one-way restrictors installed in the open and close lines ensure smooth door operation and prevent cavitation of the door-actuating cylinders. WINDSHIELD WIPER SYSTEM The windshield wiper system shown in figure 12-68 consists of a pressure reducer, speed control needle valve and drive mechanism, hydraulic actuator, two window actuator units and wiper blade assemblies, and a return line check valve. System pressure is directed to the pressure reducer, where it is reduced to 2,000 psi, and then the fluid passes to the speed control valve, which starts, stops, and controls the wiper blade speed. Hydraulic fluid is directed from the speed control unit to the hydraulic actuator, which, in turn, controls and directs fluid to the window units. The actuator alternately allows fluid flow to opposite sides of the window unit double piston. Constant speed of the wiper blades is provided by fluid from the speed control valve and is directed to the balance pistons in the hydraulic actuator. Fluid is also directed to the window units through the hydraulic actuator normal inlet port. The window units, by action of a rack and piston arrangement, convert the linear motion of the double piston to the reciprocating action of the drive shaft. When the system has completed one wiper stroke and the hydraulic pressure at the window unit pistons reaches a value equal to system pressure minus 200 psi, the actuator will then reverse the flow to the opposite side of the window unit piston and repeat the wiper stroke action in reverse. Any obstruction on one windshield will stop that blade, but allow the other to continue until it completes its stroke (or meets an obstruction), at which time the pressure in the window units buildup and the actuator reverses the action of both blades. The mechanical locking device is provided to hold the blades in the parked position when the needle valve is closed. NOTE: Do not operate the windshield wiper blades on a dry windshield. Maintenance of the windshield wiper system consists mainly of operational checks, removal and installation of components, and troubleshooting. The operational check should be performed according to the following procedures: 1. Provide a supply of water on the outside surface of the panels when the wiper blades are in motion. 2. Check for a wiper arm force of 7 to 10 pounds on the windshield (at the blade attachment). 3. Connect external electrical power supply. 4. Energize hydraulic power system No. 1 ac pumps. 5. Slowly open the windshield wiper speed control needle valve. 6. Blades must move from parked position and begin to cycle between 100 to 300 strokes per minute. 7. Open instrument panel to gain access to window units. Bleed air from units as they cycle by cracking the B-nuts on the tubing at each end of the window units. Allow fluid to bleed into existing drip pans until it is evident that all air has been removed. 8. Check that no hydraulic fluid leak is visible on the system tubing, connections, or at any component. 9. Check that system components perform smoothly with no erratic operation and blade reversing is synchronized. Blade rotation must be 75 degrees. The wiper blades must not touch the center post, travel into the parking area, or short cycle during high-speed operation. 10. Reduce speed of blade operation, and manually stall each wiper separately. While the blade is stalled, the opposite blade should operate smoothly. 11. Park the blades by slowing down the cycling speed to permit blades to move into the park position before they reverse. NOTE: Parking area is the area between the bottom edge of the glass and the break in the contour of the fuselage. To adjust the blades, loosen the blade attaching screw and rotate blade. One serration is approximately 5 degrees of rotation. If it is not possible to install within the parking area, install the arm outboard with the blades as close to the parking area as possible, then remove the arm and adapter. Looking down on the arm, carefully remove the adapter and rotate it one serration 12-79