2 AimTo review principles of operation of the Hydraulic Systems & Undercarriage
3 ObjectivesDefine Hydraulics and discuss the principles of operation of a typical Hydraulic SystemsState principals of operation of retractable Undercarriage systems
4 1. Hydraulic SystemsHydraulics is the transmission of force by fluid pressureUtilisation benefits:AccessibilitySimplicityPower 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 viscosityLow volatilityNon-corrosivenessStabilityInflammabilityColouringNon-toxicityIncompressibilityLow freezing pointCommon types:Mineral (petroleum based)Synthetic fluidDo 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:ReservoirPumps and pressure regulator (cut-in, cut-out valve)AccumulatorCheck valvesRelief ValvesSelectorsActuators
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 systemCan be driven electrically, by the engine, air driven or by handGear-type pumpLower 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 pumpRelatively inefficientCommonly 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 PumpVaried 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.
15 1. Hydraulic SystemsHydraulic Operation - Heavy aircraft
16 1. Undercarriage Systems Undercarriage System OverviewRequirementsAbsorb the compression loads of landing.Support the weight of the aircraft while parked.Allow the aircraft to taxi and manoeuvre while on the ground.ArrangementsConventional (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 ComponentsNosewheelNosewheels are of two types:The fully castering nosewheelThe oleo-pneumatic strut nosewheelFully castering nosewheelSteering is achieved via differential braking.Oleo-pneumatic strut nosewheelUses 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 ComponentsMainwheelMainwheels are of two types:The leaf springOleo-pneumatic strut configuration
19 1. Undercarriage Systems Undercarriage ComponentsBrakesBrakes are installed on the mainwheels onlyThey 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 ComponentsCommon 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 pressureTyre conditionAircraft weightRunway surface conditionAircraft speedAircraft brake conditionLocking 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 ComponentsParking 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 UndercarriageRetractable undercarriages reduce form drag but add weightOperated most commonly hydraulically but can be electric and/or pneumaticRequired 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 ComponentsPosition IndicatorsTypically 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 WarningA 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 OperationThe 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 ComponentsUndercarriage DisableTwo 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:9tyre pressure (psi) for a rotating wheel.7.7tyre pressure (psi) for a locked wheel.