1. Hydraulic Systems & Undercarriage
Chapter 10

2. Aim
To review principles of operation of the Hydraulic Systems & Undercarriage

3. Objectives
Define Hydraulics and discuss the principles of operation of a typical Hydraulic Systems
State principals of operation of retractable Undercarriage systems

4. 1. Hydraulic Systems
Hydraulics is the transmission of force by fluid pressure
Utilisation benefits:
Accessibility
Simplicity
Power versus weight

5. 1. Hydraulic Systems Hydraulic Principle
All liquids have a high resistance to compression. For the purposes of hydraulics, hydraulic fluid is considered incompressible.
When under pressure, hydraulic fluid exerts equal force in all directions.
Hydraulic systems apply this concept through a system of cylinders, hydraulic fluid and pipes to amplify force.

6. 1. Hydraulic Systems Hydraulic Liquid Desirable characteristics:
Low viscosity
Low volatility
Non-corrosiveness
Stability
Inflammability
Colouring
Non-toxicity
Incompressibility
Low freezing point
Common types:
Mineral (petroleum based)
Synthetic fluid
Do not mix hydraulic fluids as this can cause coagulation.
Restrictors are small diameter sections within a hydraulic system that are used to control the amount of hydraulic fluid that enters at a specific pressure.

7. 1. Hydraulic Systems Hydraulic Components
The hydraulic system provides the power required to activate aircraft services that require force to move.
Each hydraulic system has a combination of the following common componentry:
Reservoir
Pumps and pressure regulator (cut-in, cut-out valve)
Accumulator
Check valves
Relief Valves
Selectors
Actuators

8. 1. Hydraulic Systems Hydraulic Components Reservoir
Stores the hydraulic liquid.
Highest point in the system.
Features include a screened filter, a vent and baffles.
Pressurised in high flying aircraft to prevent cavitation.
Any fluid beneath the stand pipe can only be used for the emergency hand pump.

9. 1. Hydraulic Systems Hydraulic Components Pumps
Supplies fluid pressure to the hydraulic system
Can be driven electrically, by the engine, air driven or by hand
Gear-type pump
Lower pressure systems up to 1500psi.
Two gears in a casing, one turned by the engine and the other is a drive gear.
At constant RPM, constant volume.
Piston-type pump
Relatively inefficient
Commonly used as a hand pump for emergencies

10. 1. Hydraulic Systems Hydraulic Components Pressure Regulator
Used with constant delivery systems allowing fluid bypass.
Maintains constant system pressure.
Relieve the pump when no load.
Variable-Displacement Pump
Varied volume of fluid dependent on angle of swash plate.
Constant system pressure, no regulator.
No pressure relief valve.
Outlet pressure adjusts swash plate angle.

11. 1. Hydraulic Systems Hydraulic Components
Accumulator – provides short-term pressure storage.
Maintains line pressure for short time following failure of the hydraulic pump or a small leak.
Assists to maintain line pressure during high utilisation.
Dampens pulsations.
Allows for thermal expansion.
Check Valves – allows hydraulic fluid to flow in one direction only.

12. 1. Hydraulic Systems Hydraulic Components Relief Valves
PRVs are safety devices that limit maximum pressure to prevent damage.
Located between pump and system components.
Thermal relief valves (TRV) relieves pressure from the effects of thermal expansion.
Selectors – selector valves or directional control valves control the distribution of hydraulic fluid to systems. Valves can be simple on/off, four-way for reserve flow or more complex arrangements.

13. 1. Hydraulic Systems Hydraulic Components
Actuator (Hydraulic Jacks) - converts the force of the fluid pressure into movement.
Filtering & Cooling – filters are often included in the reservoir and the system return line. Cooling takes place in the reservoir and occasionally a cooler is installed in the return line.

14. 1. Hydraulic Systems
Hydraulic Operation – Light Aircraft

15. 1. Hydraulic Systems
Hydraulic Operation - Heavy aircraft

16. 1. Undercarriage Systems
Undercarriage System Overview
Requirements
Absorb the compression loads of landing.
Support the weight of the aircraft while parked.
Allow the aircraft to taxi and manoeuvre while on the ground.
Arrangements
Conventional (taildragger) / Tricycle.
Fixed / Retractable.
Components include wheels, shock absorbers (spring steel or gas-strut) and less commonly skis, floats and skids, dolly’s/trolley’s.

