 40 is equipped with dual LITEF (LCR-93) Attitude Heading
Reference Units (AHRUs) with a memory module, located in the air-
craft's nose section.
Each system (#1 and #2 AHRS) incorporates a flux valve located in their
respective wing tips. The AHRUs contain three Fiber Optic Gyros
(FOGs) which sense angular rotation about the three principle axis
(pitch, roll, and yaw) thus, computing the airplane's attitude and head-
ing. When slaved to magnetic, the flux valves provide a magnetic head-
ing reference. The memory module stores calibration data. This data is
used to compensate AHRU inaccuracies caused from installation errors
and local disturbances to the earth's magnetic field created by the air-
craft's structure.
The AHRUs receive true airspeed (TAS) information from the on-side
ADC. However, if a single ADC failure occurs, they will receive TAS
from the operating ADC. True airspeed information is used to compute
pitch and roll attitude. If TAS inputs to #1 AHRS or #2 AHRS are lost, a
CAS will illuminate. Although system operation will be degraded, the
AHRS still retains the same accuracy as a conventional spinning mass
type gyro. AHRU's data output is received through their correspond-
ing IC for attitude and heading displays on the PFDs/MFD. Attitude
and/or heading information from the AHRS is used by the Flight Guid-
ance System (FGS), Flight Management System (FMS), weather radar
system, and the fuel quantity indicating system. In addition, AHRS #2
provides heading information through DAU #2 for the backup naviga-
tion display on the RMU.
The following CAS illumination is specific to the AHRUs:
CAS
AHRS 1-2 BASIC
5-38
Color
White Attitude Heading Reference System (AHRS 1
or 2) has reverted to basic mode due to a loss
of true airspeed from both air data comput-
ers.
Description
PM-132A