TM 1-411, Airplane Hydraulic Systems and Miscellaneous Equipment: 3 - Airplane Hydraulic Systems

SECTION III: AIRPLANE HYDRAULIC SYSTEMS

25. General.-A system wherein fluid is used to transmit a force to operate a mechanism is called a hydraulic system. A suitable pump may be used to develop the force. The force is transmitted through a column of fluid confined in pipes to a piston against which the force is directed. The piston is contained in a cylinder, and means is provided for preventing fluid from escaping past the piston. The piston rod is attached to some mechanism so that movement of the piston may operate the mechanism. The force applied to the piston, if of sufficient magnitude, will operate the mechanism. The fluid is forced into and out of the cylinder. The expelled fluid is returned to its original source or reservoir. The direction of flow to and from the cylinder, and consequently the direction of travel of the piston in the cylinder, are controlled by a manually operated selector valve. The fluid circulates throughout the system. Such are the elements of a hydraulic system of the type used in airplanes.

26. Elementary system.-a. An elementary hydraulic system is illustrated in figure 30(1). This system is presented for the purpose of stressing certain inherent faults and is not intended to represent a hydraulic system suitable for installation in an airplane. The system shown is limited in application and is operative under certain conditions only. The restricted conditions under which such a system is operative preclude its use for practical purposes. The system is a sealed circuit, and, as such, a change of fluid volume will affect its successful operation. It is characteristic of fluids to expand with an increase of temperature and to contract with a decrease of temperature. Since it may be assumed that the system in this case was completely filled with fluid to start with, any tendency of the fluid to expand with an increase of temperature must result in an increase of fluid pressure. Although an increase of fluid pressure will not in itself render the system inoperative, it might, if excessive, rupture the lines. In the case of a. decrease in temperature and in the case of fluid leakage, the resulting decrease in fluid volume will starve the pump and decrease its efficiency. In case the decrease in fluid volume becomes excessive, the pump will perform spasmodically or may cease to pump entirely. To overcome the inherent weakness referred to, a fluid reservoir is incorporated in the system as shown in figure 31. This reservoir is vented to the air and is kept partially filled with fluid at all times. It serves as an expansion chamber as well as a source of supply of fluid to replace that lost through seepage and leakage.

b. The system including a reservoir as shown in figure 30(2) is limited in application for it can be employed only in conjunction with an intermittently operated pump. The inherent weakness in the system lies in the fact that the fluid delivered to the piston is in terms of the pump operation and not in terms of the piston requirements. In the case of fluid pumps installed in airplanes, the pump is often engine-driven and it is therefore in constant operation with the engine. In this case fluid is being constantly circulated whether it is required or not and some means must be provided for bypassing fluid in excess of that required to operate the piston. Provision must also be made for relieving the pressure when the piston reaches the end of its stroke and also when the piston requires a pressure to move it in excess of a safe operating pressure. Relief valves installed as shown in figure 30(3) will bypass fluid back to the reservoir after a predetermined pressure for which the valves are set to open has been built up.

27. Improved system.-The system has now been improved to the point where it is practical and safe to operate; however, it lacks some of the refinements desirable in a system designed for use on airplanes. For the purpose for which hydraulic systems are used on airplanes, it is often desirable to hold the piston, and thereby the mechanism attached to the piston, fixed in position after the pump has been stopped. Such is accomplished by incorporating a check valve in the system as shown in figure 30(4). This unit permits fluid to flow toward the actuating cylinder, but not away from it. A check valve installed between the pump and the selector valve thus traps the pressure in the cylinder. To release the pressure, the selector valve must be reversed.

28. Complete system.-a. The system has now been developed to the point where a limited amount of mechanism can be successfully operated. In the event that several groups of mechanism must be operated simultaneously, the fluid required to operate them may exceed the capacity of the pump to supply it. In such cases a pressure tank is resorted to as shown in figure 31. Such a pressure tank stores up energy and serves the same function in a hydraulic system as a storage battery does in an electrical system. As auxiliaries to a pressure tank, two additional units are required. These are a pressure regulator and a system relief valve. The pressure regulator is installed between the pump and the pressure tank and the relief valve between the pressure tank and the reservoir. This relief valve serves as a safety vent in case the pressure regulator fails to function properly and stop the flow to the pressure tank and pressure manifold as it should. This relief valve is set to open at a pressure somewhat higher than the normal operating pressure required to operate the system.

b. A further refinement shown in figure 32 is the inclusion of a pressure gage, installed in a convenient location in the pressure side, of the system.

c. To provide an auxiliary source of power in case of failure of the engine-driven pump, a hand pump is essential in a hydraulic system intended for installation in an airplane. (See fig. 32.) This pump draws fluid directly from the reservoir and forces it into the pressure manifold. The hand pump must be resorted to in case the engine-driven pump is inactive and the pressure in the pressure tank is too low to operate the mechanism unaided. The hand pump may also be used to raise the pressure in the pressure tank. A bypass check valve, installed between the hand pump and the pressure tank will direct the output of the hand pump to the pressure tank when it is turned to "tank" position. The valve is normally left in "system" position.

d. Where an automatic pilot is hooked into a hydraulic system, it is desirable to use two engine-driven pumps, one pump serving the automatic pilot and the other the remaining mechanisms. A selector valve installed in the pressure lines from the engine-driven pumps directs the flow two ways. Either pump may be made to serve either system by reversing the setting of the selector valve. A relief valve installed in the automatic pilot pressure line will bypass fluid back into the general return line to the reservoir when the pressure exceeds the normal operating pressure required by the automatic pilot..

e. To facilitate the removal of the engine and engine mount, line disconnects are installed in the lines leading from the pumps. These units permit breaking the lines at these points without the loss of fluid.

