6.3 Hydraulic Systems: Components, Fluids, and Troubleshooting

Hydraulic Systems: Pressure, Cleanliness, and Compatibility

Aircraft hydraulic systems use fluid under pressure to operate brakes, landing gear, flaps, flight controls, steering, and other loads. The key principle is that hydraulic fluid is nearly incompressible, so pressure can transmit force efficiently. That same pressure creates maintenance risk. A technician must control stored energy before servicing, removing components, or searching for leaks.

Major components may include a reservoir, pump, filters, pressure regulator, relief valve, accumulator, selector valve, check valve, restrictor, actuator, hoses, lines, fittings, seals, and gauges. The exact arrangement depends on the aircraft. The Airframe ACS expects you to understand component function, servicing requirements, fluid types, seals, filters, accumulators, and leak troubleshooting.

Fluid identity matters. Hydraulic fluids are not freely interchangeable. The wrong fluid can swell, shrink, harden, or destroy seals. It can also affect fire resistance, lubrication, and system performance. Cross-contamination is a risk-management item because a small amount of the wrong fluid may require major corrective action. Always use the aircraft maintenance manual and container identification.

Use this hydraulic troubleshooting chain:

• Verify the complaint and the affected function.
• Check fluid quantity, correct fluid type, contamination, and aeration.
• Inspect for external leaks, damaged hoses, loose fittings, chafing, and improper routing.
• Check filters and screens for blockage or debris.
• Confirm pump output, pressure regulation, relief valve operation, and accumulator precharge where applicable.
• Evaluate selector valves, check valves, actuators, and internal leakage.
• Purge air, perform operational checks, and inspect again after pressure is applied.

Symptoms narrow the search. Slow actuator movement may indicate low pressure, restricted flow, internal leakage, cold fluid, air, or binding in the driven mechanism. Rapid pump cycling can suggest accumulator issues, leaks, or pressure-control problems. Foaming or spongy operation points toward air in the system. Overheating can come from relief valve bypass, restrictions, pump faults, or continuous operation.

Leaks require careful judgment. Never use a hand to find a high-pressure leak. Fluid under pressure can penetrate skin and cause severe injury. Relieve pressure before tightening or disassembly unless the procedure specifically requires a pressurized check. After repair, clean the area and recheck so fresh leakage is not confused with old residue.

Seal installation is another common failure point. Backup rings, O-rings, packings, and gaskets must be installed in the correct orientation, material, and size. A nicked seal or twisted backup ring may fail quickly. Compatibility between seal material and hydraulic fluid is not optional.

For the exam, choose answers that control pressure, prevent contamination, verify fluid and seal compatibility, and troubleshoot from symptom to source. Replacing a pump before checking fluid, filters, leaks, and pressure regulation is rarely the best maintenance judgment.

Maintenance records and placards can also guide troubleshooting. If the wrong fluid was added, if a component was recently replaced, or if a filter shows metal, the fault path changes. A good mechanic preserves that evidence, samples or inspects as directed, and avoids flushing, adjusting, or replacing parts before the contamination or failure source is understood.