6.hydraulic-and-pneumatic. Hydraulic and Pneumatic Systems
Hydraulic and pneumatic systems transmit force from one location in the aircraft to another using a confined fluid (liquid for hydraulic systems, compressed gas for pneumatic systems). Both rely on Pascal's law, which states that pressure applied to a confined fluid is transmitted undiminished in all directions. This allows a small input force on a small piston to produce a large output force on a larger piston, providing the mechanical advantage needed to operate heavy aircraft components.
Hydraulic Systems
Most general aviation airplanes use hydraulic systems in a limited role — typically for wheel brakes — while larger and more complex aircraft use them to operate landing gear, flaps, retractable steps, spoilers, constant-speed propellers, and primary flight controls. The basic hydraulic system contains the following essential components:
- Reservoir — stores the hydraulic fluid and provides space for thermal expansion and air separation.
- Pump — pressurizes the system. May be hand-operated, engine-driven, or electrically driven.
- Filter — removes contaminants that could damage close-tolerance components.
- Selector (control) valve — directs pressurized fluid to the appropriate actuator.
- Actuator (or actuating cylinder) — converts fluid pressure into linear or rotary mechanical motion.
- Relief valve — limits maximum system pressure to protect components.
- Lines and fittings — rigid tubing or flexible hoses that route fluid between components.
When the pilot selects an action — for example, lowering the flaps — the selector valve routes pressurized fluid to one side of the actuator while allowing fluid on the opposite side to return to the reservoir. The piston extends or retracts to move the surface, and the relief valve protects the system if pressure exceeds design limits.
The fluid itself is critical to system operation. Aircraft hydraulic fluid must have a high boiling point, low freezing point, low viscosity change with temperature, and good lubricating qualities. Three principal types are used in aviation:
- Vegetable-base fluid (MIL-H-7644, blue) — older systems; mostly obsolete.
- Mineral-base fluid (MIL-H-5606, red) — common in light aircraft brake systems and many older transport aircraft. Petroleum-derived; flammable.
- Synthetic, phosphate-ester fluid (Skydrol/Hyjet, purple or amber) — fire-resistant; used in most large transport-category aircraft.
These fluids are not interchangeable. Mixing fluids or using the wrong fluid will destroy seals and contaminate the system. Always service hydraulic components with the fluid specified in the Pilot's Operating Handbook (POH) or maintenance manual.
Brake Systems
The hydraulic brake system on a typical light airplane is independent of any other aircraft hydraulic system. Each main wheel has its own master cylinder connected by a brake line to a slave cylinder at the brake assembly. Depressing the top of a rudder pedal pushes the master cylinder piston, sending pressurized fluid to the brake caliper, which clamps the disc and slows the wheel. Because each side is independent, differential braking (left or right pedal alone) provides directional control during taxi, takeoff, and landing rollout. Common brake malfunctions include fluid leaks (mushy or spongy pedals), worn brake pads, and overheating from excessive use during long taxi or aborted takeoffs.
Pneumatic Systems
Pneumatic systems perform similar functions to hydraulics but use compressed air instead of liquid. Air is plentiful, free, non-flammable, and leaves no residue, but it is compressible, which means pneumatic actuators are less precise and respond more slowly under load. Pneumatic systems in aircraft are used to operate:
- Brakes (some aircraft)
- Landing gear (a few designs and as emergency backup)
- Doors
- Pressurization systems
- Engine starting (in turbine aircraft, using bleed air)
- Gyroscopic flight instruments driven by vacuum or pressure
- Deicing boots on the wing and tail leading edges
The basic pneumatic system includes an air compressor (often an engine-driven vane pump), a relief valve, a control valve, an air filter, a check valve, a storage bottle, and the actuating units. The check valve prevents back-flow toward the compressor, and the storage bottle holds pressurized air for emergency use — for example, blowing the landing gear down if the primary system fails.
In light single-engine airplanes, the most common pneumatic application is the engine-driven vacuum (or pressure) pump that drives the attitude indicator and heading indicator. Suction is typically maintained between 4.5 and 5.5 inches of mercury (in. Hg), and the suction gauge should be checked during runup and in cruise. A failing pump produces low or zero suction and unreliable gyro indications — a known cause of spatial disorientation accidents in IMC.
Pilot Considerations
During preflight, check hydraulic fluid level (where accessible), inspect brake lines and assemblies for leaks or stains, verify proper brake pedal feel, and confirm the suction gauge reads in the green during runup. In flight, monitor system pressure or suction gauges as required. If a hydraulic or pneumatic failure occurs, refer to the POH emergency procedures — most aircraft provide a manual or alternate means of extending the landing gear and flaps.