29. A-17A airplane.-a. General.-There are two separate hydraulic systems in this airplane; one is used to operate the landing wheel brakes while the second operates the landing gear retracting mechanism and the landing flaps. The system is shown schematically in figure 33.

(1) A fluid reservoir is located on the left inside wall of the overturning structure. The filler cap is accessible from the outside.

(2) A power pump is mounted on the engine and driven by the left gun synchronizer shaft. It is driven at crankshaft speed.

(3) A hand pump is located at the left side of the front cockpit for convenient operation by the pilot, should the engine pump become inoperative.

(4) A power control valve, mounted on the, upper left side of the fuselage just aft the fire wall, is connected to the engine pump. The control knob is located at the left of the center instrument panel. With this knob pushed forward, the fluid from the pump is diverted from the bypass to the system pressure lines. The power control valve is set to open when the back pressure in these lines reaches approximately 950 pounds per square inch. The power control valve knob must always be back when valve is not in use.

(5) A surge cylinder, mounted on the rear side of the fire wall just above the center of the fuselage, is connected to the power-control valve. The purpose of this cylinder is to absorb momentary rises in pressure above 950 pounds and thus prevent disengaging the control valve before the landing gear is completely up.

(6) There are two selector valves, one for the landing gear hydraulic system and one for the wing flap hydraulic system. The control handles are located on the left side of the cockpit.

(7) A check valve in the pressure line just below the engine pump control valve prevents flow through the engine pump or power-control valve when the hand pump is used.

(8) A pressure gage is located beside the hand pump, and is connected to the main pressure line.

(9) There are no special bleeder fittings on the landing gear or the flap operating cylinders as repeated operation of these systems forces any air up to the fluid reservoir in which it escapes from the system.

b. Brake system.-The brakes are operated by individual master cylinders which are connected by a linkage to the brake pedals. Each brake is operated independently of the other. Compensators, or springs, are included in the linkage, between the brake pedals and the master cylinders. These springs, which are held in compression when the brakes are locked, obviate the possibility of the brakes becoming loose through contraction of the hydraulic fluid and prevent damage to the system in case of temperature expansion.

(1) To lock brakes, press both brake pedals, pull brake lock handle, and release pedals.

(2) To release brakes, press both brake pedals until brake lock snaps in, and release pedals. In case the brakes have been set and locked and a considerable rise in temperature should then occur, the fluid may expand enough to compress completely the compensator springs to such an extent that the brakes cannot be released by foot pressure on the pedals. If this condition should occur, the pressure in the system may be relieved by allowing some of the fluid to escape from the bleeder valve of each wheel.

c. Landing gear system.--Rotation of torque shafts retracts the landing gear by swinging the wheels and struts inboard and up into the center section of the wing. The torque shafts are rotated by individual hydraulic operating cylinders connected to arms on the shafts, the pressure for which is furnished by an engine-driven pump or by an auxiliary hand pump at the left side of the pilot's seat. A selector valve directs the flow for extension or retraction of the landing gear. A control handle adjacent to the hand pump unlocks the latches and operates the selector valve located in the left landing gear nacelle.

 (1) A vibrator warning device attached to the left rudder pedal operates when the throttle is closed and the landing gear is in any position except fully extended and locked. The vibrator may be made inoperative by pushing the small handle on the warning switch box at the left side of the pilot's cockpit. The vibrator automatically engages again when the throttle is reopened.

(2) To retract landing gear, have flap valve in the locked position. Push back the spring loaded lock which holds the selector valve handle, in the down position, and pull up on the handle. Push forward on the power control valve knob and allow the knob to remain forward until both red capped, luminous, mechanical indicator arms protrude through the upper skin of the wing center section over the wheels and the pressure gage on the left-hand side of the cockpit remains constant at 950 pounds per square inch pressure, or higher. Pull the knob completely back if it fails to kick out automatically. The indicators, easily visible from the cockpit, are a positive indication that the wheels are fully retracted.

(3) To extend landing gear, push down on the selector valve handle beside the hand pump, and push forward on the power control valve knob. Allow the knob to remain forward until the gear is completely extended and locked and the pressure gage remains constant at 950 pounds per square inch pressure or higher. When both wheels are locked in the extended position and the engine is idling, the vibrator will be inoperative. Furthermore, green capped luminous indicator arms will protrude through the upper skin of the wing center section.

(4) To retract or extend landing gear with the engine pump inoperative, move the selector valve handle beside the pump to the desired position (up for retraction, down for extension), and operate the hand pump until the landing gear is in the proper position as indicated by the signals. If the signals should not be in order, continue pumping until extreme stiffness is experienced. This will indicate that the gear is completely up or completely down.

d. Wing flap system.-An actuating cylinder, rigidly attached near the trailing edge, of the wing center section, moves the flap control rods.

(1) A neutral position is provided in the wing flap selector valve in order that the flaps may be kept. at any angle desired.

