Introducing the mechanism of hydraulic system in landing gear of civil aircraft, Up-down process and steering operation.

1. The University of Jordan
Faculty of Engineering & Technology
Mechanical 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

2. 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.

3. 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.

4. Basic Hydraulic System

5. 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.

6. 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.

7. Landing Gear System

8. Retraction Process of Landing Gear

9. Extension System Operation
i. 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.

10. 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.

11. Emergency Extension System
Landing Gear Positioning Indicator
i. ‘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.

12. Hydraulic system flow diagram of large
aircraft nose wheel steering system

14. 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.