Hydraulic and pneumatic systems

Hydraulics move gear, flaps, brakes and flight controls because a little fluid at high pressure gives a lot of force (force = pressure × area), and the fluid can be piped anywhere.

The parts

• Reservoir: holds fluid, often pressurized to keep the pump fed.
• Pump: hand, engine-driven or electric; constant or variable displacement.
• Relief valve: opens if pressure goes too high, protecting the system.
• Pressure regulator: unloads a constant-displacement pump when pressure is reached.
• Accumulator: a sphere or cylinder with nitrogen on one side of a piston or bladder; stores pressure, dampens surges and gives emergency power. Its nitrogen preload is checked with the system's fluid pressure bled off to zero.
• Check valve: flow one way only.
• Selector valve: sends fluid to one side of an actuator or the other.
• Actuator: a cylinder (single or double acting) or a motor that does the work.
• Filters: keep the fluid clean; many have a bypass that opens if they clog.

Fluids

There are different fluids, and their seals are made for one type only: mixing them or using the wrong seals ruins the system.

• Mineral based (MIL-PRF-5606): dyed red, flammable, used in light aircraft; seals of neoprene or Buna-N.
• Synthetic hydrocarbon (MIL-PRF-87257 and similar): also red, more fire resistant, used in military aircraft.
• Phosphate ester (Skydrol and similar): purple or other light colors, fire resistant, used in airliners; seals of butyl or ethylene-propylene. It irritates eyes and skin and strips paint, so wear protection.

Pneumatics

Some aircraft use compressed air or nitrogen bottles for emergency gear extension and brakes. Lower pressure air from the engines (bleed air) or pumps runs de-ice boots and gyro instruments (vacuum systems).