ATTITUDE HEADING
REFERENCE SYSTEM
(AHRS)
GENERAL
As illustrated in Figure 16-24, the Learjet 45
is equipped with dual attitude heading refer-
ence systems (AHRS). Each AHRS consists of
an attitude heading reference unit (AHRU)
with a memory module, located in the nose sec-
tion. Additionally, each system (No.1 and
No.2 AHRS) has a flux valve located in their
respective wing tips.
Each AHRU computes airplane attitude and
heading. When slaved to magnetic, the flux
valves provide the magnetic heading reference.
The memory module is used to store calibra-
tion data, which is used to compensate AHRU
inaccuracies due to mounting (installation er-
rors) and local disturbances to the earth's mag-
netic field created by the airplane structure.
The AHRUs contain 3 Interferometer Fiber
Optic Gyros (IFOG) that sense angular rota-
tion in the 3 principle axis (pitch, roll, and
yaw). They also contain 2 high performance
tilt sensors and a servo accelerometer that
senses linear acceleration. The central pro-
cessor unit within the AHRU receives all of the
sensor inputs and performs the mathematical
computations required to calculate aircraft at-
titude and heading.
In addition, the AHRUs normally receive true
airspeed (TAS) information from the on-side
ADC. In the event of single ADC failure, they
can both receive TAS from the operating ADC.
True airspeed information is used to compute
pitch and roll attitude. When TAS inputs to ei-
ther AHRS is lost, the CAS will display an
"AHRS 1 or 2 BASIC " message (Table 16-2).
Although system operation may be degraded
somewhat in this mode, the manufacturer
claims the AHRS still retains the same accu-
racy as a conventional spinning mass type
gyro.
The AHRUs output data through their corre-
sponding IC-600 for the attitude and heading
displays on the PFDs/MFD (Figure 16-24).
Attitude and/or heading information from the
AHRS is also used by the flight guidance sys-
tem (FGS), flight management system (FMS),
weather radar antenna, and the fuel quantity
computer. AHRS No.2 provides heading in-
formation through DAU2 for the backup nav-
igation display on the RMU.
SLAVED/FREE MODES
The heading indicators on the PFDs, MFD
and the RMU standby navigation page are all
driven by the AHRS. The heading reference
is normally set to display magnetic, but can be
selected to the directional gyro (non-slaved)
mode with the "HEADING FREE" switch.
The directional gyro (DG) mode might be used
when operating in charted areas of unreliable
magnetic variation/heading or when a flux
valve has failed.
The AHRS "HEADING" switches (one for
each AHRS) are located toward the outboard
end of the pilot's and copilot's lower switch
panels (Figure 16-1). These are latching type
switches (Figure 16-24) which are dark when
slave is selected and contain a "FREE" an-
nunciator that illuminates when free or di-
rection gyro mode is selected. Located beneath
each heading free switch is a three position (L
- SLAVE - R), slewing switch that is spring-
loaded to the center position. While in the DG
(FREE) mode, the heading card can be man-
ually set to any heading using the L/R switch.
In the DG mode, as it is with any directional
gyro, it is necessary to update the airplane's
heading periodically.
When AHRS reversion is selected, an amber an-
nunciation "MAG1 or 2" will appear above the
current heading display on the HSI displays if
not in the FREE mode (Figure 16-32). If the
AHRS reversion switch is in NORM, when the
heading free switch is selected, a white "DG1
or 2" will appear above the heading lubber line
on the on-side compass rose(s). If AHRS 1 or
2 reversion was selected the annunciation will
LEARJET 45 PILOT TRAINING MANUAL
16-33
FOR TRAINING PURPOSES ONLY
FlightSafety
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