(2) To lower flaps, move the flap selector valve handle to the down position and push forward on the power control valve knob. Allow the knob to remain forward until the indicator shows that the flaps are lowered and the pressure gage remains constant at approximately 950 pounds per square inch. Then pull the knob completely back if it fails to kick out automatically.

(3) To raise flaps, move the flap selector valve handle to the up position and push forward on the power control valve knob. Allow the knob to remain forward until the indicator shows that the flaps are raised and the pressure gage remains constant at approximately 950 pounds per square inch. Then pull the knob completely back if it fails to kick out automatically. In flight, when the flaps are in the down position, if the selector valve handle is placed in the up position, the force of the air stream will close the flaps to approximately 15°.

(4) To raise or lower the flaps or landing gear without use of the engine pump, move selector valve to desired position and operate the hand pump.

e. Maintenance.- (1) To fill reservoir, have the landing gear extended and the flaps up to avoid subsequent overflow.

(2) To bleed the landing gear and wing flap systems, support the airplane so the wheels clear the ground. Fill the reservoir and alternately pump the landing gear and then the flaps completely up and down at least four times. There is no special bleeding arrangement in this hydraulic system a repeated operation of the system forces air up to the reservoir where it escapes from the system.

(3) To adjust relief valves

(a) Have the landing gear completely extended. Keep the landing gear selector valve in the down position.

(b) On the top of the hand pump, back off the relief valve adjusting screw lock nut and turn the adjusting screw to the right until snug. This is to raise the release pressure of the hand pump temporarily until the flap selector relief valve is adjusted.

(c) Set the flap selector valve in the up position and operate the hand pump until the flaps are closed. Back off the adjusting screw lock nut on the flap selector relief valve. Operate the hand pump and turn the flap selector relief valve adjusting screw clockwise to raise the pressure, and anticlockwise to reduce the pressure until the gage reading is 1,000 (+100-0). Tighten the lock nut and safety.

(d) Set the flap selector valve control handle in neutral position and adjust the hand pump relief valve until the gage reading is 1,000(+50-0). Tighten the lock nut and safety.

(e) Start the engine and place the flap selector control handle in neutral position.

(f) On the automatic power control valve, back off the adjusting screw lock nut on the kick-out relief valve (the adjustment farthest from the wall of the fuselage) and turn the adjusting screw a few turns to the right or until snug. Care must be exercised that the adjusting screw is not turned in too far as the composition seat of the valve piston may be sheared off.

(g) Back off the adjusting screw lock nut of the system (overload) relief valve (the adjustment nearest the, wall of the fuselage) and with the engine throttle advanced to approximately 1,500 r. p. m., turn the adjusting screw until the gage reading is 1,100 (+50-0). Tighten the lock nut and safety.

(h) Back off the kick-out relief valve, several turns. Place the flap selector control handle in the down position and with the engine throttle advanced to approximately 1,500 r. p. m., push the power control knob forward. Note the gage reading at which the control knob returns to its former aft position. Turn the adjusting screw to raise the kick-out pressure to 950 (+100-0). Make several tests by raising and lowering the flaps to verify the valve setting. When the adjustment is satisfactory, tighten the lock nut and safety.

30. BC-1 airplane.-a. General.-The BC-1 hydraulic system, shown schematically in figure 34, is employed for operation of the landing gear retracting mechanism and wing flaps. An engine-driven hydraulic pump is provided. A fluid reservoir is installed at the left side of the fuselage between the two cockpits. An emergency hydraulic hand pump and a pressure gage are installed on the left side of the front cockpit only. A power control knob is located at the left side of each cockpit. A separate hydraulic system is provided for operating the brakes.

(1) A compound landing gear and wing flap selector valve is located to the left of the pilot's seat. It has six ports: a pressure inport, a return outport, two ports to the wing flap, and two ports connected in parallel to the two landing gear retracting cylinders.

(2) A power control knob for controlling the operation of the flaps and landing gear by means of the engine-driven hydraulic pump is located forward of the flap and landing-gear control handles. An instruction plate for operation of the hydraulic system is mounted in the front cockpit forward of the control handles and adjacent to the position indicators. After the control handles are set at the desired positions, the power control knob marked "Push" is pushed down. It is not necessary to hold the knob down after it has been pushed; however, sudden inertial loads due to bumpy air may cause the power control to disengage before a given action is completed, in which case it is necessary to push the knob down again. The power control automatically disengages when the operation is completed. If both power control knobs are depressed at the same time, they may both fail to disengage. In this event, both valves may be disengaged by firmly pushing down on the front power control knob for a few seconds and then quickly removing the hand. Should either valve individually fail to disengage, this method can also be employed on that valve. Manually pulling the push knob out does not disengage the valve since the knob is not directly attached to the valve mechanism.

