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
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
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
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
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