Hydraulic systems

What aircraft systems will need to use hydraulics?

• Landing gear

• Flaps and slats

• Flight controls

• Nosewheel steering

• Wheel brakes

• Windshield wipers

• Role equipment (cargo doors etc)

Principle of operation

Hydraulic systems provide a means of transmission of power through an incompressible fluid via pipelines and actuators.

What are the six main components common to all hydraulic systems?

1. Reservoir of oil, which delivers oil to the pump and receives oil from the actuators

2. A pump, either hand, engine or electrically driven

3. A selector or control valve, enabling the operator to select the direction of flow of fluid for a required service, and a return path to the reservoir

4. A jack to actuate the component

5. A filter, to keep the fluid clean

6. A relief valve, as a safety device to relieve excess pressure

Active systems

• A pump is required to deliver a flow of fluid into the system, and some form of restriction is required to obtain pressure

• This restriction is provided by moveable pistons known as jacks or actuators

Constant pressure system

• Supplies fluid at a constant rate

• Single or double stage gear pump

• Pressure applied all the time

• Large flow at a small pressure

• Fast reaction time when hydraulic services are selected

• Needs and ACOV

Demand system

• Supplies fluid at a variable volume and controls its own pressure

• High pressure built up on demand

• Reaction time is much slower

• Components last longer as they are not subject to constant pressure

Automatic Cut-Out-Valve (ACOV)

Returns the fluid to the reservoir when:

• jacks ready their end of travel

• system is not operating

High pressure systems

• Weight: smaller actuators, lighter

• Force delivered: able to deliver extreme forces

• Ease of construction: complex

• Pressure delivered: above 2,000psi

Low pressure systems

• Weight: more fluid needed, heavier

• Force delivered: unable to deliver extreme forces

• Ease of construction: simple

• Pressure delivered: below 2,000psi

Types of jacks (actuators)

• Single acting - normally used as a locking device, the lock being engaged by spring pressure and released by hydraulic pressure

• Double acting - because of the presence of the piston rod, the wire of the top of the piston is greater than the area under it. Consequently, more force can be applied during extension of the piston rod. Therefore, the operation which offers the greater resistance is carried out in the direction in which the piston rod extends, eg raising the landing gear

• Balanced actuator - equal force can be applied to both sides of the piston, often used in applications as nosewheel steering

Sources of hydraulic pressure

Pumps may be

• hand operated

• Engine driven

• Electric motor driven

• Pneumatically driven - air turbine motor (ATM)

• Ram air turbine (RAT)

• Power transfer unit (PTU) - hydraulic motor driving a hydraulic pump

Hand pump

The hand pump is usually a double-acting pump (delivers oil on both strokes) in a very compact body.

Reservoirs

• Stores the hydraulic fluid

• Normally pressured but may be vented to atmosphere 

• It allows for fluid returning from the low pressure side

• Holds excess fluid to compensate for leaks 

• Has a sight glass to check the level when

• Pressurised to prevent compression 

• Fins help to reduce random movement

Accumulators

• to store hydraulic fluid under pressure. • to dampen pressure fluctuations.

• to allow for thermal expansion.

• to provide an emergency supply of fluid to the system in the event of pump failure.

• to prolong the period between cut-out and cut-in time of the ACOV, and so reduce the wear on the pump.

• to provide the initial fluid when a selection is made, and the pump is cut-out.

Filters

• Remove foreign particles from the fluid which will protect the seals and working surfaces in the components

• Relief valve which allows unfiltered fluid to pass the system when the element becomes clogged

Hydraulic motors

Converts hydraulic pressure into rotary motion. The speed of a hydraulic motor is dependent on the flow rate of hydraulic fluid into it

Pressure control

Act as safety devices

1. Thermal relief valves: Prevents overheating, allows pressure to escape 10% tolerance

2. Full flow relief valves: Fitted downstream and can completely bypass the system

3. Flap relief: Prevents mechanical overload eg flow relief 

4. Pressure reducing valve: Reduces pressure for devices that don’t need high pressure like 3000psi

5. Pressure maintaining valve: Sacrifices secondly services to keep the primary services running

6. Brake control valves: Control pressure in the braking system to prevent skidding

Relief valve

This type of relief valve is used as a final system protection, not as the system pressure controller

Flow control

• relief valve

• Restrictor valve

• Pressure maintaining valve

• Shuttle valve

• Hydraulic fuse

• Shut off valves

• Non returning/check valve

Restrictor valve (choke valve)

• Reduces fluid flow to slow speed of operation of a service

• A one-way restrictor allows full flow in one direction and restricted flow in the other

• This can either balance out the speed of operation of an unbalanced ram or allow for difference of operational speed

Pressure maintaining valve

• Ensures sufficient fluid flow to the primary controls and wheel brakes

• Isolates non-essential circuits if system pressure drops below a set limit

Shuttle valve

• One service to be operated by two independent supplies

• Guarantees an adequate supply if either system fails

• Operates on differential pressure

• Typically used to switch to the emergency system for landing gear or flap lowering

Hydraulic fuse

• Upstream components shut off flow and prevent fluid loss

• Once operated service downstream of fuse will be isolated

• System upstream of the fuse should still function normally

Shut off valves

• Similar to fuses in that they can shut-off services.

• Isn't self regulating but is operated by an external controller. Eg a control on the flight deck.

• Typically electrically operated solenoid valve.

Non return/check valve

A non return valve, or check valve only allows flow in one direction. If the pressure down stream increase to that of the supply the valve closes.

Redundancy

• Cannot allow loss of hydraulic services

• Multiple systems

• Transfer to PTU, ATM, RAT if needed

Pressure relays

In the reservoir to warn of overheating of electrical motors

Pressure gauges

Fitted after a constant delivery pump to warm or pump failure

Pressure switches

Transmits fluid pressure to a direct reading pressure gauge

Flow indicator valve

Used to illuminate a warning lamp

Temperature indication

Electrially operated to register main and emergency system pressure