(3) In the event of engine-driven hydraulic pump failure, the emergency hand pump located on the left side of the front cockpit may be used. In the event of both engine-driven pump and emergency hand pump failure in the hydraulic system, the landing gear will fall when the control handle is set for the down position. After the landing gear has reached the down position during this emergency operation without the use of either hydraulic pump, the latches may not engage until the wings of the airplane are waved to throw the gear into the locked position. The pressure gage will not show pressure; however, the landing may be completed if the vibrator warning device indicates a safe landing.

b. Brake system.-Brake pedals are incorporated in the rudder pedal assemblies. The brakes are hydraulically controlled and are actuated selectively from either front or rear cockpit. The parking brakes may be applied from the front cockpit only; however, they may be released from either cockpit by pressing the brake pedals. The parking brakes are set by means of the control knob located in the front cockpit below the instrument panel, which locks the brakes after they have been applied by the brake pedals.

a. Landing gear system.-The landing gear is pivoted at the upper portion of the oleo strut and is retracted inboard into the wing center section structure. The retraction is actuated by means of an engine-driven hydraulic pump, which may be operated from either front or rear cockpit; or a manually operated emergency hand pump, operated from the front cockpit only. Positive mechanical locks for both up and down positions are incorporated in the retracting mechanism.

(1) A selector valve control handle for the landing gear retracting mechanism is located on the left side of each cockpit. The control handle may be set for either "up" or "down," as desired. In the event that the landing gear is down and the latch pins are not in place, an unsafe landing condition is indicated by the warning device. It is then necessary to move aside the emergency plate, located on the forward portion of the landing gear control quadrant in the front cockpit only, and push the control handle through this extreme forward travel, which forces the down position latch pins in place. This extra portion of the control handle travel is to be used in emergency only, and not for normal operation. The landing gear position indicator should not be used as . safe landing indicator, since it does not indicate the position of the down latch pins. The position indicator is located at the left side of the front cockpit.

(2) When the throttle is closed to a position below approximately 1,000 r. p. m., and the landing gear is in the up position, or the down position lock pins are not in place, an electrically controlled warning device is set into operation. A release switch is located on the pilot's switch box for rendering the warning device inoperative when it is desired to close the throttle with the wheels in the up position. Opening throttle to approximately 1,200 r. p. m. automatically reinstates the warning device.

d. Wing flap system.-A flap selector valve control handle is located on the left side of each cockpit, next to the landing gear control handle. The control handle may be set in either the up or down position. The lock position is used only when it is desired to stop the flaps at an intermediate position as indicated by the flap position indicator. If flap selector valve handle is left in lock position and temperature expansion occurs, it will cause leakage or damage as flap temperature expansion valve will only relieve with selector valve in up position. The flap position indicator is calibrated in degrees and is located in the front cockpit, just forward of the control handle.

e. Maintenance.-(1) With airplane in three-point position, fill reservoir with hydraulic fluid. The reservoir should be filled to overflowing, inasmuch as a visual inspection of the fluid level as seen in the filler neck is misleading.

(2) When the lines have been drained of fluid and the reservoir refilled, some difficulty may be experienced in getting the engine pump to prime. To check for satisfactory operation of the engine pump, operate the wing flap control handle and power control knob as required to lower and raise the flaps. If satisfactory operation of the flaps is obtained, the engine pump is functioning properly. If the flaps do not operate satisfactorily, proceed as follows:

(a) Stop engine and connect a globe valve to the pressure line T located on the front of the fire wall.

(b) Close the globe valve and start engine.

(c) Allow a small leakage of fluid from globe valve.

(d) Close globe valve and test for proper functioning of pump by operating flaps with power control.

(e) Allow leakage at globe valve until pump has primed itself.

(f) Stop engine, remove globe valve, and plug T fitting.

(3) To adjust system relief and pressure control valves, proceed as follows:

(a) From. the plugged branch of the T fitting on the pressure line at the fire wall, run a line to a second T fitting.

(b) Connect a pressure gage to the second branch of the additional T fitting.

(c) Connect a globe valve to the third branch of the additional T fitting.

(d) Connect globe valve to plugged branch of suction line T at fire wall.

(e) The above piping simply connects the pressure line with the suction line through a globe valve and also incorporates a pressure gage into the system.

(f) Start engine and open globe valve.

(g) Back off pressure control adjusting screw lock nut and turn adjusting screw to the right several turns. This is to raise the kick-out pressure of the pressure control temporarily until the system relief valve is adjusted.

(h) Slowly close globe valve and note gage reading. Adjust system relief valve adjusting screw clockwise to increase the pressure and anticlockwise to reduce the pressure until the gage reading is 1,400. Tighten adjusting screw lock nut. Open globe valve to relieve the system of pressure.

(i) Back oil pressure control adjusting screw several turns and slowly close globe valve, thus building up pressure until pressure control disengages. Note gage reading at which pressure control kicks out.

(j) In the event that the control fails to disengage automatically, give knob a firm push and then release it.

(k) The globe valve should be closed slowly enough to require approximately 1/2 minute to build up the pressure from 0 to 1,200 pounds per square inch.

(7) After pressure control has disengaged, open globe valve again.

(m) Thus for every trial, open and close valve until the pressure control will consistently disengage at 1,200 pounds per square inch.

31. P-36A airplane.-a. General.-The P-36A hydraulic system, shown schematically in figure 35, consists of an electrically driven hydraulic pump for operating the retractable landing gear, tail wheel, and wing flaps, a hand pump for emergency operation, a toggle switch mounted on the left-hand side of the cockpit under the cabin sill for operating the electric pump, a reserve tank for hydraulic fluid, and a hydraulic valve connected to two operating handles on the left-hand side on the floor of the cockpit.

