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Landing Gear

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LANDING GEAR SYSTEM Prepared by: KHALID WALI MOHAMMAD 53259209393 Date: OCTOBER 12, 2011

TABLE OF CONTENT:

TOPIC | PAGE | INTRODUCTION | 2 | GULFSTREAM G200 LANDING GEAR SYSTEM (GENERAL, MAIN LG) | 3 | NOSE LANDING GEAR AND EMERGENCY GEAR EXTENSION | 5 | LANDING GEAR SYSTEM CONROLS AND INDICATORS | 7 | WHEEL AND BRAKES | 8 | ANTI SKID SYSTEM | 10 | NOSE WHEEL STEERING SYSTEM | 13 | AIR BRAKES | 14 | BOEING 747 LANDING GEAR SYSTEM (GENERAL, MAIN GEAR AND DOORS) | 15 | NOSE GEAR AND DOORS | 16 | LANDING GEAR EXTENSION AND RETRACTION, WHEEL AND BRAKES AND STEERING | 18 | POSITION AND WARNING | 19 | CONCLUSION AND REFERENCES | 20 |

INTRODUCTION:

The undercarriage or landing gear in aviation is the structure that supports an aircraft on the ground and allows it to taxi, takeoff and land. Typically wheels are used, but skids, skis, floats or a combination of these and other elements can be deployed, depending on the surface.
Landing gear usually includes wheels equipped with shock absorbers for solid ground, but some aircraft are equipped with skis for snow or floats for water, and/or skids or pontoons(helicopters).
The undercarriage is a relatively heavy part of the vehicle, it can be as much as 7% of the takeoff weight, but more typically is 4-5%.

Gulfstream G200
Landing Gear System

General

The G200 has 4 main landing gear tires and two nose landing gear tires. The landing gear is tricycle type with two wheels on each gear strut. Each unit retracts into its’ own well and is fully covered by doors, mechanically connected to the landing gear. Landing gear extension and retraction time is 9 ± 1 seconds. In emergency, landing gear may be extended by compressed nitrogen.

Main Landing Gear

Each main landing gear wheels are mounted on a trailing beam pivoted on the main landing gear strut. Its shock absorber is pin-connected at both ends, absorbing the beam energy and transmitting the ground loads to the upper portion of the main landing gear structure. The ground loads are reacted by the wing structure by the journal bearings mounted coaxially in the wing, cordwise direction, and by the bracing actuator attached to the main landing gear strut. The main landing gear retracts inwards into the fuselage wheel well. Each main landing gear has two doors. The outboard door is rigidly attached at the strut. The inboard door operates by mechanical linkage to the strut and is held in up position by two uplock cylinders. This uplock is released when landing gear lever is placed in down position by hydraulic pressure, or by nitrogen pressure in emergency. The main landing gear is held, while airborne, in retracted position by hydraulic pressure in the actuator and by the inboard door mechanical uplock if pressure drops. As the gear is extended, an internal lock in the actuator automatically locks it in fully extended positions.

The main landing gear actuator also serves as the gear brace. The downlock is released when landing gear lever is placed in UP position by hydraulic pressure.

Fig. 1: Main Landing Gear

Nose Landing Gear

The nose landing gear strut is cantilevered, sliding telescopically in a rotating tube. The rotating tube is positioned by a rack and pinion steering system. The steering movement is transmitted to the wheel axle by torque links. The steering angle is ± 60°, permitting runway width of 15 meters. Towing angle is ± 100° without disconnecting the torque links. A towing adapter is mounted on the strut with an integral safety shear pin. Ground loads are applied to the fuselage structure by means of two bearings and a drag brace. The drag brace is locked automatically in fully extended position by jury brace; it is unlocked by a hydraulic unlock actuator. The nose landing gear retracts forwards and locks up by spring-loaded mechanical lock. It is unlocked by hydraulic pressure. The hydraulic retracting actuator is sized to retract and extend the landing gear and doors. When the nosewheel is off the ground, the nose landing gear is centered by internal cams in the sliding tube, while the hydraulic supply is cut off from the steering system.

