Aircraft Hydraulic Systems: The A&P Exam Guide

What the ACS Tests in Hydraulic and Pneumatic Systems

Hydraulic and pneumatic systems are Subject F of the Airframe section (AM.II) of the Aviation Mechanic ACS, and the knowledge items cover hydraulic system components and fluids. Expect questions on reservoirs, pumps, valves, accumulators, actuators and the fluid families. The Airframe written test is 100 questions in two hours, with 70 percent to pass under 14 CFR 65.17.

Pascal's Law and Force Multiplication

Hydraulics works because a liquid is almost incompressible. Pascal's law states that pressure applied to a confined liquid is transmitted undiminished in every direction. That is why a reading of 3,000 psi at the pump is also 3,000 psi at the actuator, no matter how the lines bend. The force an actuator produces is pressure times the area it acts on, so fitting a larger piston on the output side multiplies force while the smaller input piston travels a longer distance.

Hydraulic Fluids: Three Families You Must Recognize

The fluid questions are about base stock, fire resistance and color, so learn the family before the brand name.

MIL-PRF-5606 is the petroleum-base fluid found on most light aircraft. It flows well at low temperature but it is flammable. MIL-PRF-83282 is a synthetic hydrocarbon developed as a fire-resistant replacement, and it is compatible with MIL-PRF-5606 systems. Phosphate-ester fluids such as Skydrol 500B-4 are the fire-resistant choice on civil transport category aircraft: they do not sustain a flame, but they are not fireproof, and they readily absorb moisture from the atmosphere, so contamination control matters more than it does with mineral fluid.

Seals and Fluid Incompatibility

Never top off a system with the wrong fluid. Petroleum-base and phosphate-ester fluids are not interchangeable, and the seals are matched to the fluid the manufacturer specified. If the two are mixed, or if mineral fluid contacts a phosphate-ester system, drain and flush the system and replace the seals. Skydrol also attacks many paints, plastics and adhesives, and it irritates skin on contact, so gloves and eye protection are required whenever you service a phosphate-ester system. The aviation maintenance glossary is a quick place to confirm the abbreviations you will see on a fluid specification.

The Components the Questions Target

Reservoirs and filters

The reservoir stores the fluid, lets it expand and contract with temperature, gives air and moisture a chance to separate, and keeps a positive head at the pump inlet. Filters are installed in the pressure line, the return line and the pump case drain line, and their rating is given in microns.

Pumps

Every system has at least one power-driven pump, which may be engine driven, electric motor driven or air driven, with a hand pump as a backup on some aircraft. Electric motor pumps are generally fitted for ground operations or emergencies. Pumps are classified as constant displacement or variable displacement: a constant-displacement pump needs a pressure regulator, while a variable-displacement pump uses an internal compensator to change output.

Pressure regulators and relief valves

These two are not the same thing. A pressure regulator manages pump output to hold system pressure in range and unloads the pump by bypassing fluid to return when that pressure is reached. A relief valve is a safety device that opens only if pressure exceeds the set maximum, plus a thermal relief valve for fluid expansion. A relief valve cannot act as a regulator on a large engine-driven system because the energy spent holding it off its seat turns into heat that cooks the fluid and the seals.

Selector valves, actuators and fuses

Selector valves direct fluid to the actuator, which converts hydraulic pressure into linear or rotary motion. A hydraulic fuse is the line-level safety device: a rate fuse closes when flow suddenly exceeds a threshold, and a volume fuse closes after a set quantity of fluid has passed. Both isolate a ruptured branch so the rest of the system keeps working.

Accumulators and Nitrogen Precharge

An accumulator stores hydraulic energy, smooths pressure fluctuations and provides a short emergency supply if a pump fails. It contains a gas chamber and a fluid chamber separated by a piston or a bladder. The gas charge is dry nitrogen, never oxygen or shop air, and the precharge is set lower than system operating pressure: a typical example is a 1,300 psi preload in a 3,000 psi system. When fluid pressure exceeds the precharge, it compresses the gas and stores energy that expands back out on demand. Release both the gas and the fluid pressure before you take an accumulator apart.

Troubleshooting: Leaks, Air, and a Pump That Will Not Unload

• External leaks show fluid at fittings, seals or a chafed line, and they show up as a falling reservoir level.
• Internal leaks bypass fluid past a piston or valve spool inside a component. Power drops and the system runs hot because pressure energy becomes heat.
• Air in the system makes actuation spongy or erratic and requires bleeding the affected branch.
• A pump that will not unload keeps the system at pressure with no demand, which points at the pressure regulator or compensator and overheats the fluid.

The same pressure-times-area logic that drives force multiplication shows up here. Look at our aircraft electrical systems FAQ for the other large AM.II systems subject, and our weight and balance guide for the General test's calculations.

The Safety Rules That Score Marks

• Release the accumulator gas and the system pressure before breaking any hydraulic connection.
• Use only dry nitrogen to charge an accumulator. Never oxygen or shop air.
• Never mix fluid families, and match every seal to the fluid the manufacturer specifies.
• Wear gloves and eye protection for phosphate-ester fluids such as Skydrol.
• Cap and plug open lines: particulate contamination is a leading cause of valve and pump failure.

Work the orals with the same structure as the written test. Our A&P oral and practical exam guide walks through how a DME turns these component questions into a spoken check, and how to answer when you are asked why a system behaves the way it does.

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