Hogeschool van Amsterdam, Technology, AviationThe 747s body gear

Project Report

Projectgroup 2A2RProjectleader

Ivar van CuykKevin van der PlasSander van der Pijl

Jos FrijmannRoy van Schagen

Wytze HilgersRoy Wassink

Anouk LelijPeter van Woudenberg

Amsterdam, October 2010

Table of contents

TABLE OF CONTENTS ....................................................................................................................2

SUMMARY ...................................................................................................................................4

INTRODUCTION ............................................................................................................................5

LANDING GEAR ANALYSIS .......................................................................................................6

LANDING GEAR PRINCIPLES .........................................................................................................6

FUNCTION LANDING GEAR .................................................................................................................6

TYPES ............................................................................................................................................6

MAIN LANDING GEAR BOEING 747 ....................................................................................................8

NOSE GEAR ...................................................................................................................................10

BODY GEAR SYSTEMS ..............................................................................................................12

HYDRAULIC SYSTEM .......................................................................................................................13

EXTENSION AND RETRACTION SYSTEM ...............................................................................................13

ALTERNATE EXTENSION AND SYSTEM .................................................................................................15

SHOCK STRUT ................................................................................................................................15

BODY GEAR STEERING SYSTEM ..........................................................................................................16

AIR/GROUND SYSTEM .....................................................................................................................17

HYDRAULIC BRAKE SYSTEM ..............................................................................................................17

BRAKE CONTROL SYSTEM .................................................................................................................18

REGULATIONS .......................................................................................................................19

GENERAL REGULATIONS ABOUT THE DESIGN .......................................................................................19

DIFFERENT CONDITIONS ..................................................................................................................20

NOSE WHEEL AND STEERING SYSTEM .................................................................................................21

Literature Review

The second year Aviation students received an assignment to make an analysis of the landing gear of a modern aircraft. The engineering department of Amsterdam Leeuwenburg Airlines [ALA] wanted the project group to discover any interference in the landing gear of the chosen aircraft. This to make an extension of the fleet possible. The project group has chosen the Boeing 747, this because the landing gear of this aircraft is more complicated. The project group also found a proper incident of the Boeing 747 that could be investigated.

First, the project group has made an analysis of the various landing gears. The various landing gears have been described each with its own advantages and disadvantages. The landing gear of the Boeing 747 had been described more precisely after it became clear what kind of landing gear is used on the Boeing 747. The landing gear of the Boeing 747 is called a multi bogey gear. This landing gear consists of a main landing gear and a nose landing gear. The main landing gear of the Boeing 747 contains the body gears and the wing gears, this because the Boeing 747 is an aircraft with a great mass. The body gear consist of two bogeys and is places under the fuselage, the wing gear contains also two bogeys and is placed under the wings of the aircraft. The body gear and the wing gear have, related to each other, a different construction, although almost all components are used in both systems. The nose gear contains one bogey and is placed under the fuselage near the cockpit.

There are many laws and requirements, which have to be taken into account when developing a landing gear. The landing gear must be able to deal with great forces that develop in extreme situations. These forces are described and have been taken into account when making calculations of the forces that are acting on the landing gear. The dimensions of the landing gear are needed to make a proper calculation of these forces. The dimensions of the main gear have been described, because during the incident the problem occurred in the main gear of the landing gear. The landing gear must be able to deal with forces acting on it. Therefore, it is necessary to know the properties of the materials used in the landing gear. So the properties of materials that are normally used in aviation are described. There is also described how the properties of the materials can be improved. The calculation is made, after all dimensions and the properties of the used materials are clear. The cause of the problem is made clear by making different calculations of the forces acting on the landing gear. When the cause is made clear the effects of the incident can also be made clear. The financial implications have been examined. Finally a conclusion is made.Introduction

The airplane has four main landing gear and a single nose gear. The nose gear is aconventional steerable twowheel unit. The main gear consist of two steerablebody gear and two non-steerable wing gear. Each main gear has four wheels pertruck in tandem pairs. The main gear trucks must be tilted and centered to allowretraction into the wheel wells.Hydraulic power for nose and body gear retraction and extension is supplied byhydraulic system 1. Power to retract and extend the wing gear is provided byhydraulic system 4. An alternate extension system is also provided.The normal brake system is powered by hydraulic system 4. The alternate brakesystem is powered by the hydraulic systems 1 or 2. Pressure-operated selectorvalves provide automatic brake source selection. Antiskid protection is providedwith both systems, but the autobrake system is available only through the normalsystem.Hydraulic accumulators provide reserve brake pressure in case of normal hydraulic pressure loss. Tire pressure and brake temperature are monitored continuously and displayed on the configuration page of the System Display(SD). Alerts on the Engine and Alert Display (EAD) abdSD will warm the flight crew of anti-skid, brake, and tire malfunctions. The BTM/TPI system consists of a BTM/TPI computer in the center accessory compartment and pressure and temperature sensors on the landing gear. The system monitors tire pressures and individual brake tempeatures. These temperatures and pressure are displayed on the SD when the configuration page is selected with the config cue switch. Abnormal trie pressure and temperatures ara annunciated by EIS alerts. A blanked out portion of the configuration page tire/brake display inducates a component failure in the system. The display of an individual tire or brake will be blank if data for that tire or brake is not availabe.

