(e) Tail Wheel Lock Actuator System

Figure 315. Tail Wheel Lock/Unlock Actuator Electrical Operation

TM-1-1520-238-T-4 Army Model AH-64 Helicopter (NSN
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TM 1-1520-238-T-4 3–18 3–7. SYSTEM DESCRIPTION (cont) 3–7 (e) Tail Wheel Lock Actuator System (fig. 3–15). When the actuator is in the locked position, fuselage reaction is minimized to rotor torque and crosswind effects. The lock actuator also prevents the tail wheel from swerving during normal landings, or when landing on a sloped terrain, and inhibits shimmy of the tail wheel during takeoff or landing. The locking pin shears under high side load conditions to prevent damage to the fork, trailing arm assembly or tailboom. The tail wheel lock actuator system can also be manually controlled. The handle attached to the lock actuator is used to manually unlock and lock the tail wheel. A quick–release safety pin, with an attached warning flag, is used to hold the lock actuator in the unlocked position during ground handling which prevents accidental locking of the tail wheel. When the handle is placed in the full up position, it seats the lock pin which secures the wheel in the center position. Placing the handle in the full down position unseats the lock pin, allowing the fork assembly freedom of movement. (f) Tail Wheel Proximity Switch. The working circuit of the tail wheel lock/unlock actuator electrical operation consists of a TWHL LOCK circuit breaker (CB59) that applies 28 VDC to the pilot TAIL WHEEL UNLOCK/LOCK switch, to the open contacts of the tail wheel lock relay (K34) and the tail wheel lock proximity switch. The tail wheel lock relay is controlled by the tail wheel lock proximity switch. When the tail wheel is unlocked, the proximity switch supplies an electrical ground to the tail wheel lock relay. Placing the TAIL WHEEL switch in the UNLOCK position applies 28 VDC to the tail wheel lock control valve solenoid allowing hydraulic pressure to unlock the tail wheel. When the tail wheel lock/unlock actuator is in the the unlock position, the tail wheel lock proximity switch is actuated and supplies an electrical ground to the tail wheel lock relay. When the tail wheel lock relay is energized, 28 VDC control voltage is applied to the multi–channel dimming controller which supplies voltage to illuminate the TAIL WHEEL UNLOCK advisory light. (3) MLG Brake System. The MLG brake system (fig. 3–16) provides braking and directional control for taxiing the aircraft. The brake system holds a maximum alternate gross weight of 17,650 lbs. on a 12 slope. A separate system is provided for the left and right wheels. The MLG brake system can be operated from the pilot or CPG station by depressing the top of the pilot or CPG directional control pedals. The parking brake is locked from the pilot station only, but can be released from either crew station. (a) Transfer Valves. The transfer valves allow the pilot or CPG master cylinder pressure to operate the brakes without mixing fluids. The two transfer valves provide pressure which interconnects between the pilot and CPG master brake cylinders. The bleed screws on the transfer valve are used to bleed air from the CPG master cylinders. When the brakes are operated from the pilot station, the transfer valve does not function mechanically. Fluid pressure passes from the pilot’s master brake cylinder through the transfer valves to the wheel brakes. The tranfer valve functions only when the CPG’s brakes are operated. Fluid pressure from the CPG master cylinder moves the piston, opening the CPG inlet port and poppet, thus closing the pilot’s inlet port which prevents fluid from flowing back into the pilot’s reservoir. The CPG piston transmits pressure to the hydraulic fluid forcing it through the outlet port into the transfer valve. This action operates the MLG wheel brakes. Pressure from the pilot’s master cylinder overrides the poppet valve. Unseating the poppet opens the pilot’s inlet and seals the CPG’s inlet to allow the pilot’s pressure to operate the wheel brakes. (b) Wheel Brake Assembly. The wheel brake assembly converts hydraulic pressure into a retarding mechanical force that stops wheel rotation. Hydraulic pressure activates three pistons in each brake assembly to press friction linings against a floating brake disc, thus slowing or stopping wheel rotation.

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