(1) An assembly consisting of a power pump and a 12-volt motor circulates the fluid. The pump is equipped with an integral relief -valve adjusted to 1,000 pounds per square, inch.

(2) The hand pump is a single-cylinder, reciprocating, doubleacting pump. Each end of the cylinder is equipped with an intake check and a discharge check valve. A relief valve set at 1,500 pounds per square inch is integral with the hand pump. A special fitting is mounted on the hand pump to accommodate a pressure gage for checking relief valve pressures.

(3) The selector valve assembly consists of two operating handles connected to two camshafts on which the cams operate a series of poppet valves for the control of fluid flow.

(4) An orifice is installed in the landing gear hydraulic system to eliminate pressure differentials and consequent undesirable flow characteristics resulting from unequal piston areas and displacements for the landing gear and tail wheel retracting cylinders. This fitting is installed on the selector valve in the T fitting for the return line from the landing gear when the landing gear is being lowered. If this fitting were not installed, the larger volume of return fluid flowing through one side of this T fitting from the landing gear cylinders would prevent the smaller volume of return fluid from the tail wheel cylinder from flowing into the other side of the T. The restriction in this fitting is 1/16 inch in diameter and 1/4 inch in length.

(5) An orifice is installed in the flap system in the selector valve boss for the return line when the flaps are being raised. Due to the air loads on the flaps in the down position they will go up automatically as soon as the selector valve is moved to the up position. The 3/64-inch orifice controls the flow of fluid and regulates the rate at which the flaps move upward.

(6) A check valve is installed in the pressure line from the motordriven pump in the fuselage. This valve is located on the left side of the airplane behind the overturning bulkhead. This check valve must have the arrow indicating the direction of flow pointing toward the selector valve assembly. Its purpose in the system is to prevent the electric pump being driven hydraulically when the hand pump is operated.

b. Brake system.--The brakes are actuated by Warner hydraulic brake master cylinders mounted on and actuated by the rudder pedals in the cockpit. The parking brake lever located below the instrument board may be engaged when pedals are depressed and disengaged by depressing pedals.

c. Landing gear system,.-The landing gear shock struts retract by rotating backward about a trunnion at the top of the strut. During retraction the strut is rotated 90°' about its longitudinal axis by gears so that the wheel lies flush in the wing.

(1) The landing gear is locked in both the up and down position by hydraulically operated mechanical locks. A warning horn will operate when the throttle is closed except when the wheels are down and locked. A cam mounted on the throttle rod may be pulled out to disengage the horn operating switch temporarily. Engagement is automatic, when throttle is next opened. Position indicator transmitters connected to the retracting mechanism are located in the wheel pockets. These transmitters are electrically connected to the position indicator on the instrument panel. An operation instruction plate is mounted in the cockpit.

(2) To retract the landing gear and the tail wheel, depress button on landing gear control handle located at left side of pilot, move control handle to UP position and operate the electrical pump with toggle switch located forward and just above the throttle quadrant. The switch must be held "on" during operation of the electric pump. An indicator on the instrument panel indicates position of the landing gear.

(3) To extend landing gear, place the control handle in the down position and operate the electric pump as before, holding the switch "on" a few seconds after the wheels are down to insure positive engagement of locks. Return the landing gear control handle to neutral except during operation of the mechanism. When the landing gear has been lowered, the control handle must not be raised beyond the neutral position due to possible loss of pressure with resultant disengagement of locks. Where any doubt exists as to pressure on the locks, the control handle will again be placed in the down position, the pump operated, and the handle returned to neutral position. The landing gear may also be operated by the handoperated pump located on the right side of the cockpit.

(4) The tail wheel is fully retractable. The retracting strut has an integral, hydraulically operated, mechanical lock.  This lock is not connected into the landing gear lock warning system. The tail wheel is locked in its down position by this mechanical lock, operated by hydraulic pressure, and is held in its up position by hydraulic pressure alone. An indicator on the instrument panel indicates position of the tail wheel. The position indicator transmitter is mounted forward of and above the retracting strut.

(5) Before starting engine, taxiing, or landing, make sure that landing gear is locked down.

d. Wing gap system.-A single hydraulic actuating strut mounted on the airplane centerline bulkhead inside the trailing edge of the wing is used to control both flaps.

(1) To lower flaps, move the control handle (at left side of the pilot's seat) forward, and operate the electric pump with the toggle switch located forward and just above the throttle quadrant. The switch must be held "on" during operation of the electric pump. An indicator on the instrument panel indicates position of the flaps.

(2) To raise flaps, the control handle is set in the back position and the electric pump operated as before. Return the flap control handle to neutral position except during operation. The flaps may also be operated by the hand-operated pump located on the right side of the cockpit. In flight, flaps will go up automatically as soon as the control handle is moved to the back position.

e. Maintenance.--(1) To fill and bleed the system, the airplane should be supported on jacks or cradles so that the landing gear and wing flaps can be operated. Fill reservoir with fluid. During filling operation, operate the landing gear twice by hand and three to four times by power and operate the flaps several times, always taking care that the reserve tank is kept at least half full. Any air in the system is carried to the reserve tank and expelled. If the system still shows signs of air after the filling operation, it may be necessary to disconnect hydraulic lines at tail wheel retracting strut and bleed the system at that point. Do not replace filler cap until filling operation is completed.

