Aircraft Hydraulic Systems: The Basics Every A&P Student Should Know

Aircraft hydraulic systems are one of those topics every A&P student needs to understand because hydraulics are used all over the airplane. Depending on the aircraft, hydraulic power may be used for landing gear, brakes, flaps, flight controls, spoilers, nosewheel steering, thrust reversers, cargo doors, and other heavy-duty systems.

The basic idea is simple: hydraulics use pressurized fluid to transmit force.

In other words, instead of using only cables, rods, gears, or electrical motors to move something heavy, an aircraft can use hydraulic pressure to do the work.

Why Aircraft Use Hydraulics

Aircraft need systems that are strong, reliable, and able to move heavy loads with smooth control. Hydraulics are useful because they can transmit a large amount of force through relatively small lines and components.

Hydraulic systems are commonly used because they can:

• Move heavy components with less physical effort
• Operate smoothly and precisely
• Transmit force around corners and through tight spaces
• Handle high loads without requiring large mechanical linkages
• Provide fast and powerful movement

For example, lowering landing gear or applying aircraft brakes requires a lot of force. Hydraulics make that possible without needing the pilot or mechanic to physically supply all of that force.

The Basic Principle: Pascal’s Law

The foundation of hydraulic systems is Pascal’s Law.

Pascal’s Law says that pressure applied to a confined fluid is transmitted equally in all directions throughout the fluid.

A simple way to think about it:

If you push on hydraulic fluid in one part of a closed system, that pressure is carried through the fluid to another part of the system.

This is what allows a small input force to create a much larger output force.

For example, if a small piston applies pressure to hydraulic fluid, that pressure can act on a larger piston and produce a greater force. This is how hydraulic systems can multiply force.

Pressure, Force, and Area

A key formula used in hydraulics is:

Pressure = Force ÷ Area

Or:

P = F / A

Where:

• P = pressure
• F = force
• A = area

The related formula is:

Force = Pressure × Area

Or:

F = P × A

This is important because the larger the piston area, the more force can be produced from the same hydraulic pressure.

For example, if a hydraulic system has 1,000 psi of pressure acting on a piston with an area of 4 square inches:

Force = Pressure × Area
Force = 1,000 psi × 4 in²
Force = 4,000 pounds

That means the actuator can produce 4,000 pounds of force.

That is why hydraulics are so useful on aircraft.

Main Parts of an Aircraft Hydraulic System

A basic aircraft hydraulic system may include:

• Reservoir
• Hydraulic pump
• Filters
• Pressure lines
• Return lines
• Selector valves
• Check valves
• Relief valves
• Actuators
• Accumulators
• Hydraulic fluid

Not every aircraft system is the same, but these components are common in many hydraulic systems.

Hydraulic Reservoir

The reservoir stores the hydraulic fluid.

It also allows room for fluid expansion and helps separate air from the fluid. Some reservoirs are pressurized, especially on larger aircraft, to help prevent pump cavitation and ensure a steady supply of fluid to the pump.

A reservoir may include:

• Filler opening
• Sight gauge or dipstick
• Vent or pressurization connection
• Outlet to the pump
• Return line connection

A low reservoir fluid level can cause poor system operation, pump noise, foaming, or complete system failure.

Hydraulic Pump

The hydraulic pump moves fluid through the system and creates flow. The resistance to that flow creates pressure.

Aircraft hydraulic pumps may be:

• Engine-driven pumps
• Electric motor-driven pumps
• Hand pumps
• Air-driven pumps
• Power transfer units, depending on the aircraft

The pump does not technically “create pressure” by itself. The pump creates flow, and pressure is created when that flow meets resistance in the system.

For example, when an actuator reaches the end of its travel or when a brake is applied, resistance increases and system pressure rises.

Hydraulic Fluid

Hydraulic fluid is the medium that transmits force through the system.

Aircraft hydraulic fluid must be able to:

• Flow at low temperatures
• Resist foaming
• Lubricate components
• Resist corrosion
• Handle high pressure
• Remain stable under operating conditions

A very important A&P point is this:

Always use the correct type of hydraulic fluid specified for the aircraft.

Mixing the wrong hydraulic fluids can damage seals, hoses, and other components. Some fluids are not compatible with certain materials.

Common types of aircraft hydraulic fluids include mineral-based fluids and phosphate ester-based fluids such as Skydrol-type fluids.

Always check the aircraft maintenance manual before servicing a hydraulic system.

Hydraulic Lines

Hydraulic lines carry fluid through the aircraft.

There are usually two main types of lines:

• Pressure lines
• Return lines

Pressure lines carry high-pressure fluid from the pump or pressure source to the component being operated.

Return lines carry fluid back to the reservoir after it has done its work.

Hydraulic lines may be rigid metal tubing or flexible hose, depending on location and system requirements.

A&P students should remember that hydraulic lines must be inspected for:

• Leaks
• Chafing
• Cracks
• Kinks
• Corrosion
• Loose fittings
• Improper routing
• Damaged clamps

Hydraulic leaks are serious because they can lead to loss of system pressure and possible system failure.

