In modern aircraft, hydraulic systems provide strong power to many mechanisms, such as flight control surfaces, landing gear, and braking systems. If you are an aircraft manufacturer, aerospace engineer, or purchasing aviation hydraulic components, it is important to understand the structure and working principles of hydraulic systems in aircraft.
What Are Hydraulic Systems in Aircraft？
The hydraulic system in aircraft is a device that uses pressurized fluid to transmit power inside the aircraft. It converts mechanical energy into hydraulic energy, providing power for the operation of aircraft components.
Why Aircraft Use Hydraulic Systems？
High Power Density
The hydraulic system in aircraft can generate enormous forces in a very small space. When the aircraft is flying at high speed, the airflow exerts very high aerodynamic loads on the control surfaces.
The hydraulic system converts the mechanical energy of the engine into driving force by pressurizing incompressible hydraulic oil, completing heavy load tasks such as landing gear retraction and loading.
Precise Control and Rapid Response
Due to the incompressibility of hydraulic oil, the hydraulic system in aircraft can achieve almost instantaneous power transmission. This makes sure that the commands issued by the pilot or flight control computer can be accurately and in real-time applied to control surfaces such as ailerons or elevators.
The rigid connection of the hydraulic system in aircraft makes the system maintain stable control under strong aerodynamic loads generated by high-speed flight.
High Reliability and System Stability
The structure of the aircraft hydraulic system is simple, with fewer moving parts. This makes sure that the hydraulic system in aircraft maintains high stability during long-term operation. Meanwhile, fewer components also mean fewer potential failure points, simpler daily maintenance, and reduced operation and maintenance costs.
Strong Adaptability
Aircraft will experience complex environments such as high altitude, low temperature, and pressure changes during operation. The components of the hydraulic system in aircraft are manufactured using aviation grade materials and processes.
It can maintain stable operation under these working conditions, and the transmission of hydraulic pressure is not easily affected by external environmental changes.
The Main Components of the Hydraulic System in Aircraft
Hydraulic Oil
In the aircraft hydraulic system, hydraulic oil is used to transmit power while also providing lubrication, sealing, and heat dissipation functions. Aviation hydraulic oil must maintain a stable viscosity within a wide temperature range. It also needs to have good wear resistance, chemical stability, and corrosion resistance.
The common types of hydraulic oil used in aircraft hydraulic systems mainly include mineral oil type, synthetic hydrocarbon type and phosphate ester type.
Mineral based hydraulic oil is usually dyed red so that it is easy to identify in case of leakage. It has good fluidity in low-temperature environments. However, mineral based hydraulic oil has a low flash point and high flammability. It is more commonly used for small and medium-sized general aircraft.
Synthetic hydrocarbon based hydraulic oil is widely used in the hydraulic systems of military aircraft, helicopters and carrier-based aircraft. It can maintain stable performance within the temperature range of -54 ℃ to 200 ℃.
Modern commercial transport aircraft and high-performance jet aircraft typically use phosphate ester based hydraulic oil. It usually appears in purple or amber colors. It has very high heat and fire resistance properties.
If a pipeline bursts under high temperature and high pressure conditions, the oil mist produced by the phosphate ester-based hydraulic oil is difficult to ignite. At the same time, it has good thermal stability and hydrolytic stability.
Hydraulic Reservoir
The hydraulic reservoir is used to store hydraulic oil and provide sufficient hydraulic fluid for the entire circuit. It has a special internal structure that can provide a space for thermal expansion of fluids and help remove air from the hydraulic oil.
In high-performance aircraft, the hydraulic reservoir has a pressurization function. This makes sure that in various flight attitudes and high-altitude low-pressure conditions, the inlet end of the hydraulic pump can always obtain a stable supply pressure, preventing cavitation.
Hydraulic Pump
The hydraulic pump can convert the mechanical energy from the engine or the electrical energy from the motor into hydraulic pressure. Depending on the driving method, aircraft are usually equipped with the engine driven pump as the main power source. The electric motor driven pump or manual pump serves as the backup power source or for ground maintenance.
These pumps are usually variable piston pumps, which can automatically adjust the displacement according to the changes in system load and maintain a constant pressure.
Hydraulic Actuator
The hydraulic actuator can convert hydraulic energy into kinetic energy to drive aircraft components. And the hydraulic actuators are divided into linear actuators and rotary actuators.
Linear actuators are mainly hydraulic cylinders. It achieves actions such as landing gear retraction and extension, flap and slat adjustment, and aircraft wheel braking through the linear movement of the piston. The cylinder body is made of aviation grade high-strength materials and can withstand pressure impacts under extreme working conditions.
