Hydraulic Power
Fluid, pumps and reservoirs, accumulators, multiple independent systems and the power transfer unit.
Hydraulic power — summary notes
Free- Hydraulic systems transmit large forces to flight controls, landing gear, brakes and thrust reversers using a near-incompressible, fire-resistant phosphate-ester fluid (Skydrol) at high pressure (3000, or 5000 psi on newer types).
- A system has a RESERVOIR (often pressurised), PUMPS (engine-driven, electric or air-driven), filters, valves and ACTUATORS; the reservoir allows 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 so no single failure disables all controls; a POWER TRANSFER UNIT (PTU) lets one system pressurise another's fluid without mixing them.
- Indication of pressure, quantity and temperature, and strict contamination control (filtration), are essential — 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 — the fluids stay separate.
- Force from pressure
- Force = pressure × piston area
Why do large aircraft have two or three completely independent hydraulic systems rather than one big one?
Hydraulic power operates the flight controls, so a single system would be a single point of failure that could leave the aircraft uncontrollable after a leak or pump failure. Independent systems (e.g. blue/green/yellow), each with its own pumps, reservoir and lines, feed duplicated actuators so that any one system can be lost while the others keep the flight controls (and essential services) working.
Hydraulic power concept map
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Hydraulic power quiz
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