In slaved mode, the heading loop of the LCR-93 attitude and heading reference unit (AHRU) is
supplied with magnetic heading data from the magnetic sensor unit (flux valve). The heading output
is magnetic heading referenced to local magnetic north. The earth rate and gyro drift correction
factors are updated continuously during slaved operation.
In DG (directional gyro) mode, the heading may be set as desired by the L SLEW and R SLEW
(right) switch of either AHRS system, after DG is selected on the appropriate system switch (DG/
SLAVE/TEST). In DG mode the system acts as a free gyro; there is no magnetic input, and no
update of earth rate and gyro drift estimation will be performed.
The AHRS basic mode is annunciated in the upper left corner of the MFD (i.e., AHRS BASIC-1-2), in
white, if the AHRS is not in a normal mode.
SYSTEM ALIGNMENT
When power is supplied to the AHRS system it automatically enters the alignment mode. Upon
completion of the alignment the appropriate modes of slaved heading or DG, and Normal or Basic
(depending upon switch selection and availability of data), will be entered. Alignment takes
approximately 30 seconds; if no internal failures are detected during the alignment self-test and all
values are within accuracy limits, heading, attitude, angular rates, and accelerations are output and
system flags are withdrawn. A pilot initiated self-test is available by pressing the DG/SLAVE/TEST
switch to the TEST position. While the system is testing, failure flags and invalid data annunciations
(amber dashes) will be in evidence, and TEST will be annunciated in red at the top left of the attitude
display and over HSI in the primary flight display (PFD). When the system is tested, the heading of
the HSI will slew to 015°, the attitude sphere will show 45° of right bank, and 5° nose up attitude.
Wind buffeting, cargo loading, and movements caused by running engines will not affect the duration
of alignment on the ground. In case of excessive movement of the airplane on the ground the
alignment may take longer or alignment may not be completed. If alignment is not completed the
warning flags will not be withdrawn and it may be necessary to restart alignment by removing power
from the system by shutting down airplane system power or by pulling the affected AHRS circuit
breakers (AHRS 1, AHRS 1 AUX or AHRS 2, AHRS 2 AUX). An AHRS battery supplies power to
AHRS 1 AUX and AHRS 2 AUX circuit breakers when the STBY PWR switch is in the ON position.
An in-flight power interruption is unlikely because a system battery will prevent an in-flight power
interruption for periods of up to 30 minutes. However, if an in-flight power interruption is experienced
or attitude and heading reference is lost during flight, an in-flight alignment can be performed. The
same system circuit breakers must be pulled, as on a ground alignment (above), to ensure that all
power has been removed from the system. By re-setting, the system will then start an in-flight
alignment which normally takes 45 seconds to 2 minutes. An in-flight alignment requires that the
airplane be flown straight and level for the duration of the alignment.
HDG REV and ATT REV
The opposite attitude source may be selected, if desired, by pressing the applicable ATT REV
(attitude reversion) button on the pilot’s or copilot’s instrument panel. The opposite heading source
may be selected by pressing the applicable HDG REV (heading reversion) button, also on the pilot’s
or copilot’s panel. Pressing either of these buttons adds the opposite system to the desired source. If
both systems are selected to the same source, or if a “cross-selection” has been made, it will be
annunciated on the EFIS displays in amber.
Takeoff with one AHRS in BASIC and/or in DG mode is not recommended.
Cessna Citation XLS - Instrumentation & Avionics
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