Hydraulic power systems

Enormous forces from compact components

Hydraulics allow aircraft to generate enormous forces using relatively small and lightweight components. Modern transport aircraft rely on hydraulic power to move flight controls, landing gear, brakes, thrust reversers, cargo doors and many other systems. Because hydraulic systems are critical to safe flight, they are designed with multiple independent power sources, extensive redundancy and strict maintenance procedures.

The basic design philosophy is simple: a single hydraulic failure must not cause the loss of essential flight controls.

Why aircraft use hydraulics

Aircraft need to move large aerodynamic surfaces and heavy components, such as ailerons, elevators, rudders, spoilers, landing gear and thrust reversers, against substantial loads. The forces involved are often far beyond what a pilot or small electric motor could generate directly. Hydraulics provide:

• High force capability
• Precise control
• Relatively low weight
• High reliability
• Compact component size

Force from pressure

Hydraulic systems are based on Pascal's law, which states that pressure applied to a confined fluid acts equally in all directions. Because fluid pressure is transmitted throughout the system, a small force applied at one location can create a much larger force elsewhere.

The basic equation is force = pressure × area, with force in pounds (or newtons), pressure in psi and area in square inches. For example, an actuator with 3,000 psi acting on a 10 in² piston produces 3,000 × 10 = 30,000 pounds of force, the equivalent of 15 tons from a relatively small actuator.

Hydraulic operating pressure

Most transport aircraft operate at approximately 3,000 psi. This became the industry standard for many decades because it provided a good balance between performance, reliability and component life.

Newer aircraft increasingly use 5,000 psi hydraulic systems. Aircraft such as the Airbus A380 and Boeing 787 use higher-pressure hydraulic systems to improve efficiency. Benefits include:

• Smaller actuators
• Smaller tubing
• Reduced fluid volume
• Lower overall weight

Hydraulic system redundancy

Hydraulic power is considered flight-critical. A single leak, pump failure or engine shutdown cannot be allowed to remove control of the aircraft. For this reason, large transport aircraft commonly employ three independent hydraulic systems, physically separated wherever practical. Each system has its own reservoir, pumps, distribution lines and fluid supply.

The objective is that the loss of any one hydraulic system still leaves sufficient control authority to fly and land the aircraft safely.

Boeing 777 example

The Boeing 777 uses three independent systems, left, center and right, commonly identified by color: left is red, center is blue and right is green.

• Left and right systems: each normally uses an engine-driven pump (EDP) for primary hydraulic power and an AC motor-driven demand pump that operates automatically when additional pressure or redundancy is required.
• Center system: designed primarily for redundancy and high-demand functions. It includes two AC motor pumps, two air-driven demand pumps and ram air turbine (RAT) backup capability, and often powers large loads such as landing gear and flaps.

Airbus A330 example

Airbus generally identifies hydraulic systems by color rather than physical location. Although the architecture differs from Boeing designs, the same redundancy philosophy applies. On the Airbus A330:

• Green system: powered by engine-driven pumps on engines 1 and 2, with the RAT for emergencies.
• Blue system: powered by an engine-driven pump on engine 1, backed by an electric pump.
• Yellow system: powered by an engine-driven pump on engine 2, backed by an electric pump and a hand pump for cargo door operation.

Reservoir

The reservoir stores hydraulic fluid and supplies the pumps. Its functions include fluid storage, accommodating thermal expansion, deaerating returning fluid and system servicing.

Most transport aircraft use pressurized reservoirs. Without adequate reservoir pressure, pumps may ingest air and lose efficiency. Advantages include:

• Improved pump inlet pressure
• Reduced cavitation risk
• Reliable operation at altitude

Pumps

Hydraulic pressure originates from pumps. Common pump types include:

• Engine-driven pumps (EDPs): the primary source of hydraulic power, operating whenever engines are running.
• Electric motor pumps: powered by aircraft electrical systems and used for backup and demand operation.
• Air-driven pumps: powered by pneumatic systems, often serving as supplementary pumps.
• Ram air turbine (RAT) pumps: deploy during severe emergencies and use airflow to generate hydraulic and/or electrical power.

Filters

Hydraulic systems require extremely clean fluid. Contaminated fluid can damage pumps, servo valves, actuators and flight controls. Hydraulic filters remove contaminants and often include bypass valves and differential pressure indicators.

Many aircraft use pop-out indicators that show when the filter becomes clogged or the pressure drop across the element becomes excessive. A popped indicator usually requires maintenance action.

