A hydraulic system uses a fluid under pressure; the fluid is the medium via which energy is transmitted. A pump pressurizes the fluid, and high-pressure fluid enters an actuator to push a sliding part connected to the landing gear, brakes, high-lift devices, spoilers, or nose-wheel steering. The actuator transforms fluid power into mechanical energy, enabling flight control surfaces like the rudder, elevators, and ailerons to move. Because aircraft have hydraulically powered components, redundancy is designed through multiple pumps, reservoirs, and fluid passages so that an auxiliary power source like the Ram Air Turbine can serve as a backup should any element fail.
What is a hydraulic system in an aircraft?
An aircraft hydraulic system is a mechanical network that uses an incompressible fluid - typically oil - under pressure to transmit power from a pump to user components. It is an open-loop variety that draws fluid from a reservoir, then furnishes that fluid to a manifold for redistribution to flight-control surfaces, braking systems, landing-gear extension/retraction, and other high-force machinery. Because the fluid is virtually incompressible, forces are transmitted instantly and efficiently. Pascal's Law ensures that pressure applied at any point is conveyed equally in all directions, so high-pressure fluid can push a sliding part inside an actuator to create precise, high-force movement. The sliding part is ultimately connected to the aircraft piece to be moved, allowing power to be delivered to specific locations where mechanical tasks are impractical. Aircraft hydraulic systems are responsible for maintaining any aircraft components or devices that use fluid or gas pressure to operate, and these systems are used in braking systems, landing gear, wing flaps, flight-control surfaces, engine pumps, air turbines, and many others.
What do hydraulic systems do on a plane?
Hydraulic systems provide power for operating flight control surfaces, flaps, nose-wheel steering, spoilers, and brakes. They extend the landing gear smoothly and absorb the forces exerted upon touchdown. By converting mechanical energy into hydraulic power, the system uses high-pressure fluid to move actuating cylinders that raise or lower the wing flaps and drive jackscrews for heavy, vertically extending landing gear. In larger aircraft, engine-driven pumps take power from the aircraft engine to generate the higher pressure needed for primary flight controls, System A powering the inboard flight spoilers, the landing gear, and the primary flight controls themselves, while a single electrical pump serves the landing gear and the brakes on smaller or backup configurations.
The landing-gear circuit illustrates the function clearly: hydraulic fluid is transmitted to the landing gear jacks, the shock absorbers, and the brake assemblies so that the aircraft can decelerate safely after touchdown. Because these tasks require force or precision, hydraulics are preferred over purely mechanical or electrical alternatives. To guarantee safety during moments like landing, a standby hydraulic system is automatically or manually activated. This redundancy guarantees that even if the main system fails, power remains available to operate the brakes and lower the gear. Thus, hydraulics are indispensable for reliable, high-force motion control throughout every phase of flight.
What do hydraulics control on a plane?
Hydraulic systems control flight control surfaces, landing gear, flaps, spoilers, speed brakes, brakes, and nose-wheel steering. Flight control surfaces - ailerons, elevators, and rudders - are hydraulically actuated so that pilot inputs, conveyed through advanced systems that integrate mechanical and electronic controls, become manageable forces. Without hydraulics, a pilot moving the control yoke does not have enough strength to move the surfaces; hydraulic power enables pilots to manipulate control surfaces precisely. Hydraulic actuator moves ailerons, elevators, and rudders, jackscrews power flaps and stabilizer trim. Selector valves control flow to flaps and spoilers. Flaps, multifunction spoilers, and speed brakes are powered by hydraulics.
The landing gear is operated via the hydraulics system: actuators raise and lower landing gear, and hydraulic motors drive jackscrews which power vertically extending landing gear applications. Brakes are powered by hydraulics; hydraulic actuators move brakes, and selector valves control flow to brakes. Hydraulic failure results in loss of speedbrakes and ground spoilers, as well as braking difficulty. Fly-by-wire replaces the physical connection between pilot controls and flight control surfaces with an electrical interface, yet hydraulic actuators remain the final authority that move the surfaces up, down, left, or right.