Landing gear analysis

The construction that the aircraft uses to make contact with the surface is called the landing gear. A landing gear is necessary to make the aircraft manoeuvrable on the surface without damaging the aircraft. To understand a landing gear and its configurations, it is necessary to know the principles of a landing gear because of the problem with the Boeing 747 was a problem with its left body gear, knowledge about the Boeing 747s body gear is needed to understand the problem.A landing gear may not be used without a certification. The landing gear must gratify to all rules in CS-25. Main source of this chapter is chapter 32, Landing Gear, of the Boeing 747s manual.

Landing gear principles

In order to make it possible for an aircraft to manoeuvre when it is not in the air, an aircraft is equipped with a landing gear. A landing gear has multiple functions. The landing gear is performed in several conditions. Because of the landing gear of a Boeing 747 is a multi bogey gear, the multi bogey gear is explained further. The multi bogey gear is divided in the main landing gear and the nose landing gear.

Function landing gear

The function of a landing gear is to carry the aircraft when it is not in the air. The landing gear makes it possible to make manoeuvres in horizontal directions and to rotate in vertical directions. Another function of a landing gear is to abate the energy that arises during a landing. In this way, a landing is more comfortable, by using a shock absorber. A shock absorber reduces the loads on the other parts of the aircraft. When an aircraft is landed, it has to decrease its speed. The only contact with the runway is by the tyres. Therefore, the landing gear is equipped with brakes. The last function of the landing gear system is the aero-dynamical function. When the aircraft is in flight, the landing gear is retracted to create a more aero-dynamical shape of the aircraft. The aircraft has lower drag, which is positive for the fuel consumption.

Types

There are many types of landing gear, each with its own advantages and disadvantages. Mostly wheels are used to make aircraft manoeuvrable on the surface, but skis and floats are also used. The different types of landing gear using wheels to manoeuvre on the surface will be explained because this type of landing gear is often used in the commercial aviation, instead of the ski's and floats. After takeoff, the landing gear is often stored to improve the aerodynamic properties of aircraft. Aircraft designers have devised various ways to store the landing gear as efficient as possible.

Types of landing gear

Aircraft designers have always been searching for better landing constructions, so there are many types of landing gear. The most used landing gears are:

1. Conventional landing gear 2. Tricycle landing gear 3. Single main wheel 4. Bicycle gear 5. Quadricycle 6. Multi-bogy gear

Conventional Landing Gear

The conventional landing system was often used between 1910 and 1950. The conventional landing gear consists of a main landing gear and a tail landing gear, the main landing gear contains two wheels and the tail landing gear one wheel. The tail wheel does not absorb shocks, so the tail wheel can be kept small. The benefits of a small tail wheel are of course less weight and less air resistance. A small tail wheel does also have a couple of disadvantages. Aircraft with a conventional landing gear have an angle with the surface, because the main landing gear wheels are bigger than the tail landing gear wheel. This ensures the pilot has a bad sight when the aircraft is standing on the runway or when the pilot is taxiing on the taxiway. Another disadvantage is ground loop. Ground loop is an effect that occurs when aircraft with a conventional landing gear makes a turn on the runway or taxiway. A destabilizing moment can occur when making a turn

Tricycle landing gear

Since 1950 the tricycle landing gear is used often. The tricycle landing gear contains a main landing gear, which had two wheels or bogies and a nose landing gear, which has one wheel or bogey. This construction has many advantages over the conventional landing gear. The pilot has a better view on the runway or taxiway and ground loop cannot occur, because the nose wheel now produces a stabilizing moment.

Single main wheel

This type contains one main wheel and one small tale wheel. It is a light system because the system is made simple. Because the single main wheel landing gear consists of two wheels the manoeuvrability is not very well. This type of landing gear is often used in aircraft with a small mass.

Bicycle gear

The bicycle gear has two main landing gear constructions. These are located in the longitudinal. Aircraft with the bicycle gear are not stable because the wheels are placed next to each other, although it was a good construction for aircraft with a narrow fuselage. Mostly auxiliary wheels are used to prevent the wing strikes the surface.

Quadricycle

This construction consists of four main landing gears. The main landing gear contains wheels, which are located under the fuselage. This is ideal for cargo aircraft because the fuselage is located near the surface, so loading and unloading can be done quickly. The stability of the aircraft is not very well.