(2) To adjust system relief valve, install a pressure gage (range from 0 to 2,000 pounds) on adapter adjacent to the hand pump, operate hand pump, and note gage reading. Regulate relief valve, to bypass fluid at 1,500 pounds per square inch. Turn adjustment anticlockwise for pressure reduction. Upon completion of adjustment, remove pressure gage, install and safety pipe plug.

(3) To adjust power pump relief valve, install a pressure gage (range from, 0 to 2,000 pounds) on adapter adjacent to the hand pump, operate the power pump and check to see that relief valve relieves at 1,000 pounds per square inch. If it is necessary to adjust the relief valve, loosen the locknut and turn the adjusting nut counterclockwise to increase pressure or clockwise to decrease pressure. Tighten the locknut and safety.

32. B-18A airplane.-a. General.-This system, less the, automatic pilot, is shown schematically in figure 36. The complete system, including the automatic pilot, contains approximately 9 gallons of fluid. The fluid flows directly from a supply reservoir to two high-pressure, engine-driven oil pumps. A manually operated selector valve located on the inboard side of the pilot's seat support allows selection of the pump for supplying pressure to the main hydraulic system controlling the operation of the landing gear, wing flaps, bomb doors, and landing gear wheel brakes; the automatic pilot system being automatically served by the other pump. When the selector valve handle is in the forward (normal) position, the left pump supplies the hydraulic system and the right pump supplies the automatic pilot. When the selector control is in the aft position, the reverse is true. The fluid for the main hydraulic system passes to a pressure tank. From this tank it continues through the main system under a pressure of 600 to 800 pounds pressure.

(1) The operation of the landing gear, wing flaps, and bomb doors is controlled by individual four-way selector valves of the piston type. Movement of the piston connects the proper ports of the valves to allow operation of the various units of the system. The valves for the landing gear and wing flaps are located in an assembly in the floor between the pilot's and copilot's seats. The bomb door valve is located in the front gunner's compartment with remote control adjacent to the bomber. An emergency bomb release pull handle is located in the pilot's cockpit near the base of the control pedestal.

(2) An emergency hand pump installed on the hydraulic panel and located adjacent to the landing gear and wing flap four-way valves is so connected to the system that any hydraulic unit, including the brakes, may be operated by the hand pump. The hand pump may also be used to increase the pressure in the pressure tank when the bypass valve is changed from the normal system to tank position. The forward port of the hand pump connects to the pressure manifold, and the aft port connects to the reservoir.

(3) Two check valves are installed in the hydraulic hand pump system. The hand pump has one check valve integral in the piston. When the pump handle is pushed forward, the ball check is unseated and fluid is forced to flow through inner passages in the piston and into the pressure manifold. When the handle is pulled aft, the ball check is seated and the fluid on the forward side of the piston head is forced out into the pressure manifold, while fluid is being sucked in through the aft side of the pump.

(4) The reservoir is made of aluminum alloy and is cylindrical in shape. The filling capacity is 2 1/3 gallons and the total capacity is 3 1/2 gallons. A sight gage located beside the reservoir, when used in conjunction with instructions and filling marks on the sight gage panel, indicates the fluid level.

(5) The pressure tank consists essentially of two hemispheres of forged aluminum alloy and a synthetic rubber diaphragm which divides the tank into two chambers, one, an air chamber and the other a fluid chamber.

(6) A pressure of 600 to 800 pounds per square inch is maintained in the pressure tank and the system by the pressure regulator. As pressure builds up, a plunger in the regulator moves inward, bearing on a coil spring at 600 pounds per square inch, the spring has been compressed to a point where an extension on the plunger bears on a ball which is held in place by the same fluid which moves the plunger. The parts are so proportioned that at 800 pounds per square inch the load on the plunger is exactly balanced by 200 pounds force on the ball and 600 pounds force on the spring. Any further increase in pressure causes the inward motion of the plunger to push the ball off its seat. Fluid from the pump rushes around the ball into the plunger chamber, flowing back to the reservoir by way of a port in the valve housing. The fluid from the pressure tank is prevented from returning to the reservoir by a check valve installed in a bypass line around the regulator. The plunger with an 800-pound load is now opposed only by a 600-pound resistance on the spring. Consequently, the ball will be held off its seat until the line pressure has dropped to 600 pounds per square inch.

(7) A system relief valve is connected to a port on the regulator and to the reservoir return line. Should the pressure regulator fail to operate and the pressure increase above 1,000 pounds per square inch, the system relief valve will open and allow fluid to return to the reservoir. If it is impossible to raise the pressure in the pressure tank to 1,000 pounds per square inch by the use of the hydraulic hand pump after carefully checking the pressure regulator, pumps and connecting line, this valve may be out of adjustment, allowing fluid to return to the reservoir before the proper release pressure is reached. This valve is located in the forward gunner's compartment and adjacent to the pressure regulator and the reservoir. To raise the pressure of the pressure tank by means of the hand pump, the bypass check valve must be set in tank position.

(8) The pressure gage indicates the pressure in the hydraulic system except that which is generated by temperature expansion in the landing gear, flap, and brake systems.