Rotary actuators are mainly hydraulic motors. It achieves operations such as front-wheel steering and engine reverse thrust through the rotational motion of the rotor. Hydraulic motors have the characteristics of high torque and fast response, which can adapt to the various power requirements of aircraft on the ground and in the air.
Valves
The various valves in the aircraft hydraulic system can accurately adjust the pressure, flow direction and flow rate of the hydraulic oil, making sure that all the actuating components operate according to the instructions.
The servo valves are mainly used in the main flight control system. It can achieve micrometer level flow and pressure regulation, accurately controlling the motion amplitude and speed of components such as ailerons and elevators.
The overflow valve can monitor the system pressure in real time. When the pressure exceeds the set value, it will automatically release pressure to avoid pipeline overload and bursting.
The one-way valve can prevent hydraulic oil from flowing back, make sure independent pressure of each branch system, and avoid cross pressure affecting the operation of other components.
Hydraulic Accumulator
The hydraulic accumulator is a steel container that contains compressed nitrogen gas and is connected in parallel with a hydraulic circuit. It can store high-pressure hydraulic oil to supplement the instantaneous flow of the system during peak demand.
The accumulator can absorb the pressure pulsation generated by the hydraulic pump and the hydraulic impact within the system. It can also provide limited emergency power for the braking or flight control systems in emergency situations.
The Safety Redundancy of the Hydraulic System in Aircraft
Multiple Independent Hydraulic Systems
Modern large aircraft usually have two to three completely independent hydraulic systems. Each system has independent reservoirs, pumps, and pipelines. When one of the systems fails or the pressure drops, the other systems can still continue to provide power for the flight control surfaces or other equipment.
Multiple Power Sources
The aircraft hydraulic system is equipped with two power sources. The main power source is often the engine driven pump. When the engine malfunctions or shuts down, the system will automatically switch to the electric backup pump.
The electric backup pump is powered by the aircraft’s emergency power supply. It can quickly start and maintain the basic system pressure, making sure the normal operation of functions such as landing gear retraction and extension, braking, and core flight control.
In addition, the accumulator in the system can store high-pressure hydraulic oil. When both the main pump and the backup pump experience temporary failures, it can release pressure for a short period of time, providing power for emergency operations and avoiding safety accidents caused by power interruption.
Cross System Compensation of the Power Transfer Unit
The power transfer unit is a device that can transfer hydraulic power between different hydraulic systems without exchanging hydraulic oil. When the engine driven pump of a certain system fails, the normally working system can drive the pump of the failed system through the power transfer unit, thereby maintaining its pressure.
Priority Valves
When the system pressure experiences abnormal fluctuations due to a fault, the priority valves will distribute and regulate the hydraulic supply.
The system will automatically cut off the fuel supply for loads such as landing gear retraction and flap operation. It will distribute the limited hydraulic pressure to the main flight control systems such as the ailerons, elevators and rudder.
How does the Hydraulic System in Aircraft Work?
The working principle of the hydraulic system in aircraft is based on Pascal’s law.
The engine drives the hydraulic pump to suck hydraulic oil from the reservoir and pressurize it, converting mechanical energy into hydraulic energy. The working pressure of modern aircraft hydraulic systems usually ranges from 3000 psi to 5000 psi. These high-pressure fluids are regulated by the control valve and then delivered to the designated actuators.
The hydraulic cylinder drives the retraction and extension of the landing gear, as well as the adjustment of the flaps, through the linear movement of the piston. The hydraulic motor achieves operations such as front wheel steering and engine reverse thrust through the rotational motion of the rotor.
After the actuator completes its operation, the hydraulic oil will return to the reservoir through the return channel and enter the next cycle.
Applications of Hydraulic Systems in Aircraft
Main Flight Control System
In large aircraft, hydraulic systems are widely used to drive the main flight control surfaces. The main flight control surfaces include the ailerons, elevators and rudder, which control the rolling, pitching and yawing movements of the aircraft.
During the flight, the system will receive control instructions from the pilot. The system will accurately regulate the pressure and flow rate of the hydraulic oil, driving the actuator to start working. The actuator then drives each control surface to make corresponding actions.
Landing Gear System
The retraction and extension of the aircraft landing gear and the turning action of the front wheels are all driven by the hydraulic system.
After takeoff, the hydraulic system drives the hydraulic cylinders to extend and retract, retracting the landing gear back into the fuselage. Before landing, the landing gear will be smoothly lowered and locked by the hydraulic power.
When taxiing on the ground, the hydraulic system drives the front wheel steering actuators to make the aircraft turn and make U-turns.
Meanwhile, the hydraulic system, in conjunction with the shock absorbers of the landing gear, can absorb the impact force during landing, reduce the vibration of the fuselage, and protect the fuselage structure and onboard equipment.