Accumulators

Hydraulic accumulators store energy in the form of compressed nitrogen gas. Think of an accumulator as a hydraulic spring. Even after pumps stop running, accumulators may supply pressure for parking brakes, emergency braking and limited flight-control operation. Functions include:

• Absorbing pressure surges
• Reducing pump cycling
• Damping pulsations
• Providing temporary emergency pressure

Priority valves

During low-pressure situations, aircraft must continue powering essential systems. Priority valves protect flight-critical functions and isolate non-essential users, so the aircraft preserves pressure for the systems that matter most. Examples of lower-priority loads may include:

• Cargo doors
• Utility actuators
• Secondary systems

Heat exchangers

Hydraulic systems generate substantial heat. If fluid overheats, viscosity changes, components wear faster and seal life decreases. Heat exchangers remove the excess heat.

Many transport aircraft place hydraulic heat exchangers inside fuel systems. Fuel acts as a heat sink and absorbs hydraulic heat, improving efficiency without adding excessive weight.

Hydraulic fluids

Most transport aircraft use phosphate-ester fire-resistant hydraulic fluids such as Skydrol. These fluids provide excellent fire resistance compared with petroleum-based fluids. Although safer around fire hazards, they can:

• Irritate skin
• Irritate eyes
• Damage paint
• Damage incompatible seals
• Deteriorate certain materials

Handling fluid safely

Always wear eye protection, gloves and appropriate protective equipment when handling hydraulic fluid.

Never mix fluids. Mixing incompatible hydraulic fluids can damage seals, degrade performance, contaminate systems and create expensive repair requirements. Always verify fluid specifications before servicing.

Working safely around hydraulics

Hydraulic systems can be dangerous because of extremely high pressure, unexpected actuator movement, stored energy in accumulators, hot fluid and chemical exposure.

Before pressurizing a system, verify that all affected areas are clear. Unexpected movement can cause serious injury. Potentially moving components include:

• Flight controls
• Flaps
• Slats
• Spoilers
• Landing gear
• Gear doors
• Thrust reversers
• Cargo doors

Beware of stored pressure

Hydraulic pressure may remain trapped after pumps stop, in accumulators, pressure-retaining valves and isolated sections of the system. Never assume pressure is gone simply because a pump is OFF.

Before disconnecting any hydraulic component, follow the maintenance procedure, relieve stored pressure, verify pressure indicators and use approved safety precautions. High-pressure hydraulic fluid can penetrate skin and cause severe injuries.

Fluid injection hazards

A hydraulic leak may appear minor but can be extremely dangerous. A fine spray from a pressurized leak can penetrate skin, cause tissue damage and require emergency medical treatment. Never search for leaks with your hands. Use approved leak-detection methods.

Hydraulic failure protection

These layers help ensure that hydraulic power remains available even after significant failures. Modern transport aircraft incorporate numerous protections, including:

• Multiple independent systems
• Isolation valves
• Priority valves
• Backup pumps
• Emergency accumulators
• RAT deployment
• Fault monitoring systems

Common hydraulic maintenance findings

Many of these issues can be detected during routine inspections before they affect aircraft operation. Technicians frequently encounter:

• External leaks
• Low reservoir quantity
• Air in the system
• Contaminated fluid
• Pump wear
• Filter bypass indications
• Accumulator precharge issues
• Chafed hydraulic lines
• Damaged seals

Key takeaways

• Hydraulic force is determined by pressure × area.
• Most transport aircraft operate at 3,000 psi, while newer designs often use 5,000 psi systems.
• Multiple independent hydraulic systems ensure flight control survivability following failures.
• Reservoirs, pumps, filters, accumulators, priority valves and heat exchangers are core hydraulic components.
• Engine-driven pumps provide primary hydraulic power, while electric, air-driven and RAT-powered pumps provide backup.
• Accumulators can retain pressure even after pumps stop.
• Phosphate-ester hydraulic fluid is fire-resistant but can damage skin, eyes, paint and incompatible materials.
• Always clear movement areas before pressurizing a system.
• Always relieve stored pressure before opening hydraulic lines.

The bottom line

Hydraulic systems allow aircraft to generate enormous forces using compact, lightweight components, making them essential for flight controls, landing gear, braking systems and many other critical functions. Through multiple independent systems, redundant pumps, accumulators, emergency power sources and strict maintenance practices, transport aircraft ensure that hydraulic power remains available even during major failures. For technicians, understanding system architecture and respecting the hazards of high-pressure fluid are essential to both aircraft safety and personal safety.