Aircraft hydraulic systems allow forces to be applied, multiplied, and transmitted from one location to another through an incompressible fluid medium. Aircraft hydraulic systems are used to operate flight-control surfaces, flaps, and to extend and retract landing gear. They provide pressure to activate wheel brakes.
The aircraft hydraulic system uses an engine-driven pump or electric pump to supply pressurized fluid. A reservoir provides fluid for the hydraulic pump, which pressurizes the fluid to drive machinery or move mechanical components. Valves direct fluid flow, while the actuating cylinder transforms fluid power into mechanical energy to perform actions within the aircraft. Since safety is pivotal in aviation, aircraft hydraulic systems require redundant systems.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is the main purpose of a hydraulic system in an aircraft?
The main purpose of a hydraulic system is to power vital flight-control surfaces, braking systems, landing gear, wing flaps, and thrust reversers. In light general-aviation aircraft its use is limited to providing pressure to activate the wheel brakes. On larger aircraft the hydraulic system drives the braking system, flaps, landing-gear, air turbines, and engine pumps, because hydraulic systems can deal with higher loads than motors of similar size.
The main advantage of hydraulic versus electrical actuation is that hydraulic systems provide an easy way of distributing power: a single electrical pump serves landing gear, and the hydraulic system pressure ranges from a few hundred psi to more than 5000 psi. Hydraulic systems provide dependability, efficiency, and response. Hydraulic systems are used to move brakes, hydraulic servos have advantages in precision and power-loss, and the sliding unit is ultimately connected to the piece of aircraft to be moved.
How are hydraulics used in airplanes?
Planes use hydraulics because of immense pressures on control surfaces during flight; the principle behind aviation hydraulics is to use pressurized liquid to move part of the airplane. Flight control surfaces like ailerons, rudders and elevators rely on hydraulics to guarantee pilots can manoeuvre aircraft effectively. Fly by wire aircraft use hydraulics for most actuators because electric motors can have high torque or high precision but they do not have both. Hydraulic servos control these surfaces because hydraulics have advantages in precision and power loss.
Larger, more intricate aeroplanes use hydraulically powered components: these aircraft utilize engine-driven hydraulic pumps, and hydraulics operate at 3000+ PSI allowing small tubes to be used. Hydraulics are almost 100% efficient with very little friction-related loss of fluid, and they can deal with higher loads than motors of similar size. Light aircraft make use of hydraulics to augment and transmit braking forces from cockpit to brake disk, while larger aircraft use hydraulics for braking and nose-wheel steering. Flaps adjust lift and drag of aircraft during takeoff and landing, and they are controlled by hydraulic servos. Landing gear is operated via hydraulics system; hydraulic actuators extend landing gear and hydraulic motors drive jackscrews that power vertically extending landing gear applications, because landing gear is an intricate and heavy system requiring large amounts of power. Brakes slow down the plane during landing, and spoilers on wings are part of high-speed braking.
I first encountered hydraulics during a pre-flight briefing, where I saw large command surfaces on the airfoils driven by hydraulic force. I learned that vital subsystems like landing equipment and brakes are operated by dual, separate hydraulic arrangements.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What are the components of a hydraulic system in an aircraft?
The components of a hydraulic system in an aircraft are listed below.
- Reservoir: Each hydraulic system has a reservoir to store the hydraulic fluid, a supply line to supply the fluid, and a return line in which the fluid comes back to the reservoir. The reservoir is pressurized for constant flow of fluid through the supply line. The pressure is maintained with bleed air from the engine.
- Pump: The pump is used to pressurize the hydraulic fluid. The pump is driven by the engine or is electrically operated.
- Filters: Filters are used for removing any particles or impurities from the fluid as they can damage the hydraulic system. Filters can be installed on the supply line, return line or critical components.
