TM 1-1520-238-T-7
11–55
11–7.
SYSTEM DESCRIPTION (cont)
11–7
(b) The squat switch receives inputs from the DASEC and provides output for DASEC yaw CAS
disengagement when the helicopter is on the ground. The longitudinal, lateral, directional, and collective
servocylinders receive positioning outputs from and provide feedback to the DASEC. The SCUs send status to
the DASEC. The pilot and CPG SPAD switches provide inputs to activate the BUCS when flight controls are
jammed. The visual display unit (VDU) receives rate-of-turn outputs from the DASEC. Force trim inputs to the
DASEC activate the attitude hover/hold (ATTD/HOVER HOLD) operation. HARS inputs rate, attitude, heading,
velocity and validity signal data to the DASEC. The fire control computer (FCC) communicates with the DASEC to
issue instructions and determines system LRU status. The ADSS inputs airspeed and sideslip information. The
pilot and CPG LVDTs transmit input from collective stick, cyclic stick and directional pedal movement. The DASEC
provides outputs to the pilot’s and CPG’s caution/warning panels to light the BUCS ON, ASE, and BUCS FAIL
indicators.
(c) The CPG collective stick grip provides switchology to enable the CPG’s flight controls when BUCS
has been activated. The pilot’s and CPG’s cyclic sticks provide inputs to disengage force trim. The pilot ASE
panel provides switches to engage PITCH, ROLL, YAW, ATTD/HOVER HOLD, and BUCS TST (test) functions.
(3) The hydraulic servocylinders (fig. 11–28) are electrically operated in the ASE and BUCS modes.
(a) When ASE is engaged, the SAS solenoid valve is energized opened. The opened SAS solenoid
valve sends pressurized hydraulic fluid to the two-stage electro-hydraulic valve (EHV). The EHV is a flapper valve
that is controlled by an electrical signal input from the DASEC. When the EHV is in the center (neutral) position,
hydraulic pressure is equal on both sides of the SAS cylinder. When the helicopter deviates from the reference
attitude, a command signal from the DASEC is applied to the EHV. The flapper valve inside the EHV moves,
causing hydraulic pressure to move the SAS cylinder.
(b) Movement of the SAS cylinder opens ports to allow hydraulic pressure to extend or retract the power
piston. The SAS LVDT provides a feedback signal of opposite polarity to the DASEC where it is summed with the
command signal. The resulting summation signal nulls out the command signal and prevents overcorrection.
When the command signal is nulled out, the EHV flapper valve returns to the neutral position. Equalized pressure
on the SAS cylinder causes the cylinder movement to stop. As the power piston moves, mechanical feedback is
applied to the pilot input lever, causing the lever to pivot around the pilot input point. The mechanical feedback
moves the manual servo valve to realign with the new SAS cylinder position. When the SAS cylinder and manual
servo valve are aligned, the hydraulic ports close and power piston movement stops.
(c) When the helicopter is returning to attitude, a DASEC command signal of opposite polarity is applied
to the EHV. The EHV causes the SAS cylinder to move back toward the neutral position. When the SAS cylinder
moves, it displaces the manual servo valve and opens hydraulic ports to the power piston. The power piston
moves back toward its original position. As the power piston moves, the mechanical feedback moves the manual
servo input arm which moves the manual servo valve back into alignment with the SAS cylinder. When the
manual servo valve is aligned with the SAS cylinder, the hydraulic ports are closed and power piston movement
stops.