The nose landing gear bay three doors are actuated mechanically by a system of bellcranks and rods connected to the levers on the nose landing gear strut trunnion. The two side doors open and close during nose landing gear extension and the rear door is opened backwards and remains open while the nose landing gear is down.

Emergency Gear Extension

Emergency extension of the landing gear requires actuating the emergency gear DOWN handle (on left side of the pedestal), releasing compressed nitrogen to drive the landing gear into down & locked position, and opening a valve to direct the upward hydraulic pressure to the return line; this enables landing gear extension even if landing gear lever is stuck in UP position. Once the gear Emergency DOWN handle is pulled, landing gear must not be retracted again.

Fig. 2: Nose Landing Gear

Landing Gear System Controls and Indicators Landing gear lever - Located on landing gear control panel on the front pedestal. It retracts and extends landing gear. Two positions: UP and DOWN Downlock Override button - Energizes solenoid of downlock plunger if ground contact switch fails Warning Messages: GEAR NOT DOWN - Landing gear is not down and locked with radar altitude less than 800 ft (400 ft with MOD 7222) and one thrust lever at or below max cruise detent or flaps position more than 25 degree

Fig. 3: Landing Gear System Control
Wheel and Brakes

Normal and emergency hydraulic wheel brake systems are controlled by conventional, dual, “tip-toe” brake pedals which actuate multi-disk self-adjusting brake units on each of the four main landing gear wheels. Normal operation of the brakes on each main landing gear strut is controlled by an anti-skid valve.

A. Normal Operation

The normal and emergency wheel brake systems are controlled by mechanical inputs from brake pedals, with the pilots and co-pilots commands mechanically interconnected. When the pedals are pressed, proportional pressure from the right hydraulic system is metered by the power brake valve to the anti-skid control valves, and then through fuse and shuttle valves, into two split brakes on each main wheel, separately. Under normal operation, hydraulic pressure is directed to the brakes; pressure to the brakes is governed by pilots metered pressure and modulated by anti-skid to minimize wheel skid. If any of the brake lines leading to the inboard wheels is broken, the hydraulic fuse on that side prevents loss of excessive amount of fluid and shuts off flow to the affected brake. The residual braking effect is 3/4 of normal. B. Emergency Operation Emergency braking is used when the right hydraulic system fails. To obtain emergency braking, the PARK/EMERG lever is placed in EMERG position. The left hydraulic system pressure is used with half of the normal braking capacity. If both hydraulic systems fail, accumulator pressure is sufficient for six applications of emergency brakes for a total of 50 seconds (or one T/R deployment). Anti-skid system is inoperative.

C. Parking

Parking is obtained when the PARK/EMERG lever is placed in PARK position. The lever closes a switch that energizes the parking valve and connects the right hydraulic system pressure and the right T/R accumulator pressure to the normal brake lines through shuttle valves.

D. Anti Rotation

The anti rotation function stops main wheels rotation after take-off during landing gear retraction before the main landing gear enters the wheel wells.

Back-pressure, developed in the return line during landing gear retraction, is directed into the power brake valve through the return port, which directs pressure through the (inoperative) anti-skid valve into the brakes (the anti-skid disengages when the landing gear goes out of down and locked position).

Fig. 4: Wheel Brakes System
Anti-Skid System Anti-skid system prevents wheel skidding by limiting application of main hydraulic system pressure to brakes, thereby permitting shorter landing roll and minimizing tire wear. Maximum braking efficiency is obtained when all wheels are at maximum rate of deceleration short of a skidding wheel. Wheel speed detectors transmit electrical signals to system control box which sends corresponding signals to anti-skid control valves, causing the control valves to continuously vary brake pressure as required. In flight, and if wheel contact with ground is not firmly stabilized, brake system is rendered inoperative. The anti-skid system consists of the following components: * Electronic control unit * Anti-skid control valves (2) * Wheel speed sensors (4) * Drive caps (4)

