How an aircraft hydraulic system powers flight controls, landing gear and brakes, with clear coverage of components, redundancy, failures and sim checks.
An aircraft hydraulic system uses pressurised, nearly incompressible fluid to transmit power from a pump to actuators. Valves route the fluid so those actuators can move high-load equipment such as flight controls, landing gear, flaps and brakes; reservoirs, filters, accumulators and relief valves keep the circuit supplied, clean and protected.
For our Aviation & Real-World Flying readers, the principle to remember is that a hydraulic pump directly creates flow. Pressure develops when a load resists that flow. Pressure determines potential actuator force, while flow rate, actuator size and load determine how quickly it can move.
What are hydraulics in a plane used for?
Hydraulics in a plane are used wherever compact components must move or hold heavy loads reliably.
- Primary flight controls, including ailerons, elevators, rudders and spoilers on many powered-control aircraft.
- Landing gear, including extension, retraction, doors and wheel-brake systems.
- High-lift devices such as flaps and slats, although some aircraft use electrical or mechanical drives instead.
- Nose-wheel steering and, on some types, main-gear steering.
- Thrust reversers, speed brakes, cargo doors and other type-specific equipment.
Not every aircraft needs a large hydraulic system. A light aeroplane may use cables and pushrods for its flight controls while retaining hydraulics only for the brakes or retractable landing gear. Larger and faster aircraft need powered actuation because aerodynamic and mechanical loads can exceed what a pilot could move directly.
How does hydraulic pressure move aircraft parts?
Hydraulic pressure moves an aircraft part by acting on the effective area of a piston or hydraulic motor.
Under Pascal’s law, pressure applied to confined fluid is transmitted through that fluid, subject to real losses in pipes, filters and valves. Linear actuator force is broadly described by Force = pressure × effective piston area. Retraction force is usually lower than extension force because the piston rod reduces the effective area on one side.
Actuator speed is broadly proportional to flow divided by effective area. A larger piston can produce more force at a given pressure, but it requires more fluid to cover the same distance. The system therefore exchanges speed and movement for force; it does not create energy for free.
- The reservoir supplies fluid to the inlet of the hydraulic pump.
- The pump creates flow using mechanical, electrical, pneumatic or manual power. Pressure rises as valves and loads resist that flow.
- A control valve routes fluid to the required side of an actuator in response to a cockpit control, mechanical linkage or flight-control computer.
- The actuator moves, while displaced fluid normally leaves its opposite chamber through the return side of the circuit.
- The valve returns to neutral when the commanded position is reached. The pump may destroke or unload, and a mechanical lock may carry a static load such as extended landing gear.
Large transport aircraft generally use closed-centre systems. Their neutral valves block supply flow while pressure remains available, and a pressure-compensated pump can reduce its output when demand is low. Simpler open-centre systems let fluid circulate back to the reservoir in neutral and build pressure when a control valve directs flow to a load.
Many transport-aircraft systems have a nominal pressure near 3,000 psi, while some designs use approximately 5,000 psi to obtain the required force from smaller components. Other aircraft operate at lower pressures. The approved aircraft documentation, not a generic figure, defines the normal range.
Are fly-by-wire and aircraft hydraulics the same thing?
No: hydraulics describes how force is produced, while fly-by-wire describes how a control command is transmitted.
In a conventional powered-control aircraft, cables or rods can move a hydraulic servo valve, after which hydraulic pressure supplies most of the muscle. In a fly-by-wire aircraft, sensors and computers send electrical commands to electro-hydraulic servo valves or actuator control electronics. Hydraulic actuators may still move the surface.
Some modern designs also use electrically powered or electro-hydrostatic actuators, so fly-by-wire does not automatically mean that every control surface depends on a central hydraulic circuit. Our explanation of how computers command electro-hydraulic flight controls covers that distinction in more detail.
What components make up an aircraft hydraulic system?
Every aircraft hydraulic system combines a fluid supply, a source of flow, control and protection devices, and components that convert hydraulic energy into movement.
| Component | Purpose |
|---|---|
| Reservoir | Stores returning fluid, accommodates volume changes and allows air or foam to separate. Many transport-aircraft reservoirs are pressurised to maintain a reliable pump supply at altitude. |
| Pump | Moves fluid through the system. Fixed- and variable-displacement pumps control output in different ways. |
| Filter | Captures contamination that could score pump surfaces, damage seals or jam close-tolerance valves. |
| Control valve | Directs, meters, isolates or stops flow. Selector valves, servo valves, check valves, priority valves and shut-off valves perform different jobs. |
| Actuator or motor | Converts hydraulic energy into linear or rotary movement. |
| Accumulator | Stores a limited volume of pressurised fluid against compressed gas, usually nitrogen. It can absorb pulsations and provide brief backup operation. |
| Pressure control and relief devices | Regulate normal operation and protect the circuit from excessive pressure. A relief valve is primarily a safety device, not necessarily the normal pressure regulator. |
| Hydraulic fuse | Limits fluid loss after certain high-flow downstream failures. It may not detect or stop a slow leak. |
An accumulator is not a replacement pump with unlimited endurance. Its usable capacity depends on gas pre-charge, system pressure and the volume demanded by the consumer. It may provide only a limited number of brake applications or one short operating cycle.
Which hydraulic fluid does an aircraft use?
Aircraft hydraulic fluid is the working medium that transmits pressure, lubricates components, carries heat and helps protect internal surfaces.
Aircraft do not all use the same fluid. Mineral-based fluids appear in many light, military and older installations; fire-resistant phosphate-ester fluids are common in transport aircraft; other systems specify synthetic hydrocarbon products. The fluid must be compatible with the system’s seals, hoses, coatings and servicing equipment.
