Jet Aircraft Maintenance Fundamentals ATA78 Thrust Reverser

ATA 78 Thrust ReverserBook No: JAMF ATA 78 ALL Lufthansa Lufthansa Base Issue: July 2000 For Training Purposes Only Lufthansa 1995 ã Technical Training GmbH Training Manual Fundamentals

ATA 78 Thrust Reverser

Book No: JAMF ATA 78 ALL

Lufthansa Lufthansa Base

Issue: July 2000 For Training Purposes Only Lufthansa 1995 ã

Technical Training GmbH

Training Manual Fundamentals

JAR-66

Jet Aircraft Maintenance Fundamentals

Beijing Ameco

Aviation College

For training purpose and internal use only.

Copyright by Lufthansa Technical Training GmbH All rights reserved No parts of this training

manual may be sold or reproduced in any form without permission of:

Lufthansa Technical Training GmbH Lufthansa Base Frankfurt

D-60546 Frankfurt/Main

Tel +49 69 / 696 41 78

Fax +49 69 / 696 63 84

Lufthansa Base Hamburg

Weg beim Jäger 193

D-22335 Hamburg

Tel +49 40 / 5070 24 13

Fax +49 40 / 5070 47 46

Aviation College

THRUST REVERSER PRINCIPLE

THRUST REVERSER INTRODUCTION

During landing run the brakes can usually bring the aircraft to rest within the

available runway distance

On a wet, icy or snow covered runway however the braking efficiency is

reduced The aircraft looses some of the adhesion between the tyres and the

runway and needs a much longer distance to come to rest

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Thrust Reverser Introduction cont.

Thrust reversers produce the braking force by deflecting the exhaust gas

stream to the opposite direction This deflection therefore causes a thrust force

opposite to the direction of movement

For the most efficient reverse thrust it would be ideal if the exhaust gas could

be deflected by 180 degrees, however, this would cause a major problem for

the engine operation

The deflection of the gas stream is approximately 45 to 60 degrees compared

to the ideal direction

This deflection angle results in a reverse thrust force that is much smaller than

the forward thrust force

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45 - 60°

RESULTINGREVERSETHRUST

Thrust Reverser Introduction cont.

On modern turbofan engines with a high bypass ratio, only the fan airflow is

reversed because this airflow causes most of the engine thrust

Note also that for safety reasons the thrust reversers can only be used on

ground The operation in flight is not permitted because it can cause very

dangerous aerodynamic changes that can lead to a crash

Later in this unit you will see how the reverser is protected against operation in

THRUST REVERSER TYPES

There are many different methods to reverse the engine airflow Turbofan

engines with a low bypass ratio, for example, usually have a common exhaust

nozzle for the cold and hot gas stream

These engines, therefore, also have a common thrust reverser that is either

installed in front off the exhaust nozzle as shown by this clamshell door type

reverser or at the exhaust nozzle like this bucket door type reverser

When the reverser is not operating, the deflection mechanism is flush inline

with the walls of the exhaust pipe so that it does not disturb the normal exhaust

gas stream

The clamshell door type reverser is installed on older jet engines with a low

bypass ratio It is usually pneumatically operated by bleed air from the HP

compressor

When the clamshell doors rotate to the reverse thrust position they block off the

normal flow path and uncover exit ducts with cascade vanes These cascade

vanes direct the gas stream in a forward direction

The bucket door type reverser is also found on older jet engines with a low

bypass ratio It is usually hydraulically operated by the aircraft hydraulic

CLAMSHELL DOOR TYPE REVERSER

BUCKET DOOR TYPE REVERSER

Thrust Reverser Types cont.

On turbofan engines with a high bypass ratio only the cold fan air is deflected

to give reverse thrust Therefore this reverser type is often called the fan

reverser

Reversers for the hot core engine gas stream were only used on some old

turbofan engines These so called turbine reversers are not used on modern

turbofan engines because they are not very efficient and due to the hot

environment they need very extensive maintenance

Fan reversers are most commonly cascade type reversers with translating

sleeves Depending on the engine type, these reversers are either

pneumatically operated by drive motors or hydraulically operated by actuators

in the engine cowling

When you pull the reverse thrust lever the translating sleeves move rearwards

and uncover the cascade vanes Blocker doors close off the fan discharge duct

so that all the fan air can only discharge through the cascade vanes

Later in this unit you will take a closer look at all reverser components

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BLOCKER DOORS

BLOCKER DOORSHYDRAULIC ACTUATOR

Thrust Reverser Types cont.

