Hydraulic Systems: Components and Monitoring
Architecture and components
Typical transport aircraft use two or three independent systems, each with its own reservoir, engine-driven pump, and often an electric pump, a power transfer unit (PTU) or ram air turbine (RAT) for backup. Reservoirs are pressurised (by bleed air or hydraulic pressure) to prevent cavitation at altitude and to feed the pumps; they contain standpipes so that a leak in one user circuit does not drain the whole reservoir. Accumulators store pressurised fluid for peak demands, emergency operation (brakes, thrust reversers) and to damp pressure pulses; they are pre-charged with nitrogen.
Valves and actuators
Know the roles of check valves (one-way flow), relief valves (overpressure protection), sequence valves (gear sequence), priority valves (shedding heavy users when pressure is low) and selector valves (direction control). Actuators may be single- or double-acting; servo-controlled actuators follow commands from the flight control computers.
Monitoring and traps
Candidates are tested on what a low quantity, low pressure or high temperature indication actually means. Traps: believing an accumulator pressure gauge shows system pressure (it shows stored pressure, which may be normal with pumps off); assuming a reservoir quantity loss is always a leak (it can follow accumulator or thermal contraction); and confusing a PTU (transfers power between systems, no fluid transfer) with a cross-feed valve.
Worked example: system pressure drops when gear is selected. Reasoning: gear actuation is a large transient demand; a correctly charged accumulator and priority valve should maintain pressure to essential users. If pressure falls below the priority threshold, non-essential users are shed - that is normal design behaviour, not a failure.