Filters

Hydraulic systems use filters to remove contamination from the fluid.

Contamination is one of the biggest enemies of a hydraulic system. Small particles can damage pumps, valves, actuators, and seals.

Filters may be located in:

• Pressure lines
• Return lines
• Pump case drain lines
• Reservoir fill ports

A clogged filter can restrict fluid flow and cause system problems. Some filters have bypass valves so fluid can still flow if the filter becomes clogged, but bypassed fluid may no longer be properly filtered.

Selector Valves

A selector valve directs hydraulic fluid to the part of the system that needs to operate.

For example, when the pilot selects landing gear down, a selector valve may direct hydraulic pressure to the extend side of the landing gear actuators. When the pilot selects gear up, the valve directs pressure to the retract side.

Selector valves are like traffic directors for hydraulic fluid.

They control where the pressure goes.

Check Valves

A check valve allows fluid to flow in one direction but prevents it from flowing backward.

This is important in hydraulic systems because backflow can cause loss of pressure, unwanted movement, or improper system operation.

A simple way to remember it:

A check valve is a one-way valve.

Relief Valves

A relief valve protects the hydraulic system from excessive pressure.

If pressure becomes too high, the relief valve opens and allows fluid to return to the reservoir or low-pressure side of the system.

This prevents damage to:

• Pumps
• Lines
• Seals
• Actuators
• Valves
• Fittings

A relief valve is a safety device. It keeps the hydraulic system from over-pressurizing.

Hydraulic Actuators

A hydraulic actuator changes hydraulic pressure into mechanical movement.

There are two common types:

• Linear actuators
• Rotary actuators

A linear actuator moves in a straight line. This type is commonly used for landing gear, flaps, cargo doors, and other components that need push-pull motion.

A rotary actuator produces rotating movement. These may be used where turning or twisting motion is needed.

A simple landing gear actuator works by applying pressure to one side of a piston. The piston moves, and that motion extends or retracts the landing gear.

Single-Acting and Double-Acting Actuators

A single-acting actuator uses hydraulic pressure to move in one direction. Return movement may be done by a spring, gravity, or another force.

A double-acting actuator uses hydraulic pressure to move in both directions.

For example:

• Pressure on one side extends the actuator.
• Pressure on the other side retracts the actuator.

Double-acting actuators are common in aircraft systems because they provide positive control in both directions.

Accumulators

An accumulator stores hydraulic pressure.

It usually contains hydraulic fluid on one side and compressed gas, often nitrogen, on the other side. The gas is separated from the fluid by a piston, bladder, or diaphragm.

Accumulators can be used to:

• Store emergency pressure
• Smooth out pressure surges
• Reduce pump cycling
• Provide pressure for limited system operation
• Absorb shocks in the hydraulic system

A common A&P point:

Accumulators are usually precharged with dry nitrogen, not oxygen or shop air.

Oxygen should not be used because it can create a fire or explosion hazard when combined with hydraulic fluid and pressure.

Hydraulic System Pressure

Different aircraft use different hydraulic pressures.

Some light aircraft systems may use relatively low pressure, while transport-category aircraft may use high-pressure hydraulic systems.

Common hydraulic system pressures include:

• 560 psi
• 1,000 psi
• 1,500 psi
• 3,000 psi
• 5,000 psi on some modern aircraft

The exact pressure depends on the aircraft design.

For A&P work, do not guess. Always check the aircraft maintenance manual.

Open-Center and Closed-Center Hydraulic Systems

Hydraulic systems can be designed in different ways.

Two common terms are:

• Open-center system
• Closed-center system

In an open-center hydraulic system, fluid can flow continuously through the system when no component is being operated. The pump keeps fluid moving, but pressure stays relatively low until a component is selected.

In a closed-center hydraulic system, fluid flow is blocked when no component is being operated. The system maintains pressure and only sends flow when needed.

A simple way to remember it:

Open-center = flow continues when no work is being done.
Closed-center = pressure is held until work is needed.

Aircraft Brakes and Hydraulics

Aircraft brakes are one of the most common examples of hydraulic power.

When the pilot presses the brake pedals, hydraulic pressure is sent to the brake assemblies. That pressure pushes pistons in the brake caliper or brake assembly, creating friction to slow or stop the wheel.

Hydraulic brakes are effective because they allow a manageable pedal force to create much greater braking force at the wheels.

Brake systems may include:

• Master cylinders
• Brake lines
• Parking brake valves
• Brake assemblies
• Reservoirs
• Anti-skid components on larger aircraft

A spongy brake pedal may indicate air in the system, low fluid, leaks, or other problems.

Landing Gear and Hydraulics

Many retractable landing gear systems use hydraulics to extend and retract the gear.

A hydraulic landing gear system may include:

• Gear selector
• Hydraulic pump
• Gear actuators
• Uplocks
• Downlocks
• Emergency extension system
• Position switches
• Gear indication lights

Hydraulic pressure moves the gear, but mechanical locks usually hold the gear in the up or down position.

