Aircraft Systems: Hydraulics, Electrical, Pneumatics and Fuel
Aircraft systems questions test how hydraulic, electrical, pneumatic, fuel and ice-protection systems generate, distribute and protect power. You need to know what happens after a pump, generator or bleed failure.
Hydraulics and electrical
- Hydraulic systems use engine-driven and electric pumps to pressurise fluid. Accumulators absorb pulses and supply limited emergency actuation; relief valves protect the system; reservoirs allow thermal expansion. Pressure is not flow: a pump can hold pressure with zero flow.
- Electrical systems use AC generators, transformer-rectifier units, batteries and bus ties. The battery is a limited emergency source. APU generators, RATs and emergency inverters vary by type, so learn the aircraft-specific priority.
- Pneumatics supply air-conditioning packs and pressurisation. Cabin altitude is controlled by the outflow valve; cabin rate of climb is limited by valve modulation. Bleed leaks may require isolation.
Fuel and ice protection
Fuel boost pumps prevent vapour lock and feed the engines; crossfeed balances tanks and manages asymmetric fuel. Ice protection may be pneumatic boot, thermal bleed or electro-thermal; holds must be protected before significant accretion.
Worked example: A hydraulic actuator has a piston area of 2.5 in² and system pressure is 3,000 psi. Force = pressure × area = 3,000 × 2.5 = 7,500 lbf. If retraction requires 6,000 lbf, the actuator has 1,500 lbf margin. If pressure falls to 2,500 psi, force falls to 6,250 lbf, still adequate but with reduced margin.
Traps
Do not treat the APU as an unlimited electrical or bleed source. Do not assume a battery can carry normal loads. Confusing pressure with flow leads to wrong answers on pump and actuator questions. In pressurisation, check whether the question asks cabin altitude, differential pressure or cabin rate.