internal fluid passages for various control valve functions. The manifold contains the bridle tensioner pilot valve and the internal tensioning inlet and outlet valves for the bridle tensioning system. The manifold also contains the advance and retract pilot valve, retract directional valve,    advance directional valve, and maneuvering valve. The solenoid-operated bridle tensioner pilot valve passes hydraulic fluid to actuate the bridle tensioner control valve, internal tensioning inlet valve, and internal tensioning outlet valve. When the solenoid is de-energized, the fluid is allowed to flow from these three valves to the gravity tank. The internal tensioning inlet and outlet valves control the flow of reduced pressure hydraulic fluid to and from the retraction engine hydraulic motor and orifice bypass piping. Fluid flows through the inlet valve to the hydraulic motor and bypass piping and is returned to the gravity tank through the outlet valve. This enables the hydraulic motor to rotate the drum slowly so that the static friction in the retraction engine and drive system is overcome. The advance and retract pilot valve controls the advance directional valve and the retract directional valve through the advance dump valve and the retract dump valve, respectively. When the advance solenoid is energized, the piston in the pilot valve is shifted, and hydraulic fluid flows through the valve and through the advance dump valve to shift the advance directional valve. When the retract solenoid is energized, the piston is shifted, and hydraulic fluid flows through it and through the retraction dump valve to shift the retract directional valve. Maneuvering Valve The maneuvering valve (fig. 4-51) is mounted on the manifold and is operated by the maneuver forward solenoid and the maneuver aft solenoid. This valve is energized automatically during the latter part of the advance and retraction operation to control the speed of the grab after braking has been completed. Hydraulic fluid flowing through the valve to and from the hydraulic motor is controlled by orifices. At times other than during normal operations, the valve can be energized to slowly maneuver the grab, shuttle, and pistons forward or aft for testing or maintenance. A manual override button on the valve can be pushed to maneuver the grab aft in case of power failure and permit disengagement of the aircraft from the shuttle. Two dump valves are mounted on the retraction engine frame. The valves are operated by cams mounted on the retraction engine frame. When the retraction engine nears the end of the advance stroke, the advance dump valve is actuated. The dump valve closes, allowing the pilot-actuating fluid from the advance valve to return to the gravity tank, initiating the advance braking stroke. When the retraction engine nears the end of the retraction stroke, the retraction dump valve is actuated. The dump valve closes, allowing the pilot-actuating fluid from the retract valve to return to the gravity tank, initiating the retract braking stroke. The vent valve panel is located on top of the retraction engine manifold assembly. Vent valves are mounted on the panel and connect to various points in the retraction engine hydraulic system. These valves are used to bleed air from the hydraulic lines. Main Hydraulic Accumulator The main hydraulic accumulator (fig. 4-52) consists mainly of a cylinder and a floating piston. The accumulator maintains a constant pressure in the hydraulic system. Hydraulic fluid on one side of the piston is maintained in a pressurized condition by air pressure on the other side. The piston prevents the mixing of the fluid and air. As hydraulic fluid is used, the air pressure causes the piston to move toward the fluid end of the accumulator cylinder maintaining pressure on the fluid. Fluid used from the accumulator is replenished by the main hydraulic pumps. Stroke-Control Actuator The stroke-control actuator is mounted in the bottom of the accumulator cylinder. The actuator is a lever-operated cam that operates two limit switches. The bottom limit switch controls the operation of hydraulic pump No. 1, and the top limit switch controls the operation of pumps No. 2 and No. 3. When the accumulator piston moves toward the top of the accumulator, the actuator causes the pumps to pump hydraulic fluid (go onstroke) to the accumulator. Bolts on the onstroke cam actuate the limit switches so that pump No. 1 will go onstroke before pumps No. 2 and No. 3. When the piston nears the bottom of the accumulator, the actuator causes the pumps to stop pumping (go offstroke). Bolts on the offstroke cam actuate the limit switches so that pump No. 1 will go offstroke before pumps No. 2 and No. 3. 4-41