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the ATTD/HOVER HOLD switch or by using
the momentary OFF (up) position of the pilot FORCE
TRIM REL switch. HAS provides the pilot with limited
station-keeping or velocity-hold during hover or
low speed flight. Position-hold accuracy is a function of
inertial velocity drift errors of the HARS which can
vary with time. The SAS authority margin will be
biased from its center position as these errors build
with time. The attitude hold is a limited authority mode
of the DASE in pitch and roll axes. This mode provides
the pilot with limited hands-off flight capabilities in
cruise flight. Attitude hold will only function if: (1) the
ATTD/HOVER HOLD, PITCH, and ROLL switches
on the ASE panel are engaged; (2) force trim is on, and
(3) longitudinal airspeed of the helicopter is greater
than 60 KTAS. CAS is removed when the attitude hold
mode is engaged.
HAS should not be used as the sole
method for station keeping. Crosschecking
obstacle clearance using
visual or NVS means shall be accomplished.
Do not activate HAS on the
ground or land with HAS on. Uncommanded
aircraft attitude changes
may result.
The initialization and stability of the HAS is affected by
the HARS velocity drift. A HARS velocity error at HAS
initialization will cause initial aircraft movement (at a
rate proportional to the HARS velocity error) that may
be trimmed using the cyclic force trim. Subsequent
changes in the HARS velocity accuracy, during prolonged
engagement of the HAS, will cause additional
aircraft movement that may be re-trimmed.
2-46 Change 4
TM 1-1520-238-10
NOTE
m the GPS system is not keyed or is
tracking less than 4 satellites, HAS
performance may degrade over time.
Hover Position Box and Velocity Vector
accuracy will be degraded.
Over time the SAS actuators may reach
saturation limit if not centered by reinitializing.
The ATTD/HOVER HOLD
features in pitch, roll, or yaw will be
lost if saturation occurs. The ATTD/
HOVER HOLD feature is a limited
hands off feature and the controls
should always be monitored carefully.
Heading hold is a function of HAS. The HAS mode will
function only if: (1) the ATTD/HOVER HOLD and all
ASE channels are engaged; (2) force trim is on, and (3)
airspeed is below 50 KTAS and ground speed is less
than 15 knots. Degraded conditions will occur in attitude
hold or HAS if individual ASE channel switches
are not engaged. The ATTD/HOVER HOLD switch
will disengage automatically when transitioning between
modes; attitude hold will disengage at approximately
55 KTAS when decelerating. HAS will disengage
when 15 knots ground speed or 50 KTAS is
exceeded; or when ADSS fails or is turned off.
NOTE
Backup Control Systems (BUCS) are not
incorporated on aircraft PV-529 (S/N
88-0199) and prior.
2.37.7 Backup Control System (BUCS). The
BUCS is a single-channel or non-redundant fly-by-wire
flight control system which electronically operates all
four flight controls. The BUCS provides full authority
flight control with identical handling characteristics to
DASE OFF flight in the affected axis. This is accomplished
through the use of LVDTs attached to the flight
controls in each crew station. The LVDT flight control
inputs are made to the DASE, which electronically controls
the BUCS servovalve on the primary side of each
flight control actuator. For a given flight control, the
BUCS will automatically engage when there is no mechanical
connection between the flight control actuator
and its flight control in both crew stations. The BUCS
will not engage for any flight control which retains mechanical
integrity between the flight control actuator
and the associated flight control in either crewstation.
The primary hydraulic system must be functional for
the BUCS to operate. The BUCS ON caution light will
illuminate when BUCS is engaged for any flight control.
Illumination of the BUCS FAIL warning
light while in flight shall be
treated as a flight control system
emergency
a. q Control Severance Engagement Logic.
The BUCS will automatically engage when a difference
or mistrack exists between both LVDTs for a given
flight control, one in each crewstation and both LVDTs
for the corresponding flight control actuator. For example:
if the lateral cyclic control was severed between the
lateral main rotor actuator and both crewstations, the
BUCS would engage when the crewmember flying
made a lateral input greater than approximately 1.75
inches. For flight control severances, the required input
　
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