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The standby compass is located at the top of the windshield center post.
It is a magnetic compass that does not require any electrical power to
provide the crew with a continuous standby heading display. The only
electrical input to the compass is 5-vdc to illuminate the compass at
night.
Pilot’s Manual
5-38 PM-126A
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 later
in this section.
The Learjet 45 is equipped with either a dual Honeywell (AH-800) or
dual LITEF (LCR-93) Attitude Heading Reference Units (AHRUs). Both
units contain a memory module and are located in the aircraft’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 heading.
When slaved to magnetic, the flux valves provide a magnetic heading
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 aircraft’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 corresponding
IC for attitude and heading displays on the PFDs/MFD. Attitude
and/or heading information from the AHRS is used by the Flight Guidance
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 navigation
display on the RMU.
The following CAS illumination is specific to the AHRUs:
CAS Color Description
AHRS 1-2 BASIC 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 computers.
Pilot’s Manual
PM-126A 5-39
ATTITUDE AND HEADING COMPARISON MONITORS
The attitude and heading comparison monitors are functions within
the IC-600s that compare the displayed data with the cross-side or secondary
source data, depending on system reversionary status. Annunciations
are provided to the crew if the attitude or heading on both sides
differ.
The attitude comparison function is made of two monitors, the roll
comparison monitor and the pitch comparison monitor. If the pitch
data displayed on each side differ, the pitch comparison monitor trips
and the PIT annunciation is displayed. The comparison threshold figure
for the pitch monitor is 5°. Similarly, if the roll data on both sides
differ, the ROL annunciation is displayed. The comparison threshold
figure for the roll monitor is 6°. If both the roll and pitch comparison
monitors trip, the ATT annunciation is displayed. If the heading comparison
monitor trips, HDG is displayed. The normal comparison
threshold figure for the heading monitor is 6°. However, if the displayed
roll information is > 6°, the heading comparison threshold figure
is increased to 12°.
All comparison monitor annunciations flash for 10 seconds on activation
and then remain steady. These comparison monitors provide an
extra safeguard to alert the cockpit crew in the event of any failures affecting
the attitude or heading data displayed.
Other annunciations for attitude and heading which are displayed on
the PFDs, not associated with the comparison monitors, are:
ATT FAIL and HDG FAIL. These red annunciations are displayed on
the affected side’s PFD whenever the heading or attitude display from
that AHRS has failed. If an AHRS fails or both primary and auxiliary
power supplies to an AHRS fail, both the red ATT FAIL and HDG FAIL
annunciations are displayed.
ATT1/2 and DG1/2. These annunciations are displayed on the PFDs
and indicate to the crew which AHRS is the source for the attitude and
　
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