- Valves: Valves control and regulate the flow of the fluid. Different types of valves can be used for different functions. There is a check valve that allows the fluid to flow only in one direction. The shutoff valve cuts off the fluid from the system during emergencies like leak, fire, or pump fault. The pressure reducing valve is used for decreasing pressure of fluid in a particular line. The selector valve changes the direction of pressurized fluid by rotation of the valve based on which side the pressure of fluid is required.
- Actuator: The actuator helps in converting hydraulic pressure into mechanical action. It can be connected to the primary or secondary flight controls, landing gear, brakes and the nose wheel steering.
- Accumulator: The accumulator holds the pressurized hydraulic fluid. A diaphragm separates the fluid and the activator. The accumulator provides fluid at constant pressure when the demand increases.
What is a hydraulic accumulator in an airplane?
A hydraulic accumulator is a pressure storage reservoir in which an incompressible hydraulic fluid is held under pressure that is applied by an external source of mechanical energy, and it is divided into two separate chambers.
A hydraulic accumulator is a pressure storage device that functions as a hydraulic battery. It is typically made of a spherical or cylindrical steel shell and contains a compressible gas - normally nitrogen - on one side of a divider and non-compressible hydraulic fluid at system pressure on the other. Because each accumulator contains an expandable and compressible bladder of nitrogen gas, it stores incompressible hydraulic fluid under pressure that can be released instantly when demand exceeds pump capacity.
What is the purpose of an accumulator in an aircraft hydraulic system? The accumulator has two main purposes. First, it adds volume to the system at a very fast rate, enabling a hydraulic system to cope with extremes of demand using a less strong pump. Second, it absorbs shock by smoothing out pulsations and dampening pressure surges, thereby protecting downstream components. All hydraulic accumulators store energy, so they also provide peak hydraulic power and extend component life by reducing peak hydraulic power fluctuations. How does a hydraulic accumulator in an airplane operate? During normal flight the hydraulic pump gradually tops off the accumulator until the pre-charge pressure between 1,200 and 4,000 PSI is reached. When the pump cannot keep up with pressure demand, the accumulator can quickly discharge its stored hydraulic oil at nominal system pressure. If pressure in the hydraulic system drops below the pressure inside the accumulator, the compressible gas expands, pushing against the fluid and temporarily boosting hydraulic system pressure. After its oil has been used up, maximum flow reverts to pump flow, so the unit must be re-charged by the next pump cycle.
What is the function of a hydraulic accumulator in an airplane? The accumulator provides backup power during interruptions, guaranteeing vital operations can continue without interruption. On carrier-based aircraft the onboard hydraulic accumulator controls the tail hook during deployment and retraction, absorbs a lot of the braking shock, and protects the aircraft and crew. The landing gear relies on a hydraulic accumulator to extend and retract the landing gear system, and the accumulator also provides emergency landing gear extension if the main system fails. The accumulator provides backup for aircraft braking; if the hydraulic braking compressor fails, the accumulator can deploy stored hydraulic oil at pressure to quickly bring the aircraft to a stop. By delivering a large flow supply at peak demand and maintaining hydraulic pressure during power failures, hydraulic accumulators guarantee smooth, reliable operation and refine overall system performance while reducing operating and maintenance costs.
How does a hydraulic system work in a plane?
A hydraulic system starts working when a pilot or crew member activates a switch or flight control device. The signal starts an engine driven hydraulic pump or an electric motor that drives the same pump. The pump develops a minimum of 3100 PSI across all engine speeds, and hydraulic fluid, being incompressible, ensures consistent performance as it transmits forces.
Fluid under pressure moves mechanical components: the sliding unit is connected to the aircraft piece to be moved, so the hydraulic system handles tasks that require force or precision. Pascal's Law applies - pressure applied to the liquid distributes equally - so every actuator receives the same 3000 PSI shown on the A320 gauge even when flight controls move. Flow demand on the open-loop hydraulic system causes the pressure bypass to progressively close, while a pressure sensor controls the pump motor so the system never over-pressurizes.
How does an actuator operate within an aircraft hydraulic system?
