Hydraulic Power

Fluid, pumps and reservoirs, accumulators, multiple independent systems and the power transfer unit.

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Main ideas

Hydraulic systems transmit large forces to operate flight controls, landing gear, brakes, thrust reversers and more, using a near-incompressible fluid — typically fire-resistant phosphate-ester (Skydrol) — at high pressure (3000 psi, or 5000 psi on newer types).
A system has a RESERVOIR (often pressurised), PUMPS (engine-driven, electric-motor-driven, or air-driven), pressure and return lines, filters, valves and ACTUATORS; the reservoir is sized to allow for actuator volume changes and thermal expansion.
ACCUMULATORS store pressurised fluid against a gas charge to meet peak demands, damp pressure pulses and provide a limited reserve (e.g. for braking) if pumps fail.
Aircraft carry MULTIPLE INDEPENDENT systems (e.g. blue/green/yellow) so no single failure disables all controls; a POWER TRANSFER UNIT (PTU) lets one system pressurise another's fluid without mixing the fluids.
Indication covers pressure, quantity and temperature; contamination control (filtration) is vital because particles jam servo valves.
⚠ Exam trap: an ACCUMULATOR stores energy and damps pulses (gas charge behind a piston/diaphragm); a PTU transfers POWER, not fluid, between two systems — it keeps the fluids separate while one system drives the other's pump.

Key formulas

Solved examples

What is the purpose of a hydraulic accumulator in the brake system?
The accumulator stores a volume of fluid under pressure (against a pre-charged gas cushion). It supplies the sudden high flow needed when the brakes are applied firmly, smooths pump pressure pulses, and — crucially — holds a reserve of pressurised fluid so several brake applications are still possible if the hydraulic pumps or system pressure are lost.

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