A. Operation The control unit receives wheel velocity signals from the wheel speed sensors of both wheels on one strut. The signals are converted to voltage and filtered to attenuate the natural frequency of the strut. The filters include a passive anti-skid drop out circuit to remove anti-skid control for wheel speeds below 10 knots. Each wheel speed signal is applied to a skid detector which compares the wheel deceleration (derived from the wheel speed) to a preset reference level. Whenever an incipient skid is detected the skid detector send a proportional signal to the anti-skid servo valve, which reduces the brake pressure. In addition to that, after each incipient skid a modulator signal brings brake pressure to a value slightly below the incipient skid threshold, and then allows pressure to increase at a controlled rate. The servo valve input circuit receives also a lead signal from a lead network which provides compensation for system lags.

The combination of skid detector, lead and modulator signals provide a highly adaptive and efficient skid control which achieves the greatest attainable deceleration while minimizing any tendency for deep skids. Locked wheel protection is provided to each wheel by a detector and an wheel arming circuit. When armed, the detector removes brake pressure on both paired wheel whenever the wheel speed on one or both of the wheels drops below 30 percent of the aircraft groundspeed. The locked wheel arming circuit is armed to groundspeeds above 30 knots or when both left and right weight-on-wheel switches indicate an airborne condition. Groundspeed is defined as the greater of the two wheel speeds. Touchdown protection prevents any pressure at the brakes prior to the wheel spin-up because locked-wheels conditions are detected (arming circuit armed in flight and wheels not rotating).

After touchdown, brake pressure can be applied only at wheel spin-up above 35 knots or 2.5 to 3.5 seconds after transition of one weight-on-wheel switch, whichever occurs first. For speeds under 35 knots, the touchdown protection is lost if one weight-on-wheel switch fails in GROUND mode, but brakes pressure loss occurs only after a double failure in the AIR position.

The control unit provides continuous monitoring of system components and signals the ANTI SKID OFF light of the corresponding system if one or more of the following conditions exist: * Opened or shorted wheel speed transducer or interface, * Opened or shorted anti-skid control valve coil or interface, * Loss of anti-skid control valve drive current from the control box, * Loss of power or failure to control power on box power on, or loss of regulated power supply, * Anti-skid control valve full dump command longer then 1 second with the weight-on-wheel switch in ground position. If the fault is removed, the light goes out automatically. After a tire burst at speeds higher than 30 knots a locked wheel condition is detected when the burst wheel speed decreases under 30% of the aircraft groundspeed and braking is lost on both wheels (ANTI SKID OFF light comes on). The anti-skid system is to be switched off and braking continues.

Anti-Skid System Controls and Indicators

ANTI-SKID Pushbutton - Located on pilot glareshield. It has two positions as follows: Pressed off - Anti-skid system is off. (ANTI-SKID and the OFF lights are on). Pressed ON - Anti-skid system is energized on ground. Switch is left in ON position during flight. Main gear downlock switch removes power from anti-skid system after take-off. OFF Caution Light - Lights come on when main gear is down and locked and: 1. ANTI-SKID pushbutton is pressed off. 2. Electrical failure exists in control system. 3. Control system produces full brake release signal for more than one second. 4. Fault exists which may cause lock-up of one main wheel during deceleration.

Fig. 5: Anti-Skid Control System

Nose Wheel Steering System

The nose wheel steering system includes a rack and pinion steering mechanism actuated by two coaxial hydraulic actuators, controlled and monitored by an electro-hydraulic servo system, which includes hydraulic control valve manifold and the electronic control unit. The nose wheel steering system provides: active steering angle of ±60° and towing angle of ±100° without need to manually disconnect the nose gear torque-links. The pilot controls the steering system by pedals, over an angle of ±3° or by a handwheel, located on the pilot console, over an angle of ±60°. The ON-OFF, nose wheel steering switch allows activation the nose wheel steering system, or to switch it off and remain with the nose landing gear in a free-swivel mode, which provides shimmy damping. PEDAL DISC pushbutton on the handwheel allows disconnection of nosewheel steering by the pedals (such as for flight controls check). The nose wheel steering modes of operation are: 1. Active steering - normal mode on ground 2. Actively centered mode - the system is actively maintaining the nosewheel in a centered position during approach and take-off when both nose landing gear weight-on-wheel switches are in air-position 3. By-pass mode - the two actuators are hydraulically interconnected by the bypass valve if nose wheel steering switch is OFF, or as a result of a hydraulic pressure failure. The bypass mode is the normal mode after nose landing gear retraction.