Multi-bogey gear

The multi bogey gear contains a main landing gear and a nose landing gear. The main landing gear consists of two rows of wheels and the nose landing gear mostly of two bogies. The multi-bogey gear is used by aircraft with a large mass.

Storage of the landing gear

Mostly the landing gear is stored to improve the aerodynamic characteristics of aircraft. There are several ways to store the landing gear. The landing gear of small aircraft is often stored in the wings or fuselage, or a combination of these places. Big aircraft often use a space, which is called the fuselage-podded. This is a part of the aircraft that strengthened the transition area of the wing and the fuselage. In the fuselage-podded is enough space for storing the landing gear. In some cases, space for the landing gear is created under the wings. This is called wing-podded. The landing gear of rotor aircraft is often stored under the engine.

Main Landing Gear Boeing 747

The main landing gear is the principle gear of an aircraft. Which type of a main landing gear is used, depends on the size of the aircraft. The Boeing 747 is a large, heavy aircraft. Therefore is used a complex landing gear, which is composed of a several main gear units. The landing gear operates through by a retractable system.

Construction

The Boeing 747 is equipped with four main gear units, consisting of two body gears, which are attached to the fuselage, and two wing gears, which are placed aft of the rear wing inboard of the engine nacelles. Each of these is featured with four-wheel bogies.

1.Wing Gear2.Body Gear

1

2

Boeing 747s Main Landing Gear

Through a trunnion and trunnion fork, supported at the forward end by the wing rear spar and at the aft end by the landing gear support beam, is each wing gear attached to the structure. Each body gear is attached to the structure through a trunnion cantilevered from backing on the aft bulkhead of the body gear wheel well.

The doors of the landing gear consist of wing gear doors and body gear doors. Both have each wheel well doors and shock strut doors. Wheel well doors operate hydraulically and can be closed when the gear is extended or retracted. These doors are hinged together. The doors of the shock strut are also hinged together. They operate mechanically and are attached by linkage rods. These doors only move when the gear is moved. All doors are of frame construction, with on the inner and outer sides skin panelling. The doors close over all gear openings, and accord with the contour of the fuselage. This provides an aerodynamic smoothness.

Several components are used to optimize the using of the landing gear. A shock absorber, or shock strut, is an item, which is used to all current landing gears. The basic function of this component is to absorb the kinetic energy during the landing and taxiing so the accelerations imposed upon the frame will be reduced to an acceptable level.

The brakes, in combination with a skid control system, are used to reduce the speed and to stop the aircraft. Brakes can also used to hold an aircraft stationary while its parked, or when the engines are running up, but also to steer the aircraft by differential action or to control speed while the aircraft is taxiing. The skid controls are used to minimize the stopping distance and to reduce the excessive tyre wear and burst tyres by unduly skidding. The available degree of friction coefficient is constantly sensing by the systems, and by controlling brake pressure to supply an almost constant brake force nearly to the skidding point. The anti-skid system is used to prevent a wheel for slipping during braking. This is a lot safer because in a large aircraft, the pilot does not notice the wheels are slipping.The landing gear position indication system shows the position of the landing gear and the doors. The information is delivered to the crew through the display units in the cockpit (EICAS).

Operation

The retraction and extension of the landing gear and their doors operates by a hydraulic system. When hydraulic power is available, an electrically powered alternate extension system unlock the gear and doors. This movement is controlled by one action on the panel in the cockpit. When the handle is placed in DN position, the doors open, gear unlocks, gear extends, and then the doors close. UP position is conversely: the gear doors open, the gear retracts and locks, and the doors close. Gear and door operation are controlled by sequence valves. There is one actuator for the extension (1), and two actuators for the retraction of the landing gear (2). Retraction happens during upwind, so there is more strength required to retract.

1.Actuators for retraction

of the L/G

2.Actuator for extraction

2of the L/G

1

Actuators for retraction and extraction

The inner wheels of the Boeing 747 can rotate and are used during towing and taxiing of the aircraft. This movement is contrary to the motion of the nose-wheel. For rotating the wheels, two actuators are used (1). The brakes are operated using the hydraulic pipes (2). The possibility of steerable body gear trucks reduce tyre scrubbing, which occurs when an aircraft makes a sharp turn.

1.Actuators for rotating2the inner wheels

2.Hydraulic pipes, brakes

1

Actuators on the wheels and hydraulic pipes

Five air-oil shock struts absorb the landing impact. These works in the first place as air springs. The variances in runway and the vibrations of rolling are absorbed by the hydraulic forces within the shock strut.

A Boeing 747 has eighteen wheels, from which two on the nose gear, eight on the body gear and eight on the wing gear. Any single wheel of the body gear and wing gear is provided with a brake unit. This is installed on the side nearest the shock strut. The brakes on a Boeing 747 are multidisc brakes, fitted with a combination of automatic adjusters and return springs. These adjusters compensate for the wastage of the brakes. When an aircraft brakes, the anti-skid system automatically operates. The wheel skid is compensated by this system, by control of brake pressure. This happens through the anti-skid valves. A brake temperature monitoring system displays the brake temperatures and make the cabin crew alert of overheated brakes.