Fluid is not the system’s original energy source: the engine, electrical system or another power source drives the pump. For a closer explanation, see how hydraulic fluid transmits pressure, lubricates parts and removes heat.
Incompatible fluid types must never be mixed. Colour can provide an initial clue, but age and contamination alter appearance; trained maintenance personnel identify and service fluid from the approved specification. Using the wrong type can swell seals, damage components and create extensive leaks.
What powers the hydraulic system in an aircraft?
Hydraulic pumps can receive power from the engines, electrical system, another hydraulic circuit or an emergency device.
- Engine-driven pumps commonly provide the main source of flow while their associated engine is running.
- Electric pumps may provide primary, supplementary, ground-service or backup power.
- Air-driven pumps use pneumatic power on aircraft designed for them.
- Power transfer units use one hydraulic system to drive a pump serving another system without transferring fluid between the circuits.
- Ram air turbines can drive a hydraulic pump directly or generate emergency electrical power, depending on the aircraft.
- Hand pumps are used in some light-aircraft systems and alternate-extension arrangements.
A failed engine-driven pump does not necessarily mean that the whole associated system is lost. An electric pump or power transfer arrangement may still pressurise it. That distinction matters: a backup pump can compensate for a failed pump, but it cannot restore a reservoir that has emptied through a leak.
A power transfer unit transfers mechanical power, not hydraulic fluid. This is the principle behind the A320 PTU and its characteristic barking sound.
Why do aircraft have more than one hydraulic system?
Multiple hydraulic systems prevent a single pump failure or isolated leak from removing every flight-control, braking and landing-gear function.
Transport aircraft commonly have separate reservoirs, pumps and plumbing circuits. Different actuators on the same flight-control surface may be powered by different systems, while check valves and shut-off valves limit fault propagation. Priority valves can preserve critical consumers when available flow or pressure is reduced.
Independence is not absolute. Physical damage, contamination or a shared power loss can affect more than one circuit, which is why system routing and backup methods matter as much as the number of coloured system labels. Some equipment may also be supplied by only one system because an alternative method, such as gravity gear extension or accumulator braking, fulfils the backup role.
The Airbus A320 is a useful example, with independent Green, Blue and Yellow systems supplied by different pump arrangements. Our system-by-system map of the A320’s pumps and consumers shows how that redundancy is distributed.
What happens when plane hydraulics fail?
A hydraulic failure degrades the pumps, circuits or aircraft functions connected to the affected part of the system; it does not automatically remove every hydraulically powered service.
| Indication | Likely interpretation | Useful distinction |
|---|---|---|
| Low pressure with normal quantity | A stopped or failed pump, missing power source, open relief path or internal leakage | Check whether the pump should be operating before treating the indication as a leak. |
| Continuing quantity loss | An external leak or fluid being lost into another part of the aircraft | Starting another pump may simply discharge more fluid. Small position- or temperature-related quantity changes can be normal on some types. |
| High temperature | Heavy continuous demand, low quantity, internal leakage, restriction or abnormal pump operation | A normal pressure reading does not rule out a damaging internal leak. |
| One slow or inoperative consumer | A local valve, actuator, lock or branch problem | Normal operation of other consumers suggests that the entire hydraulic system has not failed. |
| Several consumers move slowly | Reduced pump flow, restricted supply, low quantity or excessive combined demand | Actuator speed depends on available flow, not pressure alone. |
| Jerky or noisy operation | Aerated fluid, cavitation, contamination or an inlet restriction | Repeated cycling can aggravate overheating or fluid loss. |
Available backups are type-specific. They may include another hydraulic circuit, an electric pump, an accumulator, alternate braking, gravity-assisted landing-gear extension, a ram air turbine or limited manual control reversion. Gravity extension releases and locks the gear; it does not necessarily restore normal door sequencing or retraction.
Real flight crews use the aircraft’s checklist rather than attempting improvised resets. If the checklist directs isolation of a leaking or overheating system, repeatedly switching its pump back on can worsen fluid loss or component damage.
How should you diagnose a hydraulic fault in a flight simulator?
Diagnose a simulated hydraulic problem by separating pump power, pressure, fluid quantity and consumer behaviour instead of treating them as one indication.
- Establish the model’s system depth. Some aircraft simulate separate pumps, reservoirs, leaks, temperature and accumulators; simpler models may use only a working-or-failed hydraulic state.
- Check the expected power source. Confirm that the associated engine, electrical bus or emergency source is available and that pump and isolation switches match the normal configuration.
- Read pressure, quantity and temperature separately. Low pressure with stable quantity points in a different direction from a steadily emptying reservoir.
- Compare several consumers. If only the nose-wheel steering or one flap channel fails, look for a local control, lock or system assignment. Multiple failed consumers supplied by the same circuit suggest a system-level problem.
- Distinguish pump failure from fluid loss. Select a backup pump when the aircraft procedure calls for it. Do not expect that pump to work after the circuit has lost its usable fluid.
- Use the type-specific checklist. Hydraulic architecture varies too much for a universal switch-reset sequence, and realistic add-ons may retain damage after the original cause is removed.
A mistake we see often in simulator flying is assuming that normal indicated pressure guarantees normal operation. A pressure-compensated system can show pressure while delivering inadequate flow, and an accumulator can briefly preserve pressure after the main pump has stopped. Hardware conflicts, control locks and lost electrical power can also imitate a hydraulic fault, so check those only after mapping which aircraft functions share the affected circuit.