17. 1. Undercarriage Systems
Undercarriage Components
Nosewheel
Nosewheels are of two types:
The fully castering nosewheel
The oleo-pneumatic strut nosewheel
Fully castering nosewheel
Steering is achieved via differential braking.
Oleo-pneumatic strut nosewheel
Uses a telescopic stainless steel cylinder as a shock absorber. A torque link provides steerability.
Nosewheels are susceptible to a vibration termed shimmy. This is a gyroscopic reaction and occurs most commonly immediately post take-off. The shimmy damper is a shock absorber that is used to prevent this oscillation.

18. 1. Undercarriage Systems
Undercarriage Components
Mainwheel
Mainwheels are of two types:
The leaf spring
Oleo-pneumatic strut configuration

19. 1. Undercarriage Systems
Undercarriage Components
Brakes
Brakes are installed on the mainwheels only
They are hydraulically operated and typically of the self-adjusting disc type.
Operated by the pilot toe pressure, fluid from a master cylinder is supplied under pressure to slave cylinders where brake callipers apply pressure to brakes pads.
The brakes are independent allowing differential braking.

20. 1. Undercarriage Systems
Undercarriage Components
Common faults with the braking system include:
Air in the hydraulic lines producing springy or spongy feel and less effective breaking.
Leaking fluid so the brake toe pedal moves without the pilot being able to apply maximum pressure.
Water or dirt between brake pad and disc causing reduced effectiveness and noisy brake operation.
The following factors affect braking feel and effectiveness:
Tyre pressure
Tyre condition
Aircraft weight
Runway surface condition
Aircraft speed
Aircraft brake condition
Locking a wheel will result in skidding, loss of directional control and contribute towards flat spots, tyre deflation or blow out.

21. 1. Undercarriage Systems
Undercarriage Components
Parking Brake - holds whatever pressure is in the brake lines. To operate, apply the toe brakes and then set the park brake.
Tyres – function to:
Support the aircraft on the ground.
Absorb part of the landing shock.
Provide traction during braking and steering.
Provide a wearing surface that is easily replaceable.
Tyres typically are electrical conductors grounding the aircraft.
Marked to indicate size, speed rating, ply rating, tubed or tubeless, creep and wear.
Tyre creep is the tendency for the tyre to slip on the rim of the wheel.

22. 1. Undercarriage Systems
Retractable Undercarriage
Retractable undercarriages reduce form drag but add weight
Operated most commonly hydraulically but can be electric and/or pneumatic
Required Components:
Up and down locks.
Position indicators.
Emergency extension method.
Emergency operation is usually via a hand pump or an emergency extension valve (legs fall and lock via momentum/air pressure).

23. 1. Undercarriage Systems
Retractable Undercarriage Components
Position Indicators
Typically three green lights, one red, and the selector are used to control and indicate the undercarriage position. Micro switches sense the position of each strut. Some aircraft have mirrors or mechanical indicators to confirm in the case of electrical failure.
Aural Warning
A warning horn which activates under specific flight conditions of low airspeed, flap extension and the gear retracted to warn the pilot prior to a possible gear up landing.
Undercarriage Operation
The selector is a lever with a small wheel for easy identification. It is operated by pulling out and then up/down.

24. 1. Undercarriage Systems
Retractable Undercarriage Components
Undercarriage Disable
Two common types of systems are:
squat switch: senses weight on the wheels.
air switch: senses dynamic pressure (IAS).
On the ground, the selector must be in the down position to prevent inadvertent undercarriage retraction.
VLO – max. IAS when raising/lowering gear.
VLE – max. IAS with gear extended.

25. 1. Undercarriage Systems
Hydroplaning (Aquaplaning)
When the runway is wet, hydroplaning can occur, causing the tyre to plane on the water. As friction is reduced, braking effectiveness suffers and there is a threat of loss of directional control.
Tyre pressure determines the speed at which hydroplaning may occur:
9tyre pressure (psi) for a rotating wheel.
7.7tyre pressure (psi) for a locked wheel.

26. Questions?