The position of the landing gear is showed by proximity switch sensors, which are located on any single landing gear and doors. The sensor signals provide the data of the position to the EICAS display units.

Nose gear

The construction of the nose gear differs from that of the main gear. The nose gear is used to support the forward end of the fuselage. The nose gear is also used for controlling the direction of the aircraft while it is moving on the ground. The nose gear suffers from vibrations (shimmy) more than the main gear does.

Construction

Starting from the ground, the nose gear of a Boeing 747 (Figure 4. ) (Appendix IV) has two tyres and wheels (1) that are attached on one axle (2). This axle is connected to the lower end of the shock strut inner cylinder (3). The shock strut is used to absorb the shocks from the landing impacts and rolling over bumps on the ground. Also connected to the shock strut inner cylinder, is the lower end of the lower torsion link (4). The upper end of the upper torsion link (5) is connected to the forward steering collar (6). Aft steering collar (7) attach lugs slip over the forward steering collar attach lugs. Both collars are locked together around the shock strut outer cylinder (8) by actuator attach pins (9). These pins also hold the rod end of the steering actuators (10). The steering actuators, steering collars and torsion links are all parts used in the steering mechanism. At the upper end of the shock strut outer cylinder there is a trunnion (11). When the nose gear extends or retracts it pivots on this trunnion. The trunnion itself rotates in bearings. The trunnion is supported by two side braces (12). The braces extend from the attach lugs at the centre of the shock strut outer cylinder. Besides the side braces, there is also a lower tripod brace (13) connected to the centre of the shock strut outer cylinder. The lower tripod brace is connected to the upper tripod brace (14) that extends forward from the centre of the trunnion. Also connected to the trunnion is the nose gear actuator (15). The nose gear actuator activates the nose gear extension or retraction movement. Between the end of the nose gear actuator and the hinge of both tripod braces there is a drag strut (16). Together with the nose gear lock actuator (17), it holds the nose gear in the up and locked, or down and locked position.

171.Tyre and wheel

2.Axle

153.Shock strut inner

Acylinder

4.Lower torsion link

165.Upper torsion link

6.Forward steering

collar

87.Aft steering collar

1178.Shock strut outer

9cylinder

149.Attach pin

10.Steering actuator

1011.Trunnion

12612.Side braces

1313.Lower tripod

brace

14.Upper tripod

brace

15.Nose gear

5actuator

16.Drag strut

See A17.Nose gear lock

4actuator

1

3

2

Nose gear

Steering operation

Both pilots have their own steering tiller that they can use for normal steering operation. One tiller is on the left of the captain and the other is on the right of the first officer. A tiller movement of 90 degrees results in a steering movement of 45 degrees. Tiller movement in either direction is transmitted to a steering metering valve by cables (Figure 5. (1). This valve directs fluid (oil) with a pressure of 3000 PSI (hydraulic system No. 1) to the nose wheel steering actuators (2). When the steering actuators are activated, they transfer their power to the attach pins (3). These pins transmit a turning moment to both collars. The forward collar (4) transfers the power to the upper torsion link. The upper torsion link transfers the power to the lower torsion link. This link turns the shock strut inner cylinder to the left or right. This action turns the axle and the axle turns the wheels. The tillers can turn the nose wheels around 70 degrees maximum.

21.Metering valve

assembly

2.Steering actuator

3.Attach pin

4.Forward steering collar

3

14

3

2

Steering operation

Besides the normal steering there is another way to steer the wheels. This is done with the help of the rudder pedals. The nose wheel steering system is connected to the rudder pedals through mechanical linkage and cables. When the nose gear is compressed by the weight of the aircraft, rudder pedal steering is available. The rudder pedals can turn the nose wheels around 10 degrees maximum. When the aircraft is towed, the wheels can turn around 65 degrees maximum without disconnecting the nose gear torsion links.

Shimmy

Because of the flexibility of tyre side walls, vibrations known as shimmy are induced into the nose gear. Especially at high speeds, excessive shimmy can cause vibrations throughout the whole aircraft, which is dangerous. Wear of the nose wheel bearings, worn torsion links and uneven tyre pressures all increase the tendency to shimmy.

In general these ways are used to reduce shimmy: Provision of a hydraulic lock across the steering jack piston;

Fitting a hydraulic damper;

Fitting heavy self-centring springs;

Double nose wheels;

Twin contact wheels.

The beside mentioned steering metering valve consists of a spring compensator. This spring compensator maintains a pressure of 205 to 325 PSI against the steering actuator pistons to act as a shimmy damper.

