Aircraft hydraulic systems are a unique and vital part of modern aviation, supplying the power and control required for numerous functions that ensure the safe and efficient flights we often take for granted. The technology, quality, and implementation of aviation hydraulics differ significantly from fluid power systems used in traditional industrial or mobile applications. Hydraulics are favored in aircraft due to their reliability, precision, and responsiveness.

If you're more familiar with aviation than fluid power, it’s important to understand that hydraulic systems work on the fundamental principle of fluid power: using pressurized fluid to generate mechanical force or motion to perform work or position devices. While the hydrostatic concept may seem simple, it is extremely effective, enabling hydraulic systems to perform powerful tasks with compact actuators.

In aircraft, hydraulic systems are responsible for controlling various critical functions, including flight control surfaces, landing gear, braking systems, cargo doors, and wing flaps and slats. You may be surprised to learn that nearly everything that moves on a modern jetliner is powered by hydraulics—except for the cabin doors.

An aircraft hydraulic system consists of the same basic components as a traditional hydraulic system: a reservoir, pump, actuators, valves, tubes, hoses, and filters. However, the design, materials, and placement of these components are very different from what you might expect.

Unique and Complex Systems

In an aircraft, the reservoir serves as the storage location for hydraulic fluid, supporting all the system's needs, as well as facilitating fluid expansion, settling of contaminants, and cooling. On the ground, typical reservoirs rely on atmospheric pressure to push fluid into the suction port of a pump, creating a suction effect. However, at cruising altitude, atmospheric pressure is only a quarter of what it is at sea level, which makes cavitation a near certainty.

To address this challenge, aircraft reservoirs are often pressurized to ensure that the pump(s) consistently receive positive flow, which is essential for reliable operation. In addition to the reduced atmospheric pressure, an aircraft experiences pitch, roll, and turbulence, which would disrupt the stable fluid level in a traditional reservoir. To prevent this, aircraft reservoirs are designed differently from their ground counterparts, often resembling propane canisters, complete with pressure gauges and adapters sprouting from them like stems on a potato.

Actuators are perhaps the most critical component in aircraft hydraulic systems, as they convert hydraulic pressure into mechanical motion and force, enabling the control of flight surfaces, deployment and retraction of landing gear, brake operation, and the movement of other essential systems. Their precise control and high power-to-weight ratio make hydraulic actuators ideal for these demanding tasks.

Common actuators are responsible for controlling flight surfaces like ailerons, elevators, and rudders, which manage roll, pitch, and yaw, respectively. These actuators are highly complex, compact, and powerful, combining cylinders, valves, sensors, and sometimes pumps in their design.

The signaling and power supply options for aircraft actuators vary based on the aircraft’s age and the builder’s technology choices. The signal can be mechanical, electrical, or digital. Mechanical signals often involve cables attached to the control stick, transmitting movement to the actuator via a linkage. Electrical signals, typically from analog joysticks, are widely used for their simplicity, reliability, and weight-saving benefits. Digital signals represent the most precise and lightweight solution, now common in modern aircraft.

Regarding the power source, actuators are typically powered hydraulically, though some can be electrically or electrohydraulically powered. Hydraulic power often comes from a hydraulic power unit or pump, which is typically connected to the aircraft’s internal combustion engine or its transmission.

Ensuring Safe, Redundant Control

Modern aircraft rarely rely on single-pump hydraulic systems due to the safety and reliability concerns. Even with a centralized hydraulic system, redundancy is necessary to maintain power in the event of a pump failure. Multiple sources of pressure and flow must be available to ensure the critical functions of the aircraft can still operate. Most commercial aircraft feature two engine-driven pumps, one or more electric pumps, and sometimes manual backups. The exact configuration of these systems can vary depending on the aircraft's design, age, manufacturer preferences, and its intended purpose and size.

Ultra-modern aircraft have embraced innovative technologies like electrohydrostatic actuators (EHAs) to streamline flight control and other critical functions. These advanced actuators eliminate the need for centralized hydraulic systems, which traditionally rely on multiple pumps, reservoirs, and an intricate network of plumbing running throughout the aircraft. In contrast, the EHA design integrates the pumps, reservoirs, and valves directly into the hydraulic cylinder. This setup reduces the need for extensive tubing and simplifies the system. The actuator requires only electrical power and control inputs, while onboard electronics, sensors, and dual pumps precisely manage the flight surface movements.

