Commuter aircraft hydraulics overhaul, maintenance of hydraulic system components, involves a number of standard practices together with specialized procedures set forth by manufacturers such as the replacement of relief valves, actuators, and other units, including tubing and hoses. Care should be exercised to prevent system contamination damage to seals, packings, and other parts, and to apply proper torque in connecting fittings. When installing hydraulic fittings, valves, etc. always lubricate the threads with hydraulic fluid.
Overhaul of hydraulic and pneumatic units is usually accomplished in approved repair facilities; however, replacement of seals and packings may be done from time to time by technicians in the field. When a unit is disassembled, all O-ring and Chevron seals should be removed and replaced with new seals. The new seals must be of the same material as the original and must carry the correct manufacturer’s part number. No seal should be installed unless it is positively identified as the correct part and the shelf life has not expired.
Hydraulic systems troubleshooting varies according to the complexity of the system and the components in the system. It is, therefore, important that the technician refer to the troubleshooting information furnished by the manufacturer.
(1) Relief valves repair… lack of pressure in a system can be caused by a sheared pump shaft, defective relief valves needing repair, regional aircraft control valves, the pressure regulators, unloading valves stuck in the “kicked-out” position, lack of fluid in the system, the check valves installed backward, or any condition that permits free flow back to the reservoirs or overboard. If a system operates satisfactorily with a ground test unit but not with the system pump, the hydraulic pumps should be examined.
(2) If a system fails to hold pressure in the pressure section, the likely cause is the pressure regulator, an unloading valve, leaking relief valves nedding repair, or a leaking check valve.
(3) If the pump fails to keep pressure up during operation of the subsystem, the pump may be worn or one of the pressure-control units may be leaking.
(4) High pressure in a system may be caused by a defective or improperly-adjusted pressure regulator, an unloading valve, or by an obstruction in a line or control unit.
(5) Unusual noise in a hydraulic system, such as banging and chattering, may be caused by air or contamination in the system. Such noises can also be caused by a faulty pressure regulator, another pressure-control unit, or a lack of proper accumulator action.