Landing Gear

Question 1

What components are used in a typical hydraulic landing gear system?

Answer 1

Engine driven and electrical hydraulic pumps, actuating cylinders, selector valves, solenoids, uplocks, down locks, sequence valves, emergency override controls, tubing, and other conventional hydraulic components

Question 2

Describe the Extension Sequence of a typical hydraulic landing gear system

Answer 2

1. A selector lever in the cockpit electrically commands the gear to extend
2. A solenoid valve directs hydraulic pressure to the extension side of system
3. Sequencing valves hold the landing gear in place until the landing gear doors have opened
4. Hydraulic pressure causes uplocks to be released and pressure is applied to the actuators to extend the gear
5. Once extended, downlocks are positioned hydraulically
6. Landing gear position switches provide indicating system with information of gear position
7. Sequencing valves direct hydraulic pressure to close the landing gear doors

Question 3

Describe the Retraction sequence of a typical hydraulic landing gear system

Answer 3

1. A selector lever in the cockpit electronically commands the gear to retract
2. Landing gear position switches provide indicating system with information on gear position
3. A solenoid valve directs hydraulic pressure to the retraction side of system
4. Sequencing valves prevent the landing gear from retracting until the landing gear doors open
5. Hydraulic pressure is applied to the actuators to retract the gear
6. Wheel rotation is stopped by hydraulic pressure routed to the brake system
7. Landing gear uplocks are positioned
8. Landing gear position switches provide where gear is
9. Sequencing valves direct hydraulic pressure to close the landing gear doors

Question 4

How does a landing gear safety switch function?

Answer 4

Short Answer: Squat Switch
Real Answer: A ground proximity switch is mounted in a bracket usually on the main gear shock struts and is mechanically actuated via the landing gear torque links. When the strut is compressed the torque links are close together, causing the adjusting links to open the safety switch. On takeoff, the weight of the aircraft leaves the struts, the torque links extend causing the adjusting links to close the safety switch. A ground is completed when the safety switches closes the solenoid then energizes the selector valve. Squat switches also provide signals to other systems, indicating whether the aircraft is in the air or on the ground is crucial to pressurization, nose wheel steering, thrust reversers, and APU.

Question 5

What is a brake anti-skid system?

Answer 5

A system in high-performance aircraft braking systems that provides anti-skid protection and subsequent maximum braking efficiency. Anti-skid system sensors monitor and compare wheel rotation speed to the expected value on a dry runway. Once the system detects a rotational value less than normal, a skid control valve removes some of the hydraulic pressure to the wheel, permitting the wheel to rotate a little faster and stop skidding. The more intense the skid, the more pressure is removed. The skid detection system is completely independent of the other wheels, each wheel has its own system.

Question 6

Explain the Touchdown protection circuit provided from an anti-skid system?

Answer 6

This circuit prevents the brakes from being applied during the landing approach, even if the brake pedals are depressed. This prevents the wheels from being locked when they contact the runway.

Question 7

Explain the Fail-safe protection circuit provided from an anti-skid system?

Answer 7

This circuit monitors operation to he skid control system. It automatically returns the brake system to full manual in case of system failure.

Question 8

Describe a typical large aircraft nose-wheel steering system.

Answer 8

Control of steering is accomplished from the flight deck through the use of a small wheel, tiller, or joystick typically mounted on the left side wall. Mechanical, electrical, or hydraulic connections transmit the controller input movement to a steering control unit, which directs hydraulic fluid under pressure to one or two actuators designed with various linkages to rotate the lower strut. An accumulator and relief valve keeps fluid in the actuators and system under pressure at all times which permits the steering actuating cylinders to also act as shimmy dampers. A follow up mechanism consists of various gears, cables, rods, drums, and/or bell cranks that returns the metering valve to a neutral position once the steering angle has been reached.

Question 9

What is the most common method of providing shock absorption during landing?

Answer 9

A typical pneumatic/hydraulic shock struct uses compressed air or nitrogen combined with hydraulic fluid to absorb and dissipate shock loads. Often times referred to as an air/oil or oleo strut. A shock strut is constructed of two telescoping cylinders that are enclosed on the external ends. The upper cylinder is fixed to the aircraft and does not move, while the lower cylinder is called the piston and is free to slide in and out of the upper cylinder. Two chambers are formed, the lower is filled with hydraulic fluid and the upper with compressed air or nitrogen. An orifice located between the two cylinders provides a passage for the fluid from the bottom chamber to enter the top cylinder chamber when the strut is compressed.