AHRS – PRINCIPLE OF OPERATIONS
The system comprises inertial sensor using Fiber Optic Gyros (FOGs) and
accelerometers to compute attitude, heading and flight information.
FOGs and accelerometers are mechanically fixed to the helicopter axes and
measure rates of change in pitch, roll, and yaw. A computer puts together
the rate of change from the sensors to find and continuously update the
values of pitch, roll, and heading.
The flux valves, the two ADS and the Global Positioning Systems (GPS)
increase the system capabilities.
HEADING MODE
Heading reference can either be selected as MAG or DG by means of the
switch on the AHRS control panel.
When MAG mode is selected, the AHRU heading value is slaved to the Flux
Valve input (Magnetic Heading) and no annunciator is displayed besides the
lubber line or the heading digital readout.
When DG mode is selected, the AHRU heading is not slaved to the Flux
Valve and can be manually set by acting on the SYNC switch on the AHRS
Control Panel. In this case the DG 1 or DG 2 annunciator is displayed on
the right side of the lubber line or of the heading digital readout.
AHRS - FAILURE
Failure of an AHRS is annunciated by the 1(2) AHRS FAIL caution message
in the CAS window and by the red flags ATT FAIL and HDG FAIL that
replace the ADI and the HSI, respectively.
AHRS failure also triggers the AVIONIC FAULT caution message in the CAS
and causes the failure of the on-side Autopilot: 1(2) AP FAIL and AFCS
DEGRADED caution messages appear in the CAS window and the aural
warning message AUTOPILOT – AUTOPILOT is played back.
A loss of valid pitch and roll information from the on-side AHRS is indicated
on the PFD by teh ATT FAIL red annunciator in the center of the attitude
indiacator and the loss of all attitude cales and pointers.
A loss of valid heading information from the on-side AHRS is indicated on
the PFD by the HDG FAIL red annunciator in the center of the HSI and the
loss of the compass card and the associated pointers, bugs and readouts.
Pilot can restore the attitude and heading indications by selecting the nonfailed
system using the AHRS selector knob on the RCP: in this case both
PFDs show attitude and heading data from the same system, so the
following singlesource annunciators are displayed:
• AHRS 1 or AHRS 2 on the top left side of the ADI
• MAG 1 (DG 1) or MAG 2 (DG 2) on the top right side of the
HSI
The single-source AHRS selection on the RCP does not restore Autopilot
operation.