(9) The bypass check valve is connected in the main pressure line between the pressure tank and the point where the hand pump discharges into the pressure manifold. It is mounted on the hydraulic panel. It is safetied in "system" position with light, soft wire, thus connecting the hand pump directly with the system without going through the pressure tank. If it is necessary to increase pressure in the pressure tank with the hand pump, break safety wire and move valve control to "tank" position. A small bleed hole is located in the valve seat to relieve pressure in the bomb door operating system due to temperature expansion. Excessive pressures generated in this part of the system are thus able to get to the pressure tank where they are absorbed.

(10) A check valve is located adjacent to the reservoir in the fluid return line. This valve allows fluid to return to the reservoir but prevents it returning to the system through this line.

b. Brake system.-Hydraulic pressure is transmitted to the brakes through a power brake control valve, each wheel brake operating independently of the other. The valve housing encloses two piston chambers with pressure inlet and reservoir return in common, and separate ports to their respective brakes. Application of toe pressure to the rudder pedals causes the corresponding piston and valve operating pin to move up and raise a ball from its seat, connecting the brake and pressure lines. Fluid will flow into the brake line until the piston pressure and brake pressures are equal. At this point the piston and pin will be forced down, allowing the ball to seat and relieve the brake from further increase in pressure. There is no noticeable lag in this operation. When the toe pressure is released, the piston moves down, unseating the valve-operating pin. A fixed pin mounted through a slot in the piston prevents the operating pin from following the piston. As soon as the piston and pin are separated, two holes in the piston are uncovered allowing the fluid to flow out to the reservoir and relieve the brake pressure.

(1) A check valve is incorporated in the pressure supply line adjacent to the power brake control valve and allows fluid to pass into the control valve, but prevents its returning.

(2) To set brakes for parking, pull plunger type parking lock control located on the upper left corner of control pedestal base, depress rudder pedals until plunger moves out, remove pressure from pedals, and release plunger. To release brakes from parked condition, press on rudder pedals and spring will release the lock. The hydraulic system pressure gage must show at least 500 pounds per square inch pressure when the airplane is parked.

e. Lading-gear system.-The landing gear consists of two independent, units; one mounted under each nacelle and so arranged that they may be folded up into the nacelles, leaving only the bottom of the wheels projecting.

(1) The landing gear selector valve is kept in either the up or down position except in case of a broken line, in which circumstance it is moved to a midway or neutral position.

(2) To retract landing gear, place the landing gear safety latch in latch raised position by pulling the control handle back until held by trip. A red warning light will go on as soon as the latch is raised. Move the landing gear selector valve control to up position and wheels will retract if hydraulic lines are under pressure. If for any reason the landing gear cannot be retracted with the engine pump, use the hand pump. After the wheels have come up into place, a hard stroke of the hand pump will insure that the axles are up against the rubber bumpers in the nacelles. Check the position of the wheels occasionally and if they are not up against the bumpers, pump them up with the hand pump.

(3) To extend landing gear, push the landing gear selector valve control to down position. This automatically places the safety latch in spring-loaded position. When the landing gear is down and latched, a green light will go on. Move the latch-control handle down against the floor, latch in positive-locked position. The clip on the floor should hold the latch handle down. The valve control should be left in down position at all times when the landing gear is extended.

(4) The landing gear indicating system consists of signal lights and a horn. The gear is down and locked only when the green light is on. When the red light is on, the gear is not down and locked. The horn will sound when either one of the throttles is closed and one or both wheels are not fully down and locked.

(5) A safety latch is mounted on the forward face of the front spar in each nacelle and is used to lock the landing gear in the extended position. The latch is controlled by a lever located forward of the landing gear selector valve. The valve handle and the latch lever are mechanically interconnected, making it necessary to raise the latch manually to unlocked position before operating the selector valve handle to retract the gear. Springs on the latch release and locking cables hold the latch lever in spring-loaded position. A clip on the fuselage structure should be used to bold the latch in positivelock position. This clip should be used only when the landing gear is fully extended. The latch lever is held in unlocked position by the interconnecting mechanism. A trip lever for releasing the latch from unlocked position is located immediately aft of the latch lever. An aft. thrust on this lever releases the latch.

(6) A check valve is located in the pressure line to the landing gear selector valve. This valve, by only allowing fluid to pass into the selector valve from the pressure line, permits the usage of other parts of the system without a drop in pressure in the landing-gear system.

(7) A relief valve is placed in the landing-gear system to compensate for temperature expansion of the fluid. It is connected to the pressure line between the check valve and the landing gear selector valve. When the landing gear is either up or down and temperature rises, causing an increase in volume of the fluid to such an extent as to cause approximately 3,600 pounds per square inch to build up in the pressure line, the relief valve will operate allowing excess fluid to return to the reservoir. This valve is located below and to the left of the landing gear selector valve.

d. Wing flap system.-The wing flaps are operated by a single actuating cylinder. A position indicator attached to the instrument panel in front of the pilot is connected by a flexible control to the hydraulic cylinder; any movement of the cylinder being shown on the indicator. Approximately 7 seconds are needed to fully lower the flaps and 10 seconds to raise them from full down position with the engine-driven pump supplying the hydraulic pressure. Hand pump operation is much slower.

(1) The wing flap selector valve differs from the landing gear valve only in that it has three positions: up, down, and neutral. The neutral position on the valve enables the pilot to lock the flaps in any desired position between the full up and full down. This is advantageous under certain take-off and landing conditions.