Another type of fan reverser that is used on modern turbofan engines is the

pivoting door type reverser

This engine shown has four large reverser doors The pivoting type reverser

doors are hydraulically actuated They are flush with the engine cowling when

the reverser is stowed and they close off the fan discharge airflow when they

EFFECTS OF REVERSER OPERATION

Thrust reversers give additional braking support but their operation can also

cause some disadvantages

The engine compressor can stall because the airflow through the engine is

disturbed by the deployed thrust reverser Another problem occurs when the

reverser operates at low aircraft speeds

At high speed, shortly after touchdown, thrust reversers are most efficient The

reverser gas stream is strongly deflected by the ambient airflow, and you can

see that the slower the aircraft becomes, the nearer the reverser gas stream

gets to the front of the engine

At very low aircraft speed the engine can ingest its own reversed airflow

Ingested exhaust gases cause severe compressor stall and high exhaust gas

temperatures

On some aircraft the reversed airflow can cause an air cushion below the wing

that decreases the load on the main landing gear and therefore the efficiency of

the aircraft brakes

The reversed airflow also whirls up a lot of dirt and dust from the runway

These foreign objects can damage the aircraft or the engine

To minimize all these disadvantages caused by the reverser operation, the

reversed airflow is matched to the engine position

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DECREASE BRAKEEFFICIENCYHIGH EGT

STALLDANGER

BRAKESUPPORT

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Figure 7 Thrust Reverser Advantage and Disadvantage

ho tro thang' a/c

vat the la (dat, da,.

) de bay vao engine

tang EGT len

chan dong khi' =>

de bi lock engine

Effects of Reverser Operation cont.

On this twin engine aircraft, for example, the outward pointing airflow is blown

at an optimum angle for reverse thrust

The inward pointing airflow is directed at such an angle it cannot blow into the

main wheel well and so that any whirled up dirt cannot cause any danger for

Effects of Reverser Operation cont.

On a four engine aircraft the angle of the reverse stream is more important,

because the reversed airflow of the inner engines can be ingested by the outer

engines

Therefore each engine reverser directs the airflow to the safest possible

direction This is especially important if the distance between the engines on

the wing is small

The reversed airflow can be matched by the correct arrangement of the

cascade vanes in the thrust reverser cowlings

This arrangement, however, has the disadvantage that a fan reverser cowling

for engine number 4 cannot be used on another engine position without

rearrangement of the cascade vanes

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Effects of Reverser Operation cont.

If the distance between the engines is large enough, as shown on this aircraft,

the reversed airflow has no effect on the neighbouring engine

On these aircraft the reverse airflow is always directed to get optimum reverse

thrust

Another advantage is that all engines have the same cowlings so that less

spare parts are needed

Note also that on all aircraft the thrust reverser is just an additional braking

support Even with a faulty reverser an aircraft can be released for service

however, the faulty reverser must be deactivated properly to make sure that

does not deploy unintentionally

In lesson 3 and 4 of this unit you will see in detail how reversers can be

SYSTEM ORGANIZATION AND OPERATION

SYSTEM ORGANIZATION

A typical thrust reverser system has three main subsystems These are the

control system, the actuation system and the airflow deflection system

Later in this lesson you will see that the purpose of these sub systems is

always the same but the methods are different from engine to engine

The control system is used to activate the thrust reverser and to increase the

engine power for reverse thrust

The reverser actuation system has either pneumatic or hydraulic components

It moves the airflow deflection mechanism by signals from the reverser control

system

The airflow deflection system directs the airflow in the optimum direction for

safe reverse thrust

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AIRFLOW DETECTIONSYSTEM

THRUST REVERSER CONTROLS

The main component of the control system is the thrust reverse lever in the

cockpit If you pull it you activate a reverser control switch below the control

pedestal It is used to start the reverser operation and to control the direction of

thrust reverser movement

The reverser control system is always protected by the air ground logic to

make sure that the thrust reversers cannot deploy in flight

Some aircraft, like the Boeing 737, also use an alternate ground signal from the

radio altimeter

This is an advantage on short runways because the reverser can be deployed

shortly before touch down when the flight altitude is less than 10 feet

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AIR / GRDLOGICGRD

Thrust Reverser Controls cont.