Any single failure in the nose wheel steering system is detected by the monitoring channel which switches the system to the bypass mode (fail-passive). In this case the aircraft directional control is achieved differential braking and/or rudder control.

Caution Messages: NWS INOP - Nosewheel is down and locked and nosewheel steering system is off

AIR BRAKES Four control surfaces on each wing, rotating upwards by hydraulic power for activation, serve as air brakes. The surfaces have a deployed angle of 45°. The inboard pair is fed from the actuators have mechanical lock for stowed position. Ground operation includes all four surfaces and is achieved by GROUND A/B switch with ON/OFF positions. When deployed, the air brakes dump excess lift from the wing when aborting take-off or on landing. The ground air brakes differs from flight air brakes operation by faster rate of extension and the fact that they pop out only after the aircraft touches the ground. To prevent erratic extension and retraction of the ground air brakes during bouncy landing, self holding relay bypasses the landing gear ground contact switch and holds the air brakes extended. Placing GROUND A/B switch in ON position causes all surfaces to extend, if aircraft is on ground and thrust lever position is below MAX CRUISE; FLIGHT and GROUND AIRBRAKES messages come on. Moving thrust lever beyond MAX CRUISE (such as for go-around) causes all surface to retract. Flight operation is achieved by FLIGHT A/B RETRACT / EXTEND switch. The switch is spring-loaded to OFF position. Asymmetry protection is operative once the switch is released and permits extension only if all four surfaces (inboard two surfaces of each wing) are out of retracted position. They may be extended, in flight, to increase drag at any airspeed. Flight airbrakes message comes on whenever the flight air brakes are out of retracted position.

Advisory Messages:

FLIGHT AIRBRAKES - Flight (inboard) airbrakes extended.

GROUND AIRBRAKES - Ground (outboard) airbrakes extended.

Boeing 747
Landing Gear System

General

The landing gear system consists of the gears which support the airplane while on the ground; a gear extension and retraction system; wheels and brakes for each main gear; means for steering the airplane; gear and door indicating and warning system; and control and operation of the gears for landing, takeoff and ground movement. The 747 have 16 main landing gear tires and two nose landing gear tires.

Main Gear and Doors A. Main Gear The main landing gear consists of four main gears; two body gears and two wing gears. Each gear is comprised of a four-wheel truck. The body gear is located at station 1463.5 in the fuselage. The wing gear is located aft of the rear wing spar inboard of the engine nacelles at station 1342.5. Landing impact is absorbed by five air-oil shock struts, functioning primarily as air springs. Rolling vibrations and variances in runway are absorbed by the hydraulic forces within the shock strut. Body gear trucks are steerable, providing directional control in conjunction with the nose gear in sharp turns during low speed taxiing and towing. This capability also reduces tire scrubbing in sharp turns. B. Main Gear Doors The landing gear doors consist of body gear doors and wing gear doors. The body and wing gear doors each have wheel well doors and shock strut doors. The wheel well doors are hydraulically actuated and can be closed with the gear extended or retracted. The shock strut doors are mechanically attached by linkage rods to the gear shock strut and move only when the gear is moved. All doors are of frame construction with skin paneling on the inner and outer sides. The doors close over all gear openings and fair with the fuselage contour to provide aerodynamic smoothness.

Nose Gear and Doors A. Nose Gear The nose landing gear is a steerable wheel assembly which supports the forward end of the airplane and provides directional control while on the ground. The gear is hydraulically actuated and retracts forward in the wheel well. Landing loads are absorbed by an air-oil shock strut. Steering is provided by hydraulically actuated cylinders. The airplane can be towed forward or aft from the nose gear. B. Nose Gear Door The nose gear doors are clamshell type and consist of four doors that fair with the fuselage contour when closed. All doors are of alclad rib and skin construction.