This is important because hydraulic pressure alone should not be the only thing keeping landing gear locked.

Common Hydraulic System Problems

Hydraulic systems are reliable, but they can have problems.

Common hydraulic system issues include:

• Low fluid level
• Leaks
• Air in the system
• Contaminated fluid
• Clogged filters
• Pump failure
• Internal actuator leakage
• Defective relief valve
• Incorrect fluid
• Damaged seals
• Overheating

A hydraulic system problem may show up as slow operation, noisy pump operation, low pressure, erratic movement, overheating, or total failure of a component.

Air in the Hydraulic System

Air in a hydraulic system is a problem because air is compressible.

Hydraulic fluid is mostly incompressible, which is why it works well for transmitting force. Air, however, compresses under pressure.

Air in the system can cause:

• Spongy brakes
• Jerky actuator movement
• Slow system response
• Noise
• Foaming fluid
• Poor system performance

This is why hydraulic systems often need to be properly bled after maintenance.

Hydraulic Leaks

Hydraulic leaks should always be taken seriously.

A small leak can become a major problem, especially in a high-pressure system.

Leaks may occur at:

• Fittings
• Hoses
• Tubing
• Actuator seals
• Pump seals
• Valve bodies
• Reservoir connections

When inspecting for leaks, never use your hand to search for a high-pressure hydraulic leak.

High-pressure fluid can penetrate the skin and cause serious injury.

Use proper inspection methods and follow the aircraft maintenance manual.

Hydraulic Safety Precautions

Hydraulic systems can be dangerous because they may contain very high pressure.

Important safety practices include:

• Relieve system pressure before disconnecting lines
• Use eye protection
• Keep hydraulic fluid away from skin and eyes
• Clean spills immediately
• Use the correct hydraulic fluid
• Never mix incompatible fluids
• Do not use oxygen to service accumulators
• Follow lockout and safety procedures
• Support aircraft components before working on hydraulic actuators
• Keep contamination out of the system

A landing gear door, flap, or flight control surface can move suddenly if hydraulic pressure is applied unexpectedly.

Always think about stored pressure before working on a hydraulic system.

A&P Oral and Written Test Style Questions

Here are some common A&P-style questions related to hydraulic systems.

What law explains the operation of hydraulic systems?

Pascal’s Law.

Pressure applied to a confined fluid is transmitted equally in all directions.

What does a hydraulic pump do?

A hydraulic pump creates fluid flow. Pressure develops when that flow meets resistance.

What component stores hydraulic pressure?

An accumulator stores hydraulic pressure.

What gas is normally used to precharge a hydraulic accumulator?

Dry nitrogen is normally used.

Why should oxygen not be used in a hydraulic accumulator?

Oxygen can create a fire or explosion hazard when exposed to hydraulic fluid under pressure.

What is the purpose of a relief valve?

A relief valve protects the hydraulic system from excessive pressure.

What is the purpose of a check valve?

A check valve allows fluid to flow in one direction and prevents reverse flow.

What can cause spongy hydraulic brakes?

Spongy brakes may be caused by air in the hydraulic system, low fluid, leaks, or improper bleeding.

Why is contamination bad in a hydraulic system?

Contamination can damage pumps, valves, seals, actuators, and other precision components.

What should you check before servicing a hydraulic system?

Always check the aircraft maintenance manual for the correct fluid type, pressure, servicing procedure, and safety precautions.

Quick Memory Tips

Here are a few simple ways to remember hydraulic system concepts:

Hydraulics = force through fluid

Pascal's Law = pressure applied to fluid is transmitted equally

Pump = creates flow

Resistance to flow = creates pressure

Accumulator = stores pressure

Relief valve = protects from too much pressure

Check valve = one-way valve

Actuator = turns pressure into movement

Final Thoughts

Aircraft hydraulic systems may seem complicated at first, but the basic idea is not too bad:

Hydraulic fluid carries pressure, and that pressure is used to move aircraft components.

Once you understand Pascal’s Law, pressure, force, area, pumps, valves, actuators, and accumulators, the rest of the system becomes much easier to understand.

For A&P students, hydraulics are important because they connect theory with real aircraft maintenance. You are not just memorizing parts. You are learning how force is created, controlled, directed, and safely maintained in an aircraft system.

Whether you are working on brakes, landing gear, flaps, or flight controls, the same basic hydraulic principles apply.

References for Further Study

For A&P study, review the FAA Aviation Maintenance Technician Handbook sections related to aircraft hydraulic and pneumatic systems.

Recommended references:

• FAA-H-8083-30B, Aviation Maintenance Technician Handbook — General
• FAA-H-8083-31B, Aviation Maintenance Technician Handbook — Airframe
• FAA-H-8083-32B, Aviation Maintenance Technician Handbook — Powerplant
• FAA AC 43.13-1B, Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair

Always use the current aircraft maintenance manual when performing actual maintenance.