Understanding the intricacies of aircraft hydraulic systems provides insight into the engineering marvels that keep aircraft flying safely through varying conditions.
By Josh Cosford, Contributing Editor
Aircraft hydraulic systems are a special breed of essential components in modern aviation, providing the power and control necessary for various functions that provide the safe and efficient flights we all take for granted. The technology, quality and execution of aviation hydraulics differ surprisingly from the fluid power systems in either traditional industrial or mobile designs. Hydraulics is a preferred technology in aircraft because of its reliable, precise and responsive nature.
Image courtesy of Adobe Stock
If you’re a reader with more aviation than fluid power experience, know that hydraulic systems operate on the basic principle of fluid power: using pressurized fluid to generate mechanical force or motion to achieve work or position devices. The hydrostatic concept, while simple, is highly effective, allowing hydraulic systems to perform powerful tasks using compact actuators.
In aircraft, hydraulic systems are used to control various critical functions, such as flight control surfaces, landing gear, braking systems, cargo doors, and wing flaps and slats. You might be surprised to learn that nearly everything that moves on a modern jetliner is hydraulic, aside from the cabin doors.
The primary components of an aircraft hydraulic system include a reservoir, pump, actuators, valves, tubes, hoses, and filters, just as with traditional hydraulic systems. However, how they look, what they’re made from and where they’re located are vastly different than you’d expect.
Unique and complex systems
The reservoir is where hydraulic fluid is stored, providing the storage location for all the system’s needs, as well as for expansion, contamination settling and cooling. Typical ground-level reservoirs use atmospheric pressure to push fluid into the suction port of a pump due to the perceived suction effect. But at cruising altitude, atmospheric pressure is a quarter that at sea level, making cavitation all but guaranteed.
To overcome the lack of ambient pressure, aircraft reservoirs are frequently pressurized to guarantee the pump(s) always see consistent, positive flow, which is critical to reliable performance. Besides the lack of atmospheric pressure above the clouds, the aircraft is subject to pitch, roll and turbulence that would prevent a stable fluid level in a traditional reservoir. Rather than a rectangular prism, aircraft reservoirs resemble propane canisters with pressure gauges and adapters sprouting out like an old potato growing stems.
Illustration of the construction of a bootstrap hydraulic reservoir
Actuators are perhaps the most crucial component within aircraft hydraulic systems, as they are responsible for converting hydraulic pressure into mechanical motion and force. This conversion enables the control of flight surfaces, the deployment and retraction of landing gear, the operation of brakes, and the movement of other critical aircraft flight systems. The precise control and high power-to-weight ratio offered by hydraulic actuators make them ideal for these functions.
Some of the common actuators are used to operate flight surfaces, such as ailerons, elevators, and rudders, which control the roll, pitch and yaw, respectively. Aircraft actuators are highly complex, compact and powerful devices that include a mixed bag of cylinders, valves, sensors and even sometimes pumps.
Various combinations of signaling and power supply options are available for aircraft actuators, and how they’re employed depends on both the aircraft’s age and the builder’s preferred technology. The signal to the actuator can be mechanical, electrical or digital. A mechanical signal may come via a cable attached to the control stick and fed to a linkage at the actuator at the other end. Electrical signals via analog joysticks are widespread for their simplicity and reliability, which also saves weight. The digital signal is the ultimate in precision and lightness and is now the state of the art for modern aircraft designs.
Taking each of these three primary signals, the actuator is powered either electrically, hydraulically or electrohydraulically, and you know we’re going to skip discussing the prior since this is a fluid power article, after all. By “powered,” I mean the source of the hydraulic energy, which we know typically comes from a hydraulic power unit or a hydraulic pump mounted directly to an internal combustion engine or its transmission.
Ensuring safe, redundant control
Aircraft rarely use single-pump hydraulic systems anymore, as the safety and reliability are problematic. Even with a centralized hydraulic system, we require sufficient redundancy to provide power in the case of a single pump failure; there must be a reliable source of pressure and flow to handle the critical functions of plane operation. Commercial aircraft may operate with two engine-driven pumps, one or more electric pumps and even manual backups. Note that none of these configurations are gospel, as implementation varies based on the age of the aircraft design, manufacturer preferences, and size and purpose of the plane.
Parker Aerospace’s electrohydrostatic actuation (eha) systems eliminate the need for central hydraulics. They use electric power for aircraft flight control-surface actuation that results in reduced aircraft weight, efficient power consumption, and improved maintainability.
Ultra-modern aircraft have evolved to use technologies such as electrohydrostatic actuators. These actuators used for flight controls and other functions do away with the centralized hydraulic systems which require multiple pumps, reservoirs and a web of plumping routed throughout the aircraft. The EHA design mounts the pumps, reservoirs and valves to the body of the hydraulic cylinder, reducing the need to run tubes throughout the plane. The actuator simply requires electrical power and control inputs, and the onboard electronics, sensors and dual pumps do the work to control the flight surface precisely.