Body Gear Systems In this paragraph the systems of the Boeing 747's body gear is being discussed. The body gear is being discussed because the left body gear of the Boeing 747 was the landing gear that malfunctioned during take-off and causing the Boeing 747 to make an emergency landing at Schiphol Airport. Multiple systems used in the Boeing 747 are using the hydraulic system to support. The extension and retraction system is one of those systems, because a failure in the hydraulics may not cause the Boeing to be disable to retract the landing gear, there is also a alternate extension and retraction system. When the Boeing 747 touches down with a malfunctioned body gear, the shock struts of the three remaining gears now has to absorb the impact. The body gear of the Boeing 747 is also equipped with an Air/Ground system a Body gear steering system a hydraulic braking system and a brake control system.

Hydraulic System The Boeing 747 has a hydraulic system to provide multiple systems in the Boeing 747 to work properly. The hydraulic system is applied in the entire aircraft. In this paragraph only the hydraulics in the body gear and the hydraulic actuator will be discussed.

The hydraulic system in the body gear

The hydraulic system of the Boeing 747 is applied to decrease the work pressure of the pilots. It uses a fluid to transport pressure applied by an engine driven pump. The hydraulic system is based on hydrostatic laws, such as Pascal's law tells that the force the pilot applies into the hydraulic system can be increased by decreasing the surface of the applied force. This makes the pilots use a small amount of strength to do heavy tasks, such as, retracting the landing gears. The Boeing 747 has four independent hydraulic systems, so if one fails another can take over. The landing gear relies on the hydraulic systems No.1 and No.4.

The hydraulic systems No.1 and No.4 are available for multiple systems applied in the body gear of the Boeing 747. The extension and retraction system, the body gear steering system and the hydraulic brake system use the hydraulic system to increase the force applied by the pilot.

Hydraulic actuator

The hydraulic system uses hydraulic actuators to turn the hydraulic pressure into a movement. Actuators are cylinders (1) with two chambers. If the hydraulic pressure increases in one of the chambers the hydraulic the piston will move creating a larger chamber for the high pressure. The piston (2) will move the piston rod (3) which will perform the action that is required. If the hydraulic pressure is increased in the red chamber and the piston rod will move in the direction of the red arrow, if the pressure is increased in the blue chamber, the piston rod will move in to the direction of the blue arrow

1.Cylinder

2.Piston3.Piston rod

Hydraulic actuator

Extension and retraction system

The landing gears on Boeing 747 are designed to retract during flight to decrease drag and to increase the aerodynamic stability of the aircraft. This makes the plane use less fuel and a increase manoeuvrability. The body gear will be raised and lowered simultaneous with the wing gear and the nose gear by a lever in the cockpit. To raise and lower the landing gear the extension and retraction uses a gear actuator to retract and lower the body gear. The sequence valves make sure all components will retract in correct order.The landing gear will be locked into place when raised and when extended.

Gear actuator

To retract the body gear during flight and extend the body gear for landing the Boeing 747 uses hydraulic powered actuator. The gear actuator will be attached to the aircraft structure and the shock strut of the landing gear. When the actuator is retracting (1) the body gear will retract (3). The actuator will turn the shock strut around the trunnion (2) and covers the body gear in the wheel well.

1. Gear actuator

2. Trunnion 3. Shock strut

Retracting body gearRetracted body gear

Sequence valves

The wheel well doors are linked to the body gear with sequence valves, these valves make sure the wheel well doors are opened before the body gear will be lowered and closed after the body gear is locked into place when retracted. The sequence valves steer the hydraulic fluid from the hydraulic system to the doors actuator, the up lock actuator, the down lock actuator and the body gear actuator. By retracting the landing gear using the landing gear lever, the hydraulic fluid will enter the door sequence valves and goes through a valve to the door actuator. The pressure builds up and the wheel well door will open. When the doors are opened the hydraulic pressure in the sequence valve is also increased and now opens another valve and allows the hydraulic fluid to flow to the lock mechanism, when the body gear is unlocked the hydraulic fluid will now raise the body gear and finally the hydraulic fluid will lock the gear in the up position.

Up lock mechanism

To make sure the body gear stays retracted the landing gear will be locked into place, which is done by an up lock mechanism. The body gear of the Boeing 747 is locked by locking a hook around the up lock roller that is attached to the shock strut. The hook will be kept in the locking position by a spring and can be unlocked by a hydraulic actuator or electric actuator to release the body gear or to receive the up lock roller in the hook.

Down lock mechanism

Next to an up lock mechanism there is also a down lock mechanism, to make sure the body gear stays in to place when extended and not retract when touching down at a high impact. The down lock mechanism, that has to lock the

body gear into place mechanical, makes sure the Boeing 747 is able to land on its body gear when multiple systems are malfunctioned. The down lock mechanism is an over centre knee joint on the jury strut that is connecting the drag strut with the aircrafts structure.The jury strut will over centre the joint allowing it to take forces in direction of the jury strut, because the jury can only move to the mechanical limit of the knee joint and makes sure the jury strut will not fold and allows the body gear to rise. The joint can be unlocked out of the over centre position by the hydraulic actuator.