Within an aircraft hydraulic system, the actuator helps convert hydraulic pressure into mechanical action. The process begins when the pump pressurizes the system, activating the motion of the actuator. High-pressure hydraulic fluid enters the actuator and pushes against a sliding unit housed inside a cylinder. This unit consists of a piston connected to a rod that is ultimately linked to the piece of the aircraft to be moved. Valves route pressure to the appropriate side of the actuator: pressure sent to one side raises the flaps, while pressure applied to the other side lowers them. The same principle governs landing gear extension or retraction, cargo ramp movement, nose wheel steering, brake application, and thrust reverser deployment, assuring safe takeoff, landing, and ground operations.
Linear hydraulic actuators are among the most common types used in aircraft systems because they can push, pull, and hold components of all sizes with more power, speed, and precision. Fluid power is turned into work as the piston extends or retracts, transferring actuator movement directly to the control surface, landing gear, brakes, or cargo ramp. Failure of either hydraulic system does not disable the actuator, since many cylinders are arranged in tandem and fed by two independent systems. For flight control, the pilot deflects the stick, rotating a summing bar that opens the servo valve, allowing pressurized fluid to reach the actuator. The closed-loop system contains the liquid within hoses and the actuator itself, returning excess fluid to be filtered and recirculated.
How many hydraulic systems are in an aircraft?
Modern commercial aircraft power flight control surfaces from three independent hydraulic systems. The Boeing 757 has three fully independent hydraulic systems - Left, Center, and Right. According to SAE AIR5005 the 757 has 3 hydraulic systems and 2 PTUs. Redundancy triads are used in aviation engineering so that each system has its own reservoir and can back the others up in case of a failure, assuring the aircraft can still be safely operated using the remaining systems.
Why is the hydraulic system of an aircraft classified in different colors?
An aircraft's hydraulic installation is split into independent branches, and each branch is given a name and a colour. On Airbus aeroplanes these branches are commonly identified as Green, Yellow and Blue, Historic types like the Concorde carried yellow, blue and green circuits. Colour codes of hydraulic systems are an identification system for each separate hydraulic branch; they do not refer to the colour of the fluid circulating inside the lines.
Although the circuit names are green, yellow and blue, the fluid that fills them is purple. Transport category aircraft use the purple-colored phosphate ester-based fluid, most often Skydrol LD. These purple fluids have high fire-resistant properties. Mineral-based fluids are used in smaller aeroplanes; these fluids are petroleum based and are dyed red for easy identification. Vegetable-based is a third permitted type of hydraulic fluid. The fluid color serves as an important indicator of the fluid's composition and condition: darker amber indicates normal operation, light brown indicates early signs of oxidation, while black indicates severe contamination or overheating. Changes in the color of hydraulic fluid provide early warnings of contamination, degradation, or system malfunction, so observing the color of its oil gauges the state of the hydraulic system.
Standardized color codes are applied to the hoses themselves. Red is often used to indicate high-pressure lines, blue typically denotes low-pressure return lines, and yellow is used for fluid lines carrying specific types of oil or hydraulic fluid.
What is the aircraft hydraulic ground power unit?
The aircraft hydraulic ground power unit, referred to as a mule, helps ground crews perform aircraft maintenance operations to keep their fleets in top condition and allows them to perform preventive maintenance tasks that would otherwise not be possible without starting an aircraft's engines.
An aircraft hydraulic ground power unit - commonly called an HPU - is a specialized piece of ground support equipment designed to generate, regulate and control hydraulic power for aircraft systems while the aircraft remains on the ramp or in the hangar. By connecting directly to the aircraft's hydraulic circuits, the unit supplies 3,000 psi fluid pressure to operate flight controls, landing gear, cargo doors, thrust reversers and other hydraulically driven components without requiring the engines or the on-board auxiliary power unit to run. This external source of pressurized hydraulic fluid simulates operational conditions, enabling maintenance crews to perform leak detection, functional checks, and ATA Chapter 29 ground tests that guarantee every actuator, valve and line is functioning correctly before flight, thereby safeguarding passengers and crew.