Smooth Landing with Hydraulic Systems

Another crucial function of aircraft hydraulic systems is the operation of landing gear. Hydraulic actuators extend and retract the landing gear during takeoff and landing, facilitating a safe transition between flight and ground operations. This system also improves aerodynamics compared to fixed landing gear. The landing gear mechanism combines traditional hydraulics with hydropneumatic shock absorbers to ensure a smooth and safe landing.

As expected, the deployment and retraction of landing gear are managed through cylinders, but with some clever modifications for safe and effective operation. The landing gear design incorporates multiple axes of motion, torque arms, and locking mechanisms. All three landing gear struts operate from the same control system, which includes pumps, valves, and a reservoir. It’s an all-or-nothing system, ensuring that all three landing gear components are either fully extended or retracted at the same time for optimal safety and performance.

The shock absorbers in commercial aircraft offer significant advantages over fixed landing gear systems. Upon landing, the aircraft’s weight presses the landing gear into the ground, and the dual-chamber shock absorbers progressively dampen the vertical impact at increasing rates. The top chamber, filled with nitrogen, is separated from the oil-filled bottom chamber by an orifice. As the assembly compresses under load, the orifice regulates the damping rate.

A key component in this system is the metering pin—a long, conical rod that extends through the orifice. As the shock absorber compresses under vertical load, the pin’s increasing diameter fills the orifice, effectively metering oil flow and increasing the damping rate. This mechanism provides smoother shock absorption during landing. Just like automobile suspension, the landing gear struts ensure a comfortable ride for the aircraft while taxiing on runways.

Hydraulic Systems for Precise Control at Landing and Takeoff

Hydraulic systems also play a critical role in aircraft braking, precisely controlling the pressure applied to the brakes for smooth and safe deceleration. While high-speed braking is aided by spoilers on the wings and tail, you might be surprised to learn that the wheels themselves feature powerful hydraulic brakes. These systems are designed with redundancies to ensure the aircraft can always come to a complete stop when necessary. Additionally, hydropneumatic accumulators provide backup energy in case of a pump failure, ensuring the braking system remains operational.

Hydraulic systems in aircraft are essential for the smooth operation of various components, including cargo doors, wing flaps, and slats. These systems enable cargo doors to open and close quickly, facilitating efficient loading and unloading on larger aircraft. Additionally, hydraulic actuators control wing flaps and slats, which are crucial for increasing lift during takeoff and landing. The precision and reliability of hydraulic systems are key to ensuring these functions operate seamlessly.

Maintaining High Standards

While hydraulic systems are used across many industries, their application in aviation has unique requirements. Aircraft hydraulic systems must meet rigorous safety standards due to the high stakes of flight operations. These systems need to be both reliable and redundant to ensure continuous functionality during flight. The manufacturing process of aircraft hydraulic components includes strict quality control and extensive nondestructive testing procedures.

For instance, raw materials undergo ultrasonic testing to detect any cracks or inclusions. When parts are cast, they are tested using ultrasonic scans to identify any flaws. Machined parts also go through additional nondestructive testing, such as magnetic particle or dye penetrant tests, especially for machined threads.

Aircraft hydraulic systems are designed to be lightweight and compact to reduce the impact on aircraft weight and space. This often leads to innovative design solutions not commonly found in other industries. Furthermore, hydraulic fluids used in aviation are specially formulated to withstand extreme temperatures, pressures, and are fire-resistant, ensuring they won’t support flames under any circumstances, unlike typical mineral oils which can combust.

Aircraft hydraulic systems must operate reliably in diverse environmental conditions, from the extreme cold at high altitudes to the intense heat of runways. This requires careful engineering to ensure consistent performance regardless of external factors.
Aircraft hydraulic systems are also subject to rigorous maintenance and inspection protocols to ensure they remain in optimal condition for flight safety. This level of scrutiny is unique to aviation, where safety is of utmost importance.

Hydraulic systems play a vital role in the safe and efficient operation of modern aircraft. Their advantages, including high power-to-weight ratio, precision, reliability, and compact design, make them the preferred technology in aviation. While they share similarities with hydraulic systems in other industries, aircraft systems are uniquely designed to meet the demanding standards of safety, compactness, and reliability. Understanding the complexities of these systems sheds light on the advanced engineering that ensures the safe operation of aircraft, and as aviation continues to evolve, hydraulic systems will remain a cornerstone of air travel for future generations.