Another sub system of the reverser control system is the thrust reverser

interlock system It makes sure that the engine power can only be increased

after the reverser is fully deployed

You can see the operation of the reverser interlock system in lesson 2 and 3 of

T / RINTERLOCK

ACTUATOR

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Figure 13 Thrust Reverser Interlock

chi keo duoc 1 phan cua reverser vi interlock da khoa' lai, cho den khi

phan reverser cowl mo ra het thi` moi keo them duoc reverser de tang ga

len toi da cho viec thoi nguoc.

THRUST REVERSER ACTUATION SYSTEM

You can find hydraulic actuation systems on reversers with large blocker doors

such as the bucket door type reverser or the pivoting type reverser where the

blocker doors have individual hydraulic actuators

Hydraulic reverser systems are also possible on reversers with translating

sleeves and cascade vanes but their hydraulic actuators are more complicated

because they must be synchronized

Hydraulic reverser actuation systems usually have a control valve module that

receives signals from the control system to supply the hydraulic fluid to the

deploy or stow side of the actuators

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HYDRAULIC ACTUATOR

CONTROLVALVEMODULEGRD

Thrust Reverser Actuation System cont.

Pneumatic reverser actuation systems are only used on cascade type thrust

reversers with translating sleeves

They usually have air motors that are supplied by bleed air from the engine

The air motors operate the translating sleeve via drive shafts and gearboxes

with ballscrew actuators

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Thrust Reverser Actuation System cont.

All thrust reversers must have a locking system to make sure that the reverser

is kept safe in its stowed position and that it cannot move unintentionally

Hydraulic systems often have locking devices in the actuators or separate latch

mechanisms like this one

When the reverser door is stowed, the hook of the latch mechanism secures

the blocker door in the stowed position

Pneumatic reverser systems usually have brakes at the air motors as locking

AIRFLOW DIRECTION SYSTEM

This segment shows the airflow deflection components of a cascade type

reverser in more detail

You can find these components in the reverser cowling that is made of a fixed

part and a movable part the so called translating sleeve

The blocker doors are linked between the fixed cowl and the translating sleeve

They are flush inline with the fan air duct when the reverser is stowed

The blocker doors move with the movement of the translating sleeve

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FAN REVERSER CROSS SECTION

Airflow Direction System cont.

The cascade vane segments are bolted to the fixed part of the reverser

cowling If you take a closer look at the cascade segments you can see that

they look different

All deflect the airflow in a forward direction, but some of them also deflect the

airflow 45 degrees upwards, some others also deflect the airflow 45 degrees

downwards, and some of them deflect the airflow just at 90 degrees straight

out of fan air duct Finally there are even some segments that are totally

blocked

Note that an upward pointing segment from the right hand cowling points

downwards if you put it on the left hand cowling

The cascade segments direct the reverse airflow away from the aircraft

structure and from the ground Therefore, the arrangement is different for each

engine position

It is important to remember this if you have to replace cascade segments or

reverser cowlings You must always make sure that you have the correct

cascade arrangement for the engine position

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FAN REVERSAER COWLING

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Figure 18 Cascade Vane Segments

THRUST REVERSER INDICATION

The indication for the reverser system is usually very simple The system has

switches at the actuation components or position sensors at the fixed reverser

sleeve

Reverse unlock messages indicate that the reverser is not safe in its stowed

position

These messages are always amber They are also visible when the reverser

moves to the deployed position or to the stowed position

Many aircraft also have another indication light for the reverse system A green

reverse light indicates that the reverser is fully deployed and that the throttle

interlock is released so that the engine power can be increased for reverser

Figure 19 Thrust Reverser Indication

trong qua trinh reverser cowl chay ra chu REV co mau vang

chay ra het co mau xanh Green

PNEUMATICALLY OPERATED REVERSER

PNEUMATIC REVERSER OPERATING PRINCIPLE

Here you can see a simplified pneumatically operated reverser system The

main components are

the bleed air supply ducts

control valves

one or two pneumatic drive units

gearboxes

flexible drive shafts

and ballscrew actuators

Pneumatic reversers usually operate with high stage bleed air from the

en-gine’s HP compressor When the control valve is open, the air enters an air

motor in the pneumatic drive unit

The air motor starts rotating Via flexible driveshafts and gearboxes it operates

the ballscrew actuators

There are two more air supply possibilities for the reverser system Bleed air

supply from the low stage is not used for the thrust reverser This is because at

idle power high stage air is switched on and closes the check valve to the low

stage pick up

Low stage air is only used to supply the aircraft pneumatic system You will

learn more about this in unit 36 of the airframe course

Bleed air from the pneumatic system can be used to operate the reversers for

maintenance purposes when the engine is shut down

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