Over centre knee jointAlternate extension and system

Because all the landing gears must be able to extend when the hydraulic system malfunctions, the Boeing 747 is equipped with a alternate extension system. The alternate system will simply electrically unlock the up lock mechanism of the body gear and simultaneous releases the wheel well doors. With some manoeuvring of the aircraft and the gravity the body gear will be lowered and lock into place. The alternate landing gear will be operated by a switch in the cockpit.

Shock strut

The shock strut absorbs most of the impact when the aircraft touches down and when taxiing

on the runway. The Boeing 747 is

equipped with Oleo-pneumatic shock1.Outer cylinder

struts .This is a shock2.Nitrogen valve

3.Upper chamber

absorber that uses both oil (5) and

(Nitrogen)

compressed nitrogen (3) to absorb4.Orifice support

impact. The shock strut has an innertube

and (11) an outer cylinder (1). The5.Oil

6.Orifice

outer cylinder is attached to the aircraft

7.Upper bearing

structure by the trunnion. The inner

8.Metering pin

cylinder is attached to the bogie of the9.Seal

body gear. The inner cylinder can10.Lower bearing

move in and out the outer cylinder to11.Inner cylinder

absorb impact. The orifice (6) of the

shock strut is a small passage. The oil

will be pressed through this passage at

impact and controls the absorption of

the impact. The metering pen (8) and

the orifice support tube (4) allow the

shock strut to absorb and control the

impact even better. The oil is after

compression in the upper chamber

and will return to lower chamber. To.Oleo-pneumatic Shock strut

avoid to oil and the nitrogen to leak

throw the gap between the inner and outer cylinder, the shock strut has a seal (9) attached to the lower bearing (10) to seal that gap and allows the cylinders to move into each other.

Body gear steering system

The Boeing 747 has a body gear steering system. This system helps the pilots to control the aircraft To get a better view of the body gear steering system, a description of the construction is made.

Function

Because the Boeing 747 is a heavy aircraft, it needs more gear struts to attach the wheels, which have to resist the great weight of the aircraft on the ground. The Boeing 747 has two body gears under its body. During a turn, these body gears will not follow the radius of the turn as the wing gears do. As a result of this, great forces will stress on the wheels and on the strut. Furthermore the body gear wheels will wear faster than normal as a result of tyre scrubbing. Much maintenance on the body gear wheels and struts is necessary to keep the body gear in good condition. To avoid this problem, body gear steering is introduced. Body gear steering allows the body gear of the Boeing 747 to rotate thirteen degrees whether right or left. Now body gear steering has been installed, sharper turns can be made without damaging the wheels or the strut.

Figure 11.Tyre scrubbing and how to avoid tyre scrubbing

Construction

To allow body gear steering, the wheel truck has to rotate in respect to the shock strut. To rotate the wheel truck, the inner cylinder of the shock strut is rotatable. Like at normal gears, the body gear has a torsion link that is linked to the wheel truck and the shock strut. However, instead of an upper torsion link, two steering cylinders were added. These cylinders, connected to each other on the lower torsion link, are connected to the shock strut by a yoke. Now, if one of the cylinders extends, the wheel truck can rotate. The pressure for both cylinders is delivered by hydraulic system 1.

Operation

When the nose gear turns in one direction, the body gear turns smaller proportional angles in the other direction. There are some requirements for the body gear steering. To prevent the body gear from damaging during a high speed turn, the body gear steering will switch off when a speed of twenty knots is reached. From that point, it will function as torsion link only. There is also a limit at the angle of rotation. The body gear rotates maximum thirteen degrees whether left or right. The angle of thirteen degrees will be reached when the nose gear is rotated seventy degrees. Furthermore, the body gear may not be tilted and has to be extended and locked.

Air/ground system

Some systems of the aircraft, like the spoilers, operate different in air than on the ground. This makes it important to know if the aircraft is in the air or on the ground. Therefore, sensors that measure the gear tilt are mounted on the landing gear. Those sensors will send their position data to a Proximity Switch Electronics Unit [PSEU]. The PSEU supplies the position and the control data for primary landing gear sensor and alternate landing gear sensor subsystems. The PSEU will convert the position data to control data before it will send the control data to the air/ground relays. Air/ground relays have the task to tell the aircraft systems when the aircraft is in the air (air mode) and when it is on the ground (ground mode). Some systems do not have enough information when they only know if the landing gear is tilted or not. Many more sensors are mounted on the landing gear, like: up- and down lock sensors and wheel door sensors. However, signals from the landing gear lever switch, body gear steering (body gear has to be centred during in most conditions) are also important. Together, all these subsystems create the air/ground system.