Fig. 1: Landing Gear Locations

Fig. 2: Landing Gear Geometry

Landing Gear Extension and Retraction

Extension and retraction of the wing, body and nose gears and their wheel well doors is by hydraulic power. An electrically powered alternate extension system is provided to unlock the gears and doors when hydraulic power is not available. Gear and doors then extend by gravity. An additional manual extension capability is provided for the nose gear. The gear is controlled from a single handle mounted on the pilots' center panel. Cables from a quadrant actuated by the handle transfer motion to selector valves which apply hydraulic pressure for gear actuation. A safety lock prevents inadvertent operation of the control handle to the gear up position when conditions are not right for gear retraction.

When the gear unlocks and extends and the gear doors close. When the control handle is placed in the UP position, the gear doors open, the gear retracts and locks and the gear doors close. Sequence valves control gear and door operation.

Wheels and Brakes

The airplane is supported during landing, takeoff, and ground operations on 18 wheel and tubeless tire assemblies. Eight are on the wing gear, eight on the body gear and two are on the nose gear. Main gear wheels are numbered as shown in Fig. 2. Each main gear wheel (wing and body) is provided with a brake unit installed on the axle on the side nearest the shock strut. The brakes are a multidisk type with stationary elements fitted with replaceable linings and segmented rotor brake disks. The brakes are fitted with combination return springs and automatic adjusters. The adjusters compensate for brake wear.

When the airplane brakes are applied, the antiskid system automatically assumes control of the brake pressure through the antiskid valves. A brake temperature monitoring system is included to enable monitoring of brake temperatures, especially when the wheels are retracted.

Steering

Two steering tillers, each with steering control authority of approximately | 70 degrees at the nose gear, are provided. One tiller is to the left of the captain, the other to the right of the first officer. On some airplanes, the rudder pedals are also coupled to the steering control system by an interconnect mechanism. Full rudder pedal deflection provides approximately | 10 degrees or | 7 degrees of nose wheel steering. Towing angle can range between approximately | 65 degrees without disconnecting torsion links. Disconnecting the torsion links enables towing at any angle. Hydraulic system 1 powers both nose gear steering cylinders. Steering power is available only when the gear selector lever is DOWN. The steering tillers also control steering of the body gear at angles greater than | 20 degrees of nose wheel steering. A switch operated by the nose wheel steering quadrant actuates the body gear steering system. The body gear truck actuating cylinders are then powered by hydraulic system 1. The body gear trucks turn opposite to the nose gear to assist the turn.

Position and Warning

The landing gear position and warning system reflects status of all landing gear positions, gear door positions and wing and body gear truck positions. Indicator lights on the pilots' and flight engineer's landing gear modules provide status of gear position. To assure positive status of gear position redundant electronic systems (primary and alternate) for status display are provided. The gear tilt indicators located on the flight engineer's landing gear module provide status that determines when the gears are properly tilted. Landing gear position signals and truck tilt signals are also provided to other airplane systems.

Fig. 3: Wheel Numbering Diagram

Conclusion:

As we read above the landing gear is one of the most critical and important parts for an aircraft and as we see lot of incidents happen either it can be a technical failure or human factor. One of most unpredictable issues that the companies face is called “stowaways” and this term used for unauthorized passengers. This kind of passengers has been stowaway on larger aircraft by climbing a landing gear strut and riding within the compartment and this can be deadly dangerous and sometimes it can jeopardize the flight and the passengers onboard.
Learning about LANDING GEAR SYSTEM gives us as students close view about how the system works and this assignment can show our understanding and the various resources we can use to find about any aircraft’s landing gear system.

References:

* Boeing 747 AMM (ATA chapter 32)

* GULFSTREAM G200 AMM (ATA chapter 32)

* Aircraft landing gear system design: principles and practice “NORMAN S. CURREY”

* Aircraft Design “Ajoy Kumar Kundu”

* http://en.wikipedia.org/wiki/Undercarriage.php

* http://www.tpub.com/air/12.htm

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