Smooth landing
Another vital function of aircraft hydraulic systems is the operation of landing gear. Hydraulic actuators extend and retract the landing gear during takeoff and landing, allowing for not only a safe transition between flight and ground operations but also aerodynamic improvements over fixed landing gear. The landing gear is part traditional hydraulics and part hydropneumatic shock absorber.
As you expect, landing gear deployment and retraction uses cylinders, although some clever additions to the cylinder design are required for safe and effective operation. Landing gear has multiple axes of motion, various torque arms, and locking mechanisms. All three landing gear struts are operated with the same control system of pump(s), valves and reservoir. Landing gear is an all-or-nothing affair, as you want all three up or down simultaneously.
The controlled extension and retraction of landing gear are essential for safe takeoffs and landings. Hydraulically operated landing gear systems ensure a reliable and robust mechanism for deploying and retracting landing gear under varying loads and conditions.
The shock absorbers in commercial aircraft provide a multi-function advantage over fixed landing gear options. During landing, the aircraft’s weight slams the landing gear against the ground and the dual-chamber shock absorbers progressively damp the vertical velocity at progressively increasing rates. The nitrogen-filled top chamber is separated from the oil-filled bottom chamber by an orifice and as the assembly compresses under load, the orifice controls the damping rate.
Landing gear uses a clever device called the metering pin, which is a long conical rod that extends through the orifice. As the assembly compresses vertically under load, the pin’s increasing diameter fills the space inside the orifice, thereby more aggressively metering oil flow to increase the damping rate. Like an automobile, a commercial aircraft’s struts also provide a comfortable ride while taxiing on runways.
Precise control at landing and takeoff
Additionally, hydraulic systems play a crucial role in aircraft braking systems, controlling the pressure applied to the brakes to ensure smooth and safe deceleration. Although part of the high-speed braking of an aircraft comes from spoilers on the wings and sometimes on the tail, you’d be surprised to learn that the wheels contain powerful hydraulic brakes. As you’d expect, various redundancies ensure the plane can always stop when asked to, and hydropneumatic accumulators provide backup energy should any pump fail.
Hydraulic systems play a crucial role in aircraft braking systems, controlling the pressure applied to the brakes to ensure smooth and safe deceleration. Part of the high-speed braking of an aircraft comes from spoilers on the wings as seen here.
Hydraulic systems are also used to operate an airplane’s cargo doors, allowing them to open and close quickly, which is essential for loading and unloading cargo on larger aircraft. Wing flaps and slats, which are used to increase lift during takeoff and landing, are also controlled by hydraulic actuators. The precise control offered by hydraulic systems is critical to ensuring these components operate smoothly and reliably.
Maintaining strict standards
While hydraulic systems are used across various industries, their application in aircraft has unique characteristics that distinguish them from other fields. Aircraft hydraulic systems are subject to rigorous safety standards, as the stakes in aviation are high. These systems must be reliable and redundant to ensure continuous operation during flight. Manufacturing aircraft hydraulic components include stringent nondestructive testing, strict quality control procedures and extensive testing.
For example, the raw material will be tested ultrasonically to confirm there are no cracks or inclusions within the meat of the material. When the body or component is cast, the UT test scans the material to ensure no cracks reflect the ultrasonic signal. Machined parts are also subject to an array of nondestructive testing such as magnetic particle or dye penetrant tests for machined threads.
Aircraft hydraulic systems are designed to be lightweight and compact to minimize the impact on aircraft weight and space. This constraint often leads to innovative design solutions that may be found outside of other industries. Additionally, aircraft hydraulic fluids are specialized to withstand extreme temperatures and pressures and are fire-resistant. Under no circumstances should a hydraulic fluid support flame, contrary to typical mineral oil that is happy to mimic a flame thrower when given the opportunity.
Another key difference is the environmental conditions in which aircraft hydraulic systems operate. Aircraft must function reliably across a wide range of conditions, from high-altitude cold to runway heat. This requires robust engineering and design considerations to ensure consistent performance.
Aircraft hydraulic systems are also subject to stringent maintenance and inspection routines, ensuring they are always in optimal condition for flight safety. This level of scrutiny and care is unique to aviation, where safety is paramount.
Aircraft hydraulic systems are crucial to the safe and efficient operation of modern aircraft. Their preference in aviation is justified by their high power-to-weight ratio, precision and control, reliability, redundancy, compact design, and energy efficiency. While aircraft hydraulic systems share some similarities with those in other industries, they have unique requirements for safety, compactness, and reliability that set them apart.
Understanding the intricacies of aircraft hydraulic systems provides insight into the engineering marvels that keep aircraft flying safely through varying conditions. As aviation continues to evolve, hydraulic systems will remain a fundamental technology, ensuring reliable and secure air travel for generations.
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