(2) The down line of the wing flap system incorporates a relief valve which prevents the lowering of the flaps to the full down position if the airspeed is in excess of 112 m.p h. However, if the airspeed is increased above 112 m.p.h. when the flaps are in a full down position, they will not rise because of the position of the mechanical linkage. If the flaps fail to lower fully, the relief valve is bypassing fluid because the airspeed is too high.

(3) A temperature expansion valve is connected between the flap up line and the return line to the reservoir. In case the temperature rises and causes the pressure in the line to increase above approximately 1,200 pounds per square inch, the relief valve opens and allows the excess to return to the reservoir.

(4) A check valve is incorporated in the pressure line to the wing flap selector valve. Another check valve is connected into the wing flap system to allow fluid to flow from the down to the up side of the flap strut piston through the flap down relief valve.

(5) An orifice check valve is installed in the flap up pressure line to restrict the raising of the flaps.

e. Bomb door system.-There are two door operating cylinders connected by lever arms to the doors and when the pistons are extended by the pressure from the selector valve, the doors are closed. The bomb door selector valve can be operated by either the bomb door operating lever or the pilot`s emergency release handle.

(1) The bomb door operating lever is located on the left side of the bomber's compartment. The pilot's emergency release handle is located at the base of the control pedestal within reach of the pilot and copilot. The pilot's emergency release handle is interconnected with the bomb door operating lever mechanism and the bomber's bomb control mechanism. In operating the pilot's emergency release handle the first part of the pull opens the bomb doors. When the doors open fully, a second pull on the handle, drops all bombs in salvo. Instructions are contained on a plate located below the emergency release handle. The bomber's bomb control is located adjacent to the bomb door operating lever. This control has three positions: lock, unlock, and salvo. The locking system between the bomb door operating lever and the bomber's bomb control is such that with the bomb door closed the bornber's bomb control cannot be moved from its locked position. Furthermore, when the bomber's bomb control is in either the unlock or salvo position the bomb doors cannot be closed. A bungee assembly helps to spring the doors to open position as soon as one of the three selector-valve controls is moved to open.

(2) There are no check nor relief valves in the bomb door operating system. However, a small bleed hole in the hydraulic hand pump bypass check valve located between the pressure tank and the selector valve takes care of temperature expansion of the fluid. A special orifice fitting on the pressure port of the selector valve retards the operation of the bomb door system.

f. Maiptenance.-(1) To bleed the system of air

(a) Support the airplane on wing tripods.

(b) Operate the landing gear, wing flaps, and bomb door mechanisms through two or three complete cycles by means of the hand PUMP.

(2): To bleed the brakes:

(a) Set parking brakes.

(b) With at least 100 pounds per square inch pressure in the system, open bleeder port on each brake cylinder and allow about a  pint of fluid to escape from each.

(3) To relieve the system of pressure

(a) Insert safety pin in landing gear.

(b) Operate wing flaps up and down until the pressure gage, reads zero.

(c) Place the landing gear selector-valve control in the up, then in the down position.

(4) To adjust the power brake control valve

(a) Adjust low, pressure screw:

1. Attach pressure gage to bleeder port on brake.

2. With at least 500 pounds pressure indicated on hydraulic pressure gage on instrument panel and no pressure on brake pedals, adjust low-pressure screw to give 10 to 15 pounds per square inch pressure at brakes.

3. Depress pedals halfway and release fully.

4. Back off screw until pressure drops to 5 pounds per square inch.

5. Back off 1/4 turn more and lock. (Whenever low-pressure screw is adjusted, high-pressure screw must be readjusted).

(b) Adjust high-pressure screw:

1. Adjust screw to give approximately, 1/16-inch clearance between the high-pressure adjustment and the spring yoke.

2. With a minimum of 700 pounds pressure in system, push pedals down fully and note gage reading at brakes.

3. Release pedals and readjust screw (turn clockwise to increase pressure) and reapply brakes, until pressure of 600 ( + 50-0) pounds per square inch pressure is obtained. Do not turn high-pressure screw under load.

(5) To adjust landing-gear temperature-expansion relief valve, vary the number of washers between the heavy steel washer and the spring in valve body. Adjust to 3,600 pounds per square inch.

(6) To adjust wing-flap down-line relief valve, back off locknut and turn adjusting screw in end of valve clockwise to increase pressure. Adjust to 700 ( + 20-10) pounds per square inch.

(7) To adjust wing-flap temperature-expansion and system-relief valves, remove cap and screw guide clockwise to increase pressure. Adjust flap relief valve to 1,200 (+50-0) pounds per square inch and system relief valve to 1,000 ( + 50-0) pounds per square inch.

(8) To adjust pressure regulator valve, disassemble and grind off spring to reduce pressure, or add washers under end of spring to increase pressure. If spring is ground, remove sharp edges and slivers. Adjust to 800 (+ 50-0) pounds per square inch.

(9) To clean the filter screen in the bottom of oil supply reservoir

(a) Drain reservoir by removing plug in T connected to bottom cover.

(b) Remove eight bolts holding bottom cover to body.

(c) Remove lines attached to bottom cover.

(d) Remove filter screen and clean thoroughly with a stiff wire brush.

(10) When replacing cover on reservoir, inspect vellumoid gaskets for condition and replace if necessary.