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 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.
- Pascal (force from pressure)
- Force = pressure × piston area
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.
Hydraulic power concept map
Hydraulic Power
Hydraulic power quiz
Hydraulic Power, quiz
1. In the hydraulic system shown, the component marked V is connected across the pump outlet. Its function is to:
Limit system pressure by returning excess fluid to the reservoirStore fluid under pressureFilter the fluid2. A common aircraft hydraulic fluid is:
Fire-resistant phosphate ester (Skydrol)WaterEngine oil3. A hydraulic accumulator:
Stores pressurised fluid and damps pressure pulsesMeters fuelCools the cabin4. A power transfer unit (PTU) transfers:
Power between systems without mixing the fluidsFluid directly between systemsFuel to oil5. Typical large-aircraft hydraulic pressure is:
3000 psi (or 5000 psi on newer types)50 psi100 000 psi6. Multiple independent hydraulic systems are used to:
Ensure no single failure disables all controlsSave weightIncrease drag7. The reservoir is often pressurised to:
Prevent pump cavitation and vapourStore fuelCool the fluid only8. Hydraulic filters are essential because:
Particles can jam servo valvesThey add pressureThey colour the fluid