Learn how an aircraft hydraulic system creates pressure, moves flight controls and landing gear, and uses redundancy and emergency backups.
An aircraft hydraulic system uses pressurised, nearly incompressible fluid to transmit force from engine-driven, electric or hand-operated pumps to actuators. Those actuators move high-load equipment such as landing gear, brakes, flaps and flight controls. Valves direct the flow, while reservoirs, filters, accumulators and relief devices keep the system supplied, clean and protected.
In real-world aviation—and in a systems-detailed flight simulator—the crucial distinction is that a hydraulic pump creates flow. Pressure rises when a load resists that flow. High pressure alone does not indicate how quickly an actuator can move; that also depends on pump flow, actuator size and the load being moved.
How does hydraulic pressure move aircraft parts?
Hydraulics move an aircraft part by applying fluid pressure to a piston or hydraulic motor. Under Pascal’s law, pressure applied to confined fluid is transmitted throughout that fluid, apart from practical losses caused by pipes, valves and restrictions.
Actuator force is broadly calculated as pressure × piston area. A large piston can therefore generate substantial force, but it needs more fluid to travel a given distance. Hydraulics do not create energy for free; they exchange flow and movement for force.
- The pump draws fluid from a reservoir and sends it into the pressure line.
- Pressure regulation keeps the system within its operating range. A variable-displacement pump, pressure compensator or unloading arrangement may control normal pressure, while a relief valve protects against excessive pressure.
- A control valve responds to a cockpit command or flight-control computer and routes pressure to the required side of an actuator.
- The actuator moves, while fluid displaced from its opposite side returns to the reservoir.
- The valve returns to neutral when the commanded position is reached. Mechanical locks may then carry the load on equipment such as landing gear.
Large aircraft commonly use closed-centre systems, where neutral control valves block supply flow and pressure remains available for the next demand. Simpler open-centre arrangements allow fluid to circulate back to the reservoir while no component is operating.
Many transport-aircraft systems operate at a nominal pressure around 3,000 psi, while some designs use approximately 5,000 psi to obtain equivalent force from smaller, lighter components. Light-aircraft installations may use substantially lower pressures.
What are the main aircraft hydraulic components?
A complete aircraft hydraulic system needs a fluid supply, a source of flow, control and protection devices, and something that converts pressure into movement.
| Component | What it does |
|---|---|
| Reservoir | Stores fluid, receives returning fluid and allows air or foam to separate. Transport-aircraft reservoirs are often pressurised to ensure reliable pump supply at altitude. |
| Pump | Moves hydraulic fluid. It may be engine-driven, electrically powered, air-driven or manually operated. |
| Control valves | Direct, meter, isolate or stop fluid flow. These include selector, servo, check, shut-off and priority valves. |
| Actuator or motor | Converts hydraulic energy into linear or rotary movement. |
| Filter | Captures particles that could jam valves, damage seals or score pump surfaces. |
| Accumulator | Stores limited hydraulic energy using compressed gas, usually nitrogen. It can absorb pressure surges and provide short-term or emergency operation. |
| Relief valve | Opens when pressure exceeds a safe limit, protecting pipes and components. |
| Hydraulic fuse | Restricts or stops excessive flow after certain downstream line failures, limiting fluid loss. |
Landing gear provides a clear practical example: pressure operates actuators, but uplocks and downlocks usually secure the gear rather than relying on continuous hydraulic force. Our explanation of landing-gear actuators, locks and alternate extension covers that sequence in more detail.
What powers an aircraft hydraulic pump?
Aircraft hydraulic pumps can be powered by engines, electrical systems, other hydraulic systems or emergency devices.
- Engine-driven pumps normally supply the main systems whenever their associated engine is running.
- Electric pumps may provide primary, supplementary, ground or backup pressure.
- Power transfer units use pressure from one hydraulic system to drive a pump in another without mixing their fluids.
- Ram air turbines can provide emergency hydraulic or electrical power, depending on the aircraft design.
- Hand pumps appear in smaller aircraft and some alternate-extension arrangements.
The exact combination is type-specific. A failed engine-driven pump does not necessarily mean that its hydraulic system is lost; an electric pump or power transfer arrangement may still pressurise it.
Why do aircraft have multiple hydraulic systems?
Hydraulically powered transport aircraft use independent systems so that one leak or pump failure does not remove every flight control, brake and landing-gear function.
Different actuators on the same flight-control surface may receive power from separate systems. Check valves and shut-off valves prevent a fault from draining healthy sections, while priority logic preserves the most critical consumers when capacity is limited.
The Airbus A320 illustrates this with separate Green, Blue and Yellow systems; our breakdown of the A320’s three-system hydraulic architecture shows which pumps and aircraft services each system uses.
A common misconception is that a power transfer unit exchanges fluid between systems. It normally transfers mechanical power only, leaving the hydraulic circuits isolated. This operating principle also explains the A320 PTU and its characteristic barking sound.
What happens if an aircraft hydraulic system fails?
A hydraulic failure removes or degrades only the functions supplied by the affected system, although the consequences depend heavily on the aircraft’s redundancy and the location of the fault.
- Falling fluid quantity usually suggests an external leak and may eventually cause pump cavitation or total pressure loss.
- Low pressure with normal quantity can indicate a failed pump, incorrect pump configuration, lost power source or internal leakage.
- High temperature may result from prolonged heavy demand, low quantity, internal leakage or a pump operating abnormally.
- Jerky or noisy operation can point to aerated fluid, cavitation, contamination or a restricted supply.
- Slow actuator movement often indicates reduced flow rather than simply low indicated pressure.
Backups may include a second hydraulic system, electric pump, accumulator, alternate braking, gravity-assisted gear extension or manual flight-control reversion. An accumulator has limited capacity: it may provide only a specified number of brake applications or one short operating cycle.
A mistake we see constantly in simulator flying is repeatedly resetting a failed pump without identifying why it failed. If there is a leak, overheating or mechanical damage, repeated operation can worsen the condition. The correct response is the aircraft’s type-specific checklist: confirm the indication, isolate the affected source where directed and use the designated alternate system.
Which hydraulic fluid do aircraft use?
Aircraft do not all use the same hydraulic fluid, and incompatible types must never be mixed.
Mineral-based fluids are common in many light, military and older aircraft installations. Fire-resistant phosphate-ester fluids are widely used in transport aircraft, while some designs use synthetic hydrocarbon fluids. Each type requires compatible seals, hoses, paints and servicing equipment.
Fluid colour can help identify a product, but age and contamination can change its appearance. Maintenance personnel use the aircraft documentation and specified fluid standard rather than relying on colour alone. The wrong fluid can cause swollen seals, leaks, component damage and loss of system reliability.
How are hydraulic systems represented in flight simulators?
Flight simulators range from a simple working-or-failed hydraulic switch to detailed models of pump sources, fluid quantity, accumulators, leaks, temperature and system isolation.
When diagnosing a simulated fault, check pressure, quantity and temperature separately. Then establish which pump should be running, whether its engine or electrical supply is available, and which controls belong to the failed system. A cockpit switch commands a pump or valve; it cannot make an unpowered or empty hydraulic circuit operate.