How do hydraulics work in planes?
Hydraulics work on the principle of Pascal's Law. An electrically driven pump pressurizes fluid so that a large pump generates power for multiple small light hydraulic actuators. High-pressure fluid enters an actuator that moves a sliding part ultimately connected to a piece of aircraft to be moved. Servo hydraulics include servo valves, while hydraulic motors drive jackscrews that power vertically extending landing gear applications. The same principle governs the control system: fluid sent to the brake system is destined to end up in the hydraulic actuator, and hydraulics transmit braking forces from cockpit to brake disk.
What is hydraulic pressure in aircraft?
Hydraulic pressure in aircraft is the force, measured in pounds per square inch (psi), that the hydraulic fluid exerts to move flight controls, landing gear, flaps and brakes. Most transport-category aircraft work at 3000 psi, while some large or high-performance types, like Concorde, feature a 4000 psi system and a few designs reach 5000 psi. Engine-driven hydraulic pumps or power packs generate this pressure. Smaller aircraft typically pressurize the fluid through an electrically driven pump, because larger airplanes need more pressure than an electric pump alone can provide. A ram air turbine can be extended into the airstream to generate hydraulic pressure in an emergency. Inside the circuit, the value stays within a narrow band: when demand rises, a pressure bypass progressively closes to keep 3,000 psi throughout the system, and a pressure regulator device maintains almost the same source pressure to the cylinders across the full range of flow-rate demands. An accumulator stores hydraulic energy as compressed gas; if pressure drops below the accumulator charge, the accumulator pushes against the fluid, effectively boosting the hydraulic system pressure temporarily.
What are the parts of an airplane hydraulic system?
The parts of a hydraulic system include the reservoir, pump, motor, valves, filter, and often the accumulator. The reservoir stores surplus hydraulic fluid, maintains pressure balance, and provides a ready source of fluid for the pump. From this reservoir an open-loop variety draws fluid and later returns it, keeping the circuit continually supplied. The hydraulic pump, whether an engine-driven pump that generates higher pressure or an electric pump that produces pressure only when needed, pressurizes the fluid and drives the system. A pressure-relief valve acts as a failsafe: it opens when pressure exceeds the limit, allowing excess fluid to flow from the pressure side to the return side and protecting the system from dangerous over-pressurization. An accumulator, sometimes spherical and consisting of two chambers separated by a diaphragm, acts as a hydraulic battery; it stores pressurized fluid and pushes against the fluid when system pressure drops, quickly discharging whenever the pump cannot keep up with demand.
Downstream of the power pack, control and selector valves direct fluid to where it is needed most, governing the direction, speed, and pressure of the flow. Tubes, hoses, and pipes form the system plumbing. A hydraulic hose assembly - the central unit of the flexible lines - includes an inner tube, outer covering, fittings, ferrules, adapters, protective sleeves, and quick-disconnect couplings with union nuts. These hoses transmit fluid from pump to valves and tolerate relative motion between components, whereas rigid tubing and high-strength pipes handle high-pressure runs inside the airframe. A manifold, equipped with relief valves and inlet ports for control-valve connections, combines fluid into an outlet line and distributes it to the operating branches.
At the work end, actuating cylinders - consisting a cylinder and piston - convert fluid power into linear motion to raise or lower landing gear, operate cargo doors, or power thrust reversers. Hydraulic motors, most often used to drive jackscrews for landing-gear applications, produce force and motion that operates the system. Brake master cylinders, shimmy dampeners, spoiler actuators, and uplock sequence actuators are further aircraft accessories that utilize hydraulic pressure. Throughout the circuit in-line filters remove contaminants and impurities from the fluid, while heat exchangers help guarantee the fluid remains at a safe temperature. By incorporating reservoirs, pumps, valves, accumulators, tubes, hoses, pipes, filters, actuators, and motors into a coherent design, the airplane hydraulic system delivers reliable, controllable power to every flight surface and mechanism.
What are aviation hydraulic lines?