Hydraulics International, Inc. designs and manufactures the Hydraulic Ground Power Unit model HGPU50-30-1S, a self-contained cart that incorporates a 20-gallon (75 L) easily drainable fluid reservoir, a modern touchscreen control panel, and real-time monitoring and diagnostics to reduce training requirements and refine workflow. Larger size-3 units carry a 60-gallon (227 L) reservoir for extended jobs, while software-driven operator interface logs pressure, flow and temperature data throughout the test sequence. Operating temperature ranges from -32°C (-25.6°F) to +55°C (131°F), and the entire assembly is skid-mounted or towable so it can be positioned under the wing or tail with minimal ramp space.
A hydraulic mule is akin to an HPU but is typically more mobile and versatile, offering controlled hydraulic power on-demand in compact, portable form factors that fit inside a truck bed or helicopter deck. The equipment acts as the workhorse of the ground support fleet, enabling tasks that are impossible without starting the aircraft's engines and thereby reducing fuel burn, emissions and wear during scheduled or unscheduled maintenance turns.
What is an aircraft hydraulic power pack overhaul?
Aircraft hydraulic power overhaul is the process of returning an aviation unit to ‘like new’ condition; It consists of tearing down or disassembling a part, cleaning it, inspecting it, repairing or replacing parts, and then reassembling the part and testing it under FAA Part 145 or EASA standards. Hydraulic power pack overhaul includes complete disassembly, defect inspection, parts replacement or reengineering, NDT, functional testing, and certification. Typical overhaul interval is about 10,000 flight hours with a 3-6 week lead time.After overhaul the reworked power pack will be slightly different in the position of all the AN fittings and will require a lot of work to get everything back into alignment. The new H355, H565 universal test console and H402 50 hydraulic power unit are designed to perform preemptive, post overhaul, audit testing of all hydraulics parts for proof pressure, internal leakage, cracking pressure and performance characteristic using next generation ADAS Data acquisition System.
What is the most widely used type of hydraulic system used in aviation?
A mineral-based hydraulic fluid is the most widely used type in small aircraft. A mineral-based hydraulic fluid is a kerosene-like petroleum product that has very little viscosity change with temperature. This type of hydraulic fluid has good lubricating properties and includes additives to inhibit foaming and prevent corrosion. While aviation hydraulic systems are of the "open loop" variety, open loop systems draw fluid from a reservoir, pressurize fluid, and return fluid to the reservoir before making fluid available to various user components. Military aircraft have been using 5000 psi hydraulic systems since the 1970s. Concorde features a 4000 psi system, yet 3000 psi was the first hydraulic operating pressure.
Which hydraulic system is typically found on commercial aircraft?
A centralised hydraulic system is typically found on commercial aircraft. The Boeing 737, 747 and 787 all use the same basic layout: three independent circuits, each driven by variable-displacement piston pumps, share the load on flight controls, nose-wheel steering, brakes, spoilers and high-lift devices. A standby hydraulic system backs up the circuits so that the aircraft retains its hydraulic linkages even if one pump or line is lost. By the 1960s this three-system hydraulic layout had become standardised in commercial aircraft. Today the Boeing 787 features a centralised hydraulics system largely similar to that found in the 747, while the 737 retains a three-system hydraulic layout that operates at around 3,000 psi.
Smaller aircraft employ hydraulic power, but on a reduced scale. A light business jet or turboprop normally uses a single open-loop hydraulic system to drive wheel brakes, landing-gear extension and a limited set of flight controls. Variable-displacement piston pumps are reserved for large aircraft, so the small-aircraft circuit relies on simpler gear pumps and a single reservoir. Skydrol or Hyjet phosphate-ester fluid supplies fire resistance and handles temperatures up to 160°F, the same fluid that circulates in the larger fleets, guaranteeing that every category of commercial aircraft is linked by a common, reliable hydraulic technology.