Problems can appear in the air/ground system. In case the air/ground system does not get a signal from the sensors after takeoff, the air/ground system will stay in ground mode. When the air/ground system stays in ground mode, some systems in the cockpit will not work, like the autopilot. To continue the flight with this problem, the fuses of the air/ground system can be switched off and then the relays will go to air mode. However, the pilots may not forget to put in the fuses after touch down because the spoilers and brake system will not work properly.

Hydraulic brake system

The hydraulic brake system helps the pilots controlling the aircraft on the ground and in more situations. To get a better view of the hydraulic brake system, a description of the construction of this system is made. When it is clear how this system is constructed, an explanation of how the system works can be given.

Function

The hydraulic brake system has the task to slow down the aircraft on the runway when it lands. The brake system also holds the aircraft during parking and engine run up. However, also after take-off the wheels have to be slowed down before they are fully retracted. The brake system also helps with making turns on the ground. There are different kinds of brake systems (drum brakes, single disc brakes and multiple disc brakes). The multiple disc brakes are the most used on airliners. Also the Boeing 747 uses multiple disc brakes. The choice of the brake system depends on how much heat the brakes have to resist.

Construction

The multiple disc brakes exists out of four rotor discs (1) and three stator discs

(2) that rotate besides each other. The discs are made of carbon and contain a material that creates much friction when the discs are pressed together. The rotor discs have rebated joints (3), which allow the rotor discs to sit still in the wheels. The wheels also have rebated joints. The stator discs are connected to the wheel axis, which cannot rotate (4). Now, if the wheels rotate, the rotor discs will rotate together with the wheels while the stator discs will stay still. Next to the inner rotor disc, a piston housing is mounted (5). This housing contains seven cylinders (6) that press the rotor discs and stator discs together. The cylinders are pressurized by hydraulic system 4. When hydraulic system 4 cannot pressurize the cylinders, hydraulic system 1 will take over this task. If also hydraulic system 1 cannot pressurize the cylinders, than hydraulic system 2 will do pressurize them. The pressure that the brakes need differs from 160 PSI to 3000 PSI. The cylinders that are connected to each other in the piston housing are connected to the hydraulic system by just one hydraulic line (7). To avoid too much pressure on the system, a bleeding valve (8) is mounted on the piston housing. When using the brake system, the brake discs will wear. To check how much the discs are

worn, two wear indicator pins (9) are used. If these pins are not visible anymore, the brake discs should be replaced.

1.Rotor disc (4x)

1

2.Stator disc (3x)

3

3.Rebated joints

84.Wheel axis

625.Piston housing

6.Cylinder (7x)

7.Hydraulic line

78.Bleeding valve

5

9.Wear indicator

pin (2x)

9

4

Multiple disc brake

Operation

If one of the pilots creates a force on the ends of the rudder pedals, the brake system will activate. The more force presses on the pedals, the more the brake will slow down the aircraft. Slowing down the aircraft happens when the cylinders in the piston housing extend. When the cylinders extend, the discs will be pressed together. The more the discs are pressed together, the more friction will be created. This will slow down the aircraft. When the cylinders retract, the discs will stay together. To avoid this, small leaf springs are mounted between the discs to create a small space between the discs. When activated, the auto brake system will steer the braking system without any input of the pilots.

Brake control system

The brake control system supplies some functions that help the pilots. These functions are: antiskid protection, automatic braking, and brake torque control. This system also supplies arming logic for the body gear steering system. The automatic braking system will operate when the wheel speed increases to sixty knots or more during touchdown. A selector makes it possible to allow five deceleration levels. Also for a rejected take off there is a deceleration rate. The brake torque control has to prevent the brake from too much torque. When the brake torque control measures too much torque at one of the wheels, it will send a signal to the antiskid system. The antiskid system prevents the aircraft from skid. This is done by sensors that are mounted in each wheel. Those sensors measure the wheel rotation speed. The most effective way of braking is with a wheel speed that is around 85 to 90% of the ground speed. When the wheel speed is much lower than the ground speed, the antiskid system will release the brake at the certain wheel.

The brake system will only work when the aircraft is on the ground. Therefore it gets information about the gear tilt from the PSEU. In case this system has been broken, the brake system gets information from the wheels. If they rotate, than the aircraft should be on the ground. This activates the brake system.

difficult test. These test are all the CS-25 landing gear

Retracting mechanism

The rules applied to the retracting mechanism are specified in CS 25.729:

(a) Unless there are other means to decelerate the aircraft in flight at this speed, the landing gear, the retracting mechanism, and the aircraft structure (including wheel well doors) must be designed to withstand the flight loads occurring with the landing gear in the extended position at any speed up to 067 VC.