Aviation hydraulic lines are the high-pressure fluid pathways that carry hydraulic power from pumps to flight controls, landing gear, brakes and thrust reversers. In most aircraft they take two forms: aluminium alloy tubing for the majority of rigid runs, and flexible hose assemblies where motion or access requires it. Tubing sizes range from one-quarter to one-and-a-half inches and are chosen for high strength-to-weight ratio. Aluminium alloy eliminates weight while stainless steel tubing is used in specific areas that demand extra corrosion resistance or strength. Flexible hoses are built from synthetic rubber or polytetrafluoroethylene (PTFE, commonly called Teflon), are reinforced with braided cotton, wire or synthetic fibre, and are covered with an outer layer for chafe resistance. To withstand system surges and pressures that reach 3,000 psi, hydraulic lines must have the correct inside diameter to secure required flow velocity and the wall thickness to provide sufficient strength.
Because hydraulic hose assemblies operate at high pressures, they are sensitive; twisting during installation is prohibited, the hose must not be stretched tight between two fittings, and swaging tools must be compatible with approved AN-standard fittings to achieve leak-free connections. Routing precautions include separation from air-conditioning ducts, shut-off capability to hydraulic lines, and the use of bulkhead fittings instead of forcing lines through firewall holes. Fuel, oil and hydraulic lines under the cowling are today covered with protective firesleeve so that, in the event of a line failure, the 475°C (887°F) auto-ignition temperature of the fluid is not reached by external flame. These lifelines of the hydraulic system must remain in optimum operating condition, for their condition impacts flight safety - hose rupture in flight renders the engine or flight controls out of commission, and problems with tubing and fittings contribute to aircraft delays and cancellations.
What are the advantages of a hydraulic system in aircraft?
Advantages of a hydraulic system include power-to-weight ratio, reliability, and control precision. Hydraulic systems provide a high power-to-weight ratio, delivering immense force without cumbersome equipment. This power density allows lighter tubing to replace heavy mechanical linkages, reducing overall aircraft weight and drag.
Hydraulic systems guarantee surfaces respond rapidly to pilot commands through near-instantaneous transmission of force. Liquids are not compressible, so no delay occurs in movement. This responsiveness, combined with accurate control, allows pilots to manage flight envelopes while the system provides flight envelope protection.
Compared to mechanical systems, hydraulics experience less wear and tear because fluid transmits force rather than direct contact between moving parts. They reduce maintenance requirements and offer a dependable solution for pivotal operations.
Hydraulic systems contribute to safety and efficiency of flight operations. Multiple pressure sources provide redundancy, guaranteeing that failure of a hydraulic system will not result in loss of control because of redundancy. Hydraulic accumulators store pressurized oil and can quickly discharge when the pump cannot keep up with pressure demand, allowing the system to respond more quickly to temporary demand while dampening ripple.
The system works efficiently over longer distances and must perform across a broad temperature spectrum. Hydraulic fluid must have adequate viscosity while being corrosive requires careful handling. Modern commercial aircraft power flight control surfaces from three independent hydraulic systems, with engine-driven hydraulic pumps providing pressure for larger aircraft while electric pumps serve smaller aircraft with fewer demands, like the Cessna Citation Mustang which has a single electrical pump that produces pressure only when needed for landing gear and brakes.
Do all aircraft use hydraulics?
All aircraft make use of hydraulically powered components. As airplanes become heavier more systems utilize the hydraulic system, and larger, more intricate aeroplanes use hydraulically powered components more commonly. Large commercial aircraft use hydraulically powered components and modern commercial aircraft power flight control surfaces from three independent hydraulic systems. Many aircraft have flight control surfaces hydraulically actuated, and larger aircraft utilize engine-driven hydraulic pumps instead of simpler electric drives. The Boeing 747 featured a centralized hydraulic system when it was introduced in 1968, setting the standard for wide-body jets. Light general aviation aircraft activate wheel brakes, and a single electrical pump serves landing gear in types like the Cessna Citation Mustang.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
