TM 1-1520-238-T-7
11–63
11–7.
SYSTEM DESCRIPTION (cont)
11–7
(n) Hover and attitude hold mode (fig. 11–37) operates automatically during pitch and roll SCAS
operation. Hover augmentation system is engaged when the ATTD/HOVER HOLD switch on the pilot ASE panel
is engaged, force trim is on, ADSS airspeed is below 50 knots, and ADSS ground speed is below 15 knots. The
hover augmentation system holds the helicopter in a 12-foot radius circle for 20 seconds in a 5 to 10 mph wind,
when a slow ascent or descent from 200 feet above the ground is executed. The attitude hold mode is engaged
when the ATTD/HOVER HOLD switch on the pilot ASE panel is engaged, force trim is on, and ADSS airspeed is
above 60 knots. The attitude hold mode maintains desired pitch and roll heading. Airspeed is sent to the DASEC
from the ADSS. Doppler compensated pitch and roll signals are sent to the DASEC from the HARS. Actual
attitude signals are compared to reference attitude, and the DASEC computes a command signal to the
longitudinal and lateral servocylinders EHV. The EHVs moves the SAS cylinders which moves the servocylinders,
correcting lattitude deviations. The servocylinders SAS LVDTs send position feedback signals to the DASEC. The
DASEC sums the command signals with the LVDTs position signals to null out the command signals. When the
command signals are nulled, the SAS cylinders return to center position and servocylinder movement stops.
Momentary release of the FORCE TRIM switch on the cyclic sticks resynchronizes the rate signal to the force trim
system, establishing a new reference attitude. Disengagement or failure lights the ASE indicators on the pilot and
CPG caution/warning panels.
HARS
ADSS
PITCH
AIRSPEED
MONITOR
DASEC
POSITION FEEDBACK
ROLL HOLD
LATERAL
SERVO
M70-275
FORCE TRIM
ROLL
ROLL
MONITOR
PILOT ASE PANEL
CAUTION/WARNING
PANEL
ASE
HOVER
HOLD
MONITOR
PITCH
POSITION FEEDBACK
PITCH HOLD
LONGITUDIAL
SERVO
MAIN
ROTOR
MAIN
ROTOR
POSITION FEEDBACK
PSEUDO YAW HOLD
DIRECTIONAL
SERVO
TAIL
ROTOR
Figure 11–37.
Hover Augmentation System and Attitude Hold Functional Block Diagram