Wheel Braking System
The braking power of the aircraft wheel braking system is provided by the hydraulic system. When the plane landed, the pilot issued the braking command. The hydraulic system immediately responded and rapidly supplied high-pressure hydraulic oil to the wheel braking components.
Under the pressure of the oil, the brake disc and the brake pad were closely attached, generating braking force through friction. This process makes the plane decelerate smoothly and shortens the landing taxiing distance.
The hydraulic system can accurately adjust the brake pressure according to landing speed, load and other working conditions, avoiding tire blowouts or body skidding caused by excessive braking.
Engine Thrust Reverser Device
During the deceleration process after the aircraft lands, the hydraulic system will drive the deflector doors or cascade vanes in the engine compartment. The reverse thrust device changes the direction of the engine exhaust to generate reverse thrust, helping the aircraft decelerate rapidly and shortening the taxiing distance.
Flap and Spoiler Control
The flaps and spoilers on the wings are also driven by hydraulic systems. The flaps increase lift during takeoff and landing by changing the shape of the wings. The spoiler increases drag or reduces lift by disrupting airflow.
Other Components
The cargo doors or loading ramps of large transport aircraft and passenger planes are usually driven by hydraulic pressure.
In some aircraft, the hydraulic system is also used to drive the windshield wipers, components of the air management system, and the deicing system of the leading edge flaps.
FAQs
What are the Effects of Hydraulic Oil Pollution on the Hydraulic System in Aircraft?
Hydraulic oil pollution is the main cause of aircraft hydraulic system failures. The solid particles in the hydraulic oil can wear down the mating surfaces of components such as pumps and valves. In severe cases, it can also cause components to become stuck, affecting the normal operation of the system.
The moisture in hydraulic oil can cause corrosion of metal components inside the hydraulic system. If air is mixed into the hydraulic oil, it will cause pressure fluctuations and cavitation. You should regularly check the cleanliness and moisture content of the hydraulic oil, and replace the polluted hydraulic oil in time.
How to Set the Routine Maintenance and Inspection Cycle for the Hydraulic System in Aircraft?
The maintenance of the aircraft hydraulic system needs to be based on three standards: flight hours, takeoff and landing frequencies, and calendar time. Before and after each flight, you need to check the oil level, leaks and system pressure.
Regular maintenance is carried out in four levels: A, B, C, and D. The C check and D check are deep maintenance that will conduct a thorough inspection of the entire system.
For components such as pumps, actuators, and seals, you need to disassemble and inspect them according to the fixed usage cycle. Meanwhile, system pressure testing and oil analysis must be completed after each maintenance.
What are the Impacts of High Altitude and Low Temperature Environments on the Hydraulic System in Aircraft?
Low temperature will reduce the viscosity of hydraulic oil, leading to a decrease in its fluidity and a slower response speed of the hydraulic system.
Low temperature can also cause the sealing components in the system to harden and become brittle, resulting in a decrease in sealing performance and a higher risk of leakage, which affects the pressure stability of the system.
Before being put into use, all the hydraulic systems in aircraft must undergo extreme low temperature bench tests to make sure that they can operate stably and reliably under various low-temperature conditions within the full flight envelope.
How to Determine and Handle Leakage in the Hydraulic System in Aircraft?
The hydraulic system in aircraft is permitted to have an allowable leakage that meets airworthiness standards, but the hydraulic oil dripping is strictly prohibited. During ground maintenance, you need to mark the location of the leakage first, then observe and record the form and rate of the leakage.
If the leakage is caused by loose joints, you need to tighten the interface. If the seal fails, you need to replace it in time. If the leakage is caused by pipeline damage, you need to repair or replace the pipeline according to the degree of damage.
Where does the Wear and Tear in the Hydraulic System in Aircraft usually Start?
This often starts from the positions of high-frequency motion and high pressure, such as the interior of hydraulic pumps, control valves, and sealing parts of actuators. These positions are constantly subjected to pressure fluctuations and fluid erosion, making them easy to fatigue or corrosion.
What is a Ram Air Turbine?
It is a small propeller driven device, which is usually stored under the fuselage or the lower part of the wings of the aircraft. When the aircraft engine stops and the auxiliary power system completely fails, the ram air turbine will start.
When the airflow drives the propeller to rotate, the ram air turbine will drive a small hydraulic pump, providing ram pressure to operate the flight control system, allowing the pilot to control the aircraft for an emergency landing.
Final Thoughts
The hydraulic system in aircraft transmits stable and controllable hydraulic pressure through high-pressure fluid, making the flight control, landing gear and braking functions to respond accurately in complex environments. If you need relevant components of the hydraulic system in aircraft, please feel free to contact us.
