The University of JordanFaculty of Engineering & TechnologyMechanical Engineering Department

Design of Hydraulic & Pneumatic System (0904537)Second Semester 2013/2014

Hydraulic System of Landing Gear in Aircraft

By:AHMAD NAZIH

Reg. Number: 0095626

17-April-2014

Introduction

• Hydraulics is based on the fact that liquids are incompressible.

• Hydraulic system is a system where liquid under pressure is used to transmit energy.

• In it a hydraulic pump converts mechanical power to hydraulic power.

• An actuating cylinder converts hydraulic power to mechanical power.

Aircraft Manufacturers make greater use of hydraulic systems than others, the hydraulic system of the average modern aircraft

performs many functions .

Hydraulic systems determine flight worthiness, usability and reliability .

Basic Hydraulic System

Types of Hydraulic Fluids

There are three types of hydraulic fluids currently being used in civil aircraft:

1. Vegetable Base Hydraulic Fluid; Composed of caster oil and alcohol, used in older type

aircraft and flammable.2. Mineral Base Hydraulic Fluid; Processed from petroleum and it’s flammable.3. Phosphate Ester Base Fluid (Skydrol); Non- petroleum, fire-resistance, Low corrosive side effect,

Low weight.

Functions of Landing Gear

i. Support the static load of an aircraft on ground.ii. Absorb the landing shocks.iii. Dampen the vibration.iv. Facilitate aircraft for take off and landing.v. Providing the aircraft the braking and steering

functions.

Landing Gear System

Retraction Process of Landing Gear

Extension System Operationi. To lower the landing gear, the pilot moves

the landing gear handle to the GEAR-DOWN position and these events take place.

ii. The landing gear handle actuates a switch that turns on the hydraulic pump motor in the power pack so that it turns in the direction shown by the arrows in figure A.

iii. Fluid flow through the passage and check valve on the right side of the pump and around the outside of the gears.

iv. The output from the pump moves the gear-up check valve piston to the right an unseat the gear-up check valve.

v. Fluid flow into the down side of the three actuating cylinder and forces the pistons out. The nose gear is much easier to move than the main gears, so the fluid flows into and out of the nose gear actuating cylinder through restrictor.

Retraction System Operationi. When the aircraft is in the air, the pilot can

retract the landing gear by moving the landing gear handle to the GEAR-UP.

ii. The landing gear handle actuates a switch that turns on the hydraulic pump motor in the power pack so that it turns in the direction shown in Figure B .

iii. Fluid flow through the filter and the check valve on the left side of the pump, around the gears and out the right side, down to the gear-up check valve.

iv. The fluid from the pump moves the gear-up check valve piston to the left, and the fluid unseat the ball and flow to the gear-up side of each of the gear actuating cylinders. The first movement of the piston releases the mechanical down locks and allows the gear to retract.

v. Fluid return from the gear-down side of the actuator past the shuttle valve, which the spring has forced to the right, back into the reservoir.

Emergency Extension System

Landing Gear Positioning Indicatori. ‘GREEN’ light is to indicate that the LG is ‘DOWN’ and in

‘LOCKED’ position.iv. ‘RED’ light indicates the unsafe condition such as the LG is

‘NOT DOWN’ and still in ‘UNLOCKED’ or indicates that the LG is still in transition from ‘UP’ to ‘DOWN’ position or vice-versa.

v. ‘NO LIGHT’ will indicate that the LG is fully ‘UP’ and ‘LOCKED’.

Some aircraft have an emergency release handle in the flight deck that is connected through a mechanical linkage to the gear up locks. When the handle is operated, it releases the up locks and allows the gear to free-fall to the extended position under the force created by gravity acting upon the gear. Other aircraft use a non-mechanical back-up, as pneumatic power, to unlatch the gear.

Hydraulic system flow diagram of large aircraft nose wheel steering system

References 1.Airframe & Powerplant Mechanics, Airframe handbook.

FAA, U.S. Department of Transportation. 2.Pilot’s Manual, Bombardier, Section III, Hydraulic and Landing Gear System. 3. AIRCRAFT LANDING GEAR PRESENTATION, University Kuala lumpur, Malaysian Institute of Aviation Technology.