TM 1-411, Airplane Hydraulic Systems and Miscellaneous Equipment: 4 - Inspection of Hydraulic Systems

SECTION IV: INSPECTION OF HYDRAULIC SYSTEMS

33. Operation check.-a. Operate each hydraulically operated mechanism through two or more complete cycles. In the case of the landing gear, make certain that the airplane is properly supported. Perform complete operation checks with the hand pump and also with the power pump. In case it is not convenient or safe to operate the engine driven pump, use an auxiliary source of power such as a hydraulic test stand. Use the pressure line and return line tap-ins, where such are provided; otherwise, break connections in the pressure side and in the return side, of the hydraulic system and connect corresponding lines from the test stand. Observe the fluid pressure gage and compare the operating pressures attained with the operating pressures specified. Also compare the time required to operate the mechanism with the time specified as normal. Note whether the time required to operate the mechanism decreases with successive operations, indicating that air is being worked out of the system. Note whether a spongy reaction is experienced in operating the hand pump, indicating air in the system. Note if the hand pump operates with excessive ease, indicating an internal leak in the pump. Note whether the pressure attained with the power pump fluctuates, indicating an inadequate fluid supply to the pump, or an excessive amount of air in the system.  Note whether the mechanism operates with a jerky motion, indicating binding or fouling. Check the position indicators against the positions of the mechanism through a complete cycle of operation. Observe that the signal light and warning signal go on and off at precisely the proper time.

b. Operate the mechanism so as to leave a load on the actuating cylinders. With the pump inactive, note whether the mechanism creeps, that is, whether the landing gear extends, the flaps lower, the bomb doors open, etc., indicating an internal leak in the cylinder or an internal leak in one of the valves in that section of the system.

c. Build up pressure in the pressure tank by means of either the power pump or the hand pump. With the pump inoperative, operate the mechanism so as to use up the stored pressure. Observe the pressure decrease as indicated on the pressure gage, until it suddenly drops to zero. The pressure indicated on the gage, just before the sudden drop to zero, is the air pressure in the pressure tank. In case this pressure is less than that recommended, it indicates an air leak in the pressure tank. An external leak can be detected by applying soapy water about the air valve and seam of the tank. An internal leak may be detected by noting fluid emerge from the air valve when the valve core is depressed, also by the complete and sudden depletion of pressure in the system upon any attempt to operate the mechanism with the pump inactive. In the case of systems employing power control valves, an air leak in the pressure tank will cause the power control valve to kick out prematurely and erratically as sudden pressure loads are imposed upon it.

d. Apply pressure to the brake pedals and if a spongy reaction is obtained, bleed the brake actuating cylinder. Use a rubber tube, one end of which is slipped over the bleeder fitting and the other end immersed in hydraulic fluid contained in a glass receptacle. Open the bleeder valve by turning the valve one turn to the left, and operate the brake pedal. Constantly replenish the fluid supply in the master cylinder reservoir. Pass approximately 1 pint of fluid through each brake system, or until air bubbles cease to come from the bleeder tube. Make certain that the rubber tube is tight about the bleeder valve, otherwise air will enter at this point and follow the fluid into the receptacle and give the impression that air is coming from the actuating cylinder. After bleeding, close the bleeder valve and recap it, then drain the reservoir to the proper level. In case the brake reaction is firm but the pedal creeps under applied pressure, an internal leak in the master cylinder is indicated. Either the transfer valve is partially open and requires tightening, or the compensating valve is being held open by grit. In the latter case, the master cylinder must be disassembled and cleaned. In case the parking brakes jump to the off position from the parked condition, it is a further indication of an internal leak in the master cylinder or an external leak in the system.

34. Adjusting valves.-a. In adjusting relief valves, adjust but one valve at a time. Back out the adjusting screw of the valve to be adjusted, and screw in the adjusting screws of all other relief valves in the series. Start with the valve having the highest kick-out pressure and progress in descending order to the valve having the lowest kick-out pressure. In case a power-control valve is in the series, it must also be locked or held closed, and it should be adjusted last. Operate the mechanism and, with pressure applied against it, note the pressure at which the relief valve kicks out. Screw in the adjusting screw of the relief valve until the pressure indicated on the pressure gage is that required. Take several readings and consider the average as the setting of the valve. Back out the adjusting screw of the next valve, to be adjusted and again apply pressure to the system and repeat the adjusting procedure. Screw in the adjusting screw until the desired kick-out pressure is obtained.

b. It must be realized that a check valve is usually placed between the hand pump outlet line and the system-relief and power-control -valves; therefore, these valves cannot be chocked nor adjusted with pressure furnished by the hand pump. Either the engine-driven pump must be used or else an external source of power must be resorted to.