(b) Landing gear doors, their operating mechanism, and their supporting structures must be designed for the yawing manoeuvres prescribed for the aircraft in addition to the conditions of airspeed and load factor prescribed in sub-paragraphs (a)(1) and (2) of this paragraph.

(c) Landing gear lock. There must be positive means to keep the landing gear extended in flight and on the ground. There must be positive means to keep the landing gear and doors in the correct retracted position in flight, unless it can be shown that lowering of the landing gear or doors, or flight with the landing gear or doors extended, at any speed, is not hazardous.

Emergency operation. There must be an emergency means for extending the landing gear in the event of (1) Any reasonably probable failure in the normal retraction system; or

(2) The failure of any single source of hydraulic, electric, or equivalent energy supply.Tyres and Wheels

The wheels and tyres must resist many hard forces and wearing. Therefore not every tyre and wheel can be used on an aircraft. The specificities rules are on this subject are described in CS 25.731 and CS 25.733

One gear landing conditions

The one gear landing causes extreme forces on the particular gear. Therefore CS-25 describes the following requirements about the landing.

For the one gear landing conditions, the aircraft is assumed to be in the level attitude and to contact the ground on one main landing gear, in accordance with figure 4 of Appendix A of CS 25. In this attitude

(a) The ground reactions must be the same as those obtained on that side under CS 25.479(d)(1), and

(b) Each unbalanced external load must be reacted by aircraft inertia in a rational or conservative manner.1

Nose wheel and steering system

The nose wheel and also the steering mechanism also has to function in case of an emergency or failure. CS-25 describes the following requirements about this system:

(a) The nose-wheel steering system, unless it is restricted in use to low-speed manoeuvring, must be so designed that exceptional skill is not required for its use during take-off and landing, including the case of cross-wind, and in the event of sudden power-unit failure at any stage during the take- off run. This must be shown by tests. (See AMC 25.745 (a).)

(b) It must be shown that, in any practical circumstances, movement of the pilots steering control (including movement during retraction or extension or after retraction of the landing gear) cannot interfere with the correct retraction or extension of the landing gear.

(c) Under failure conditions the system must comply with CS 25.1309 (b) and (c). The arrangement of the system must be such that no single failure will result in a nose-wheel

position, which will lead to a Hazardous Effect. Where reliance is placed on nose-wheel steering in showing compliance with CS.

Bibliography

Literature

Boom, L.C. van denTabellen en Formules, Werktuigbouwkunde niveau 4

1th druk Baarn, 2000

Van den Brink, R.

Technische leergang: Hydrauliek 6th drukAmerongen, 2008

Budinski, K.G; Budinski, M.RMateriaalkunde Technici

2nd edition Schoonhoven, 1999

Callister, William D.; Rethwisch, David G.Materials, Science and Engineering

8th druk

Currey, Norman S.Aircraft Landing Gear Design: Principles and Practices

Marietta, Georgia, 1988Lockheed Aeronautical Systems Company

Dost, FConstrueren Kernboek 1

3th druk Baarn, 2003

Dost, FConstrueren Kernboek 2

3th druk Baarn, 2005

Hieminga, Jelle; IJspeert, Simon; van Langen, PieterLanding Gear

Amsterdam, 2010Hogeschool van Amsterdam

Domein Techniek

Oxford Aviation AcademyAircraft General Knowledge

Oxford, 2010

PalletAircraft Instruments and Integrated Systems

Essex, 1992

Wentzel, TillyOpbouw Projectverslag

Amsterdam, 2009

Hogeschool van AmsterdamDomein Techniek

Manuals

Boeing & KLMAircraft Maintenance Manual for 747-400

2004

Websites

http://www.onderzoeksraad.nl/docs/rapporten/2006039_2005135_JA-01-KZ_verkort_rapport.pdf

06-09-2005

http://www.airliners.netPhotos

http://www.boeing.com/commercial/airports/747.htm 12-2002

http://www.boeing.com/commercial/airports/faqs/arcandapproachspeeds.pdfAirport Reference Code and Approach Speeds for Boeing Airplanes

10-10- 2010

http://www.easa.europa.eu/agency-measures/certification-specifications.phpEASA CS25

12-08-2010

Appendices

I TYPES OF LANDING GEAR1

II GROUND LOOP3

III LANDING GEAR STORAGE.4

IV LOCATION OF THE NOSE GEAR AND THE MAIN GEAR5

V TYRE REGIONS6

VI BODY GEAR CROSSWIND CALCULATIONS7

VII INCIDENT LANDING RATIO DETERMINATION9

VIII INVESTIGATION REPORT11

ITypes of landing gear

Conventional

Tricycle

Single main wheel

Cycle gear

Quadricycle

Multi-bogey gear

IIGround loop

IIILanding gear storage.

IVLocation of the nose gear and the main gear

1Nose Gear2Wing Gear (main gear)3Body Gear (main gear)

1

2

3

VTyre regions

1Sidewall

1