Hydraulic Power
Fluid, pumps and reservoirs, accumulators, multiple independent systems and the power transfer unit.
Hydraulic power, summary notes
- 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
Hydraulic Power
Hydraulic power quiz
Hydraulic Power, quiz
1. A common aircraft hydraulic fluid is:
Fire-resistant phosphate ester (Skydrol)WaterEngine oil2. A hydraulic accumulator:
Stores pressurised fluid and damps pulsesMeters fuelCools the cabin3. A power transfer unit (PTU) transfers:
Power between systems without mixing fluidsFluid directly between systemsFuel to oil4. Typical large-aircraft hydraulic pressure is:
3000 psi (or 5000 psi)50 psi100 000 psi5. Multiple independent systems ensure:
No single failure disables all controlsWeight savingMore drag6. The reservoir is often pressurised to:
Prevent pump cavitationStore fuelCool the fluid only7. Hydraulic filters are essential because:
Particles jam servo valvesThey add pressureThey colour the fluid8. An engine-driven pump (EDP) is:
A primary hydraulic pressure sourceA generatorA fuel pump