35. Inspecting lines.-a. Shake fluid lines to detect loose anchorage. Investigate polished areas and worn spots to detemine whether the lines have shifted or whether they have been fouled by moving parts of the airplane. Replace all lines that are deformed, dented, or kinked. Note whether flexible lines are held clear during operation of the mechanism. Squeeze flexible lines with the forefinger and thumb at points of suspicion to detect soft spots and deterioration. Keep flexible lines free of oil and grease at all times. Remove oil and grease with a cloth dampened with denatured alcohol.

b. Trace all fluid leaks to their source. Clean the equipment in the vicinity of the leak with denatured alcohol. If the tube leaks, replace it. If the leak is at a connection, tighten the connection. If this does not stop the leak, break the connection and inspect the tube flare for cracks, especially at the base of thflare. Inspect the fittings for defects. Replace defective fittings. Inspect the cone seats for burs, scratches, and imbedded grit. In making a connection it is permissible to use thread compound on the male threads only. The connection should be adjusted snug, but should not be forced excessively. A turning moment of not to exceed 100 inch-pounds applied to the nut of a 3/8-inch tube fitting should seal the connection. A turning moment of not to exceed 180 inch-pounds may be applied to 1/2-inch tube fittings.

36. Fluid supply.-a. Check fluid level and replenish with fluid of the specified type and grade. Make certain that the reservoir is serviced to the proper level. Make certain that the reservoir vent is not plugged. Attach a rubber tube to the end of the vent line and, with the filler cap loosened, suck on the tube. Open the sediment trap and remove the accumulated sediment. Clean the filter screen by rinsing it in alcohol and brushing it with a stiff brush.

b. The following fluids are used for servicing hydraulic equipment:

(1) Hydraulic fluid is used for hydraulic brakes, retracting mechanisms, and shock absorbers, except, those listed in (2) and (3) below. This fluid is supplied in three grades: light, medium, and heavy. The grades of fluid to be used for various temperature ranges are given in (a) and (b) below:

(a)     Hydraulic brakes and retracting mechanism..

        Grade                    Ground temperature.--Average

        B (medium)          + 10' F. and above.

        C (light)                + 20' F. and below.

NOTE.-In case excessive leakage is encountered when using grade B fluid in hot weather, grade A (heavy) may be substituted.

(b)     Hydraulic shock absorbers.

        Grade                     Ground temperature--Average

        A (heavy)            + 50' F. and above.

        B (medium)          + 10' P. to + 600 F.

        C (light)                + 20' F. and below.

(2). Hydraulic fluid (Edgewater ring) is used in all shock absorbers employing Edgewater ring springs. Colloidal graphite has been added, and the fluid must be thoroughly stirred immediately before use. Only one grade of fluid is used for all temperatures.

(9) Fluid, hydraulic, petroleum base, Specification No. 3580, is used in most hydraulic systems, including automatic pilot systems. No other hydraulic fluid may be substituted for this oil.

c. The temperature ranges given above, show the desirable ranges of operation. Changes of fluid are not required for temporary changes in temperature due to unseasonal fluctuation, occasional altitude flight, or temporary change of station.

d. At airplane overhaul all hydraulic equipment is disassembled, thoroughly cleaned, and refilled with new fluid.

e. (1) Except as noted in (2) below, hydraulic equipment is cleaned with denatured alcohol or butyl alcohol. Solvents other than these alcohols do not mix well with castor oil fluids and may cause deterioration of packings.

(2)  An exception to the foregoing is made in the case of hydraulic systems that are serviced with lubricating oil. These are cleaned with kerosene or naphtha, as some of the internal parts of the automatic pilot  are made of materials upon which alcohols have a deteriorating effect.

f. Every precaution must be observed in handling hydraulic fluid to prevent its contamination. The storage containers must be kept sealed. All handling equipment must be kept clean and should be used for handling hydraulic fluid only. Do not expose hydraulic fluid to the air for periods in excess of those absolutely necessary. The fluid will absorb dust and grit from the air, and in certain localities this becomes a serious menace. Fluid that has been exposed to dust contamination should be filtered before using. Fluid that has been used should also be filtered before re-use. Filtering will remove the sludge from used fluid, as well as all metal flakes and grit.

g. The simplest equipment for filtering hydraulic fluid consists of a ribbed glass or metal funnel of approximately 1-gallon capacity, a sheet of standard commercial filter paper, and a container to support the funnel and contain the filtered fluid. The filter paper is folded into a funnel shape and placed in the funnel. The fluid passes through the filter slowly and should be protected from contamination by dust particles in the air. A cloth dampened in hydraulic fluid placed over the open end of the funnel will serve to protect the fluid during the filtering operation. The filtered fluid should be placed in clean containers properly marked to identify the contents and should be kept sealed until required for use.