Power in Fluid: What are Aircraft Hydraulic Systems, and What Functions Do They Control?

Hydraulic systems are vital in multiple areas of aircraft, responsible for generating the force behind tasks ranging from fine-tuned control surface movements to powering the various tasks required of landing gear assemblies. These systems transmit energy through the circulation of pressurized fluid—typically a specialized, non-compressible oil—delivering consistent and controlled mechanical power.

What makes hydraulic systems particularly valuable in aerospace is their impressive power-to-mass efficiency, enabling compact installations that handle demanding mechanical needs without significantly adding to an aircraft’s weight. They are also built to withstand the high altitudes, temperature fluctuations, and rapidly shifting loads associated with flight. In this blog, we will go over some notable aircraft systems that utilize hydraulics, as well as take a closer look at how they are designed and integrated to support a wide array of essential functions.

Flight Control Systems

One of the most important applications of hydraulics is in the manipulation of primary flight control surfaces, including the ailerons, elevators, and rudder. Given that they govern an aircraft’s roll, pitch, and yaw motions, respectively, these surfaces must respond quickly and precisely to pilot inputs in the face of aerodynamic forces.

Secondary flight controls like flaps, slats, speed brakes, and trims also rely on hydraulics for powerful actuation within the limited confines of wing and tail structures. These systems adjust lift and drag characteristics to optimize performance during critical phases like takeoff, landing, and descent. In the case of both primary and secondary surfaces, hydraulics deliver the necessary power for stable, coordinated, and responsive handling across all flight phases.

Landing Gear Systems

The retraction and extension of landing gear demands swift action under large mechanical loads, and hydraulic systems are indispensable for this function. Hydraulic actuators power the movement of gear struts between deployed and stowed positions, while also working in close coordination with gear door mechanisms. These doors have to be opened fully before the gear extends and secured again once retracted, which is where dedicated hydraulic actuators or sequencing valves come in to provide accurate timing.

In addition to positioning, hydraulic systems also assist with shock absorption during landing. Most modern aircraft employ oleo-pneumatic struts, which use a combination of compressed gas and hydraulic fluid within a sealed cylinder to evenly absorb and dissipate vertical forces during touchdown. This system greatly reduces stress on the airframe and enhances passenger comfort.

Braking Functions

On the ground, aircraft need powerful braking systems for safe deceleration. Many aircraft use multi-disc brake assemblies housed within the main landing gear wheels, which depend on hydraulic pressure supplied through reinforced brake lines. In traditional systems, this pressure may be regulated with pilot-operated master cylinders. Meanwhile, more advanced designs tend to employ electro-hydraulic braking systems where electronic brake control units signal hydraulic servovalves.

For either case, these hydraulic systems incorporate heat-resistant seals, pressure-compensating valves, and other thermal protection features to help them continue to perform under extreme heat caused by friction.

Steering Functions

Directional control during taxiing can also be managed with hydraulically actuated nose wheel steering systems. These steering actuators are designed to respond proportionally to rudder pedal input or tiller rotation, allowing for stable and responsive maneuvering in airport environments.

Thrust Reversers

Hydraulic systems also help deploy thrust reversers on jet engines, which are utilized after touchdown to redirect exhaust forward. By doing so, they shorten landing rollout distance and minimize the load placed on brake systems.

Commonly configured as translating sleeves or clamshell doors, these systems are built with robust hydraulic actuators that deliver high force in a compact form to overcome aerodynamic resistance and time-sensitive deployment demands. They are usually closely integrated with sensors and position switches, which facilitate precise sequencing and prevent deployment unless specific ground conditions are confirmed.

Cargo Doors and Utility Functions

Various utility functions rely on hydraulic systems, one example being cargo doors and service hatches. There, robust hydraulic actuators ensure that they open and close smoothly, even in the presence of high external wind loads or pressure differentials. The systems here often also incorporate hydraulic locks to secure the doors in place, sequencing valves to coordinate the order of operations, and interlocks tied to pressure and position sensors to prevent movement unless all safety criteria are met.

Standard Design & Layout Considerations for Aircraft Hydraulic Systems

Compact and Modular Design

Due to the strict space and weight limitations of aircraft—particularly in densely packed areas like wing roots, fuselage bays, and landing gear wells—hydraulic systems are engineered to be compact. To optimize their layout and simplify maintenance access, designers incorporate features like integrated valves, multifunction actuators, and quick-disconnect fittings. These elements not only reduce the length and complexity of hydraulic lines, but also enhance serviceability of all associated components in such tight quarters.

Energy Management Components

Efficient hydraulic performance within confined aircraft structures depends on the strategic use of energy management components. Accumulators, for instance, store pressurized fluid to smooth out pressure fluctuations that might occur during peak demand or brief pump interruptions. Their compact design allows them to be installed close to high-use actuators, improving response time without adding unnecessary mass or volume. Additionally, pressure regulators and relief valves are distributed throughout the system to maintain stable flow and pressure across multiple operating conditions.

System Redundancy

Redundancy is essential for aviation hydraulic systems in the event of pump failure, fluid loss, or engine malfunction. Most large commercial and military aircraft are equipped with a minimum of three independent hydraulic systems, each one usually being powered by an engine-driven pump, an electric motor-driven pump, or both. This ensures that if one system goes down, others can continue to operate.

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