Hydraulic and Pneumatic Systems

Hydraulic and Pneumatic Systems in Aircraft

Aircraft hydraulic systems transmit force by pressurizing a fluid. A relatively small pump can generate enough pressure to move landing gear, operate flight control surfaces, actuate brakes, and extend flaps—tasks that would be impractical with mechanical linkages alone. Pneumatic systems use compressed gas (air or nitrogen) for similar purposes on some aircraft, particularly for emergency backup functions and brakes on older designs. Understanding both systems is essential for any airframe mechanic.

How a Basic Hydraulic System Works

Hydraulic systems follow Pascal's Law: pressure applied to a confined fluid is transmitted equally in all directions. The core components you will encounter on nearly every aircraft are:

• Reservoir – stores the fluid supply and allows for thermal expansion. Most reservoirs are pressurized or vented depending on design.
• Pump – moves fluid from the reservoir into the system. Engine-driven pumps (EDPs) are the primary source; electric motor-driven pumps (EMDPs) serve as backup.
• Relief valve – protects the system from over-pressure by returning excess fluid to the reservoir when pressure exceeds the set limit.
• Check valve – allows fluid to flow in one direction only, preventing reverse flow.
• Selector valve – directs pressurized fluid to the correct port of an actuator based on cockpit input (e.g., gear up or gear down).
• Actuator – converts hydraulic pressure into linear or rotary mechanical movement.
• Filter – removes contaminants from the fluid to protect close-tolerance components such as pumps and servo valves.

Fluid flows from the reservoir, through the pump, through the filter, to the selector valve, into the actuator, and returns through the return line back to the reservoir. The flow path changes direction when the selector valve shifts position.

Reading a Hydraulic Schematic

Schematics use standardized symbols defined in industry references. Lines represent fluid passages; color coding or line weight distinguishes pressure, return, and case-drain lines. Always trace the full circuit: supply → selector → actuator extend port → actuator → actuator retract port → selector → return. For a landing gear system, gear-down flow pressurizes the extend side of the actuator while gear-up flow pressurizes the retract side. Tracing both cycles on the same schematic in different colors makes the flow logic clear and catches misread valve positions before any hands-on work begins.

Hydraulic Fluid Specifications

Using the wrong fluid can destroy seals and contaminate the entire system. The most common types are petroleum-based (MIL-PRF-5606, dyed red) and phosphate ester–based (Skydrol® and similar, dyed purple). These fluids are not compatible with each other and require different seal materials and hoses. Always verify the fluid type from the Aircraft Maintenance Manual (AMM) before adding fluid. Phosphate ester fluids are toxic and require nitrile or butyl gloves and safety glasses at minimum; consult the Safety Data Sheet (SDS) for full PPE requirements.

Leak Detection and Fault Isolation

External hydraulic leaks are identified visually, by feel (never hold your hand directly in front of a pressurized port—hydraulic fluid injection injuries are life-threatening), and by UV dye. UV dye is added to the fluid, the system is pressurized to operating pressure, and an ultraviolet inspection light reveals the leak source as a bright glow. Before any repair, the system must be depressurized using the approved procedure. Record the location, approximate rate, and suspected cause before opening any fitting. Common causes include deteriorated O-rings, cracked fittings, chafed lines, and loose B-nut connections.

Brake System Bleeding

Air trapped in a brake hydraulic line causes a spongy pedal and reduced braking effectiveness—a direct safety hazard. Pressure bleeding pushes clean fluid from a pressurized bleeder tank through the system to force air out through the bleed port at the caliper. The procedure follows the AMM exactly: correct fluid type, correct sequence (typically from the point farthest from the master cylinder), and correct torque on bleeder screws when closed. After bleeding, verify firm pedal feel, inspect all fittings and the caliper for leaks, check the fluid level in the reservoir, and document the fluid type, quantity added, and results on the aircraft service record. Brake fluid and hydraulic fluid specifications vary by aircraft—do not assume.

Common Failure Modes

| Component | Common Failure | Symptom |

|---|---|---|

| Pump | Wear, cavitation | Low system pressure, slow actuator |

| Relief valve | Stuck open | System will not build pressure |

| Check valve | Stuck open | Actuator creeps or drifts |

| Seal/O-ring | Deterioration, wrong fluid | External or internal leak |

| Filter | Clogged | Differential pressure indicator (bypass indicator) pops |

| Actuator | Internal seal failure | Actuator creeps; no external leak |

Safety Rules for This Lab

1. PPE is mandatory: nitrile gloves and safety glasses throughout; face shield when connecting or disconnecting pressurized lines.

2. Depressurize before any repair – confirm zero pressure on the system gauge before loosening any fitting.

3. Place a drip pan under all work areas; hydraulic fluid on a floor is a slip and fire hazard.

4. Follow the AMM for every procedural step—no improvising torque values or sequences.

5. Dispose of contaminated fluid and rags per the facility's hazardous-waste procedure.