Air Systems, Starting, and Ignition

Question 1

Air is bled off from various
positions on the compressor of
a gas turbine engine and used
to lower the temperature of
those internal parts of the
engine which may suffer from
excessive heat.

Answer 1

bleed air

Question 2

The air bled from the
compressor which is used to
cool the internal components
of the engine is also utilized to
pressurize seals, which will
increase the efficiency of both
the compressor and the
turbine.

Answer 2

bleed air

Question 3

Air bled from the engine
compressor is also used to
service many aircraft systems.
In a modern turbofan aircraft,
these may include:

Answer 3

1. Air Conditioning /
   Pressurization
2. Hydraulic Reservoir Pressurization
3. Fuel Tank Pressurization
4. Domestic Water Tank Pressurization
5. Thrust Reverser Actuation
6. Fuel Heaters
7. Engine/Airframe Anti-Icing

Question 4

For use outside the engine
itself, air is typically bled from
two sources:

Answer 4

1. outlet on an early stage compressor
2. HP compressor through a HP bleed valve.

Question 5

During this operation
of the engine, the low pressure
bleed is usually of sufficient
pressure to maintain the air
conditioning/pressurization
system.

Answer 5

high power operation

Question 6

During this operation,
the low pressure bleed will fall
and the high pressure bleed
valve will open to ensure both
an adequate pressure and
sufficient airflow.

Answer 6

low power operation

Question 7

This
opens when the anti-icing is
selected. In this situation, the
requirement is for hot air and
lots of it,

Answer 7

high pressure bleed valve

Question 8

what is the requirement when the anti-icing is selected to open the high pressure bleed valve?

Answer 8

lots of hot air

Question 9

what is the relationship between the pressure and temperature when the high pressure bleed valve opens?

Answer 9

directly proportional

Question 10

When air is bled from the
compressor, it ________ the
thrust of the engine.

Answer 10

reduces

Question 11

When air is bled from the
compressor, it reduces the
thrust of the engine. This will
be indicated on the

Answer 11

P7 or Engine Pressure Ratio gauge

Question 12

Reducing the mass flow
through the engine will ________
the amount of cooling air
cooling the combustion
chamber. This will have the
effect of ________ the
exhaust gas temperature.

Answer 12

reduce; increasing

Question 13

Increasing the mass flow
through the engine will ________
the amount of cooling air
cooling the combustion
chamber. This will have the
effect of ________ the
exhaust gas temperature.

Answer 13

increase; reducing

Question 14

Bleeding air from the
compressor ________ the load
on the turbine, which will have
the effect of ________ engine
RPM for a given fuel flow.

Answer 14

reduces; increasing

Question 15

The specific fuel consumption
will _______ whenever an
engine bleed is opened.

Answer 15

increase

Question 16

used to cool the turbine
blades to seal both the bearing
chambers and areas either
side of the turbine discs.

Answer 16

air bled from the compressor

Question 17

2 ways of disposing used air:

Answer 17

1. dumped overboard
2. ejected into the main gas stream at the highest possible pressure

Question 18

There is a limit to the amount
of ________ which can be released
into the turbine from
combustion. This limit
imposed by the materials from
which the turbine blades and
nozzle guide vanes are
manufactured.

Answer 18

heat

Question 18

If turbine blades and nozzle
guide vanes are continuously
cooled, then the temperature
of their operating environment
can exceed the ________
of the material from which they are made.

Answer 18

melting point

Question 19

these too must be cooled as they are also heated by conduction from the turbine blades.

Answer 19

turbine discs

Question 20

works by controlling
the temperature of a tubular
shroud which is fitted closely
around the turbine casing.

Answer 20

active clearance control

Question 20

Air, extracted from the IP
compressor, flows axially
through the annular passage
between the HP compressor
discs and the IP compressor
drive shaft. This air absorbs
heat from the discs which
assists in controlling the
thermal growth of the
compressor blades.

Answer 20

selective cooling

Question 21

2 types of turbine cooling

Answer 21

1. active clearance control
2. selective cooling

Question 22

The use of ________ retained the
temperature of the turbine
blade below the critical level at
which excessive creep would
occur.

Answer 22

single pass internal cooling

Question 23

When higher gas temperatures were required, passing high pressure compressor air through the blade as well as the low pressure air achieved a reasonable ________ in the gas temperature before blade failure was experienced.

Answer 23

increase

Question 24

created its own boundary layer effect which protected the turbine blade from the onslaught of the hot gases coming from the combustion chamber.

Answer 24

film cooling which allows air to pass through small holes in the leading and trailing edge of the blade.

Question 25

If passing air through the blade once lowers its temperature, then passing the air through more than once will ________ it more.

Answer 25

lower

Question 26

The optimum number of times to pass the air through the blade was found to be

Answer 26

5

Question 27

is presently considered to be state of the art in turbine blade manufacture.

Answer 27

quintuple pass, multi-feed internal cooling with extensive film cooling

Question 28

use a similar method of cooling to that which is used to cool the turbine blades. The one major difference is that only high pressure compressor air is used.

Answer 28

nozzle guide vanes

Question 29

requirement for the turbine discs of heat conduction from the turbine blade to it

Answer 29

cooled and prevented from suffering thermal fatigue

Question 30

The front and rear faces of each of the turbine discs are cooled by _______, the actual pressure in each disc cavity being controlled by ________

Answer 30

high pressure compressor air; inter-stage seals

Question 31

prevents the leaking of oil or air into spaces where it should be excluded

Answer 31

sealing

Question 32

consists of fins which rotate within an annulus of oil, or where the exterior of the seal is static, the annulus can consist of either a soft abradable material or a honeycomb structure.

Answer 32

labyrinth seal

Question 33

are used to either prevent or control leakage of air between sections of the engine which are operating at different pressures.

Answer 33

inter-stage seals

Question 34

During operation, there is a pressure drop across each fin which results in a _______ from one side of the seal to the other.

Answer 34

restricted flow of air

Question 35

The amount of pressure dropped across the seal depends upon the _______ over which the air must pass.

Answer 35

number of fins

Question 36

which drops more pressure? (a) air passing over the five pins before the LP zone (b) over the two fins preceding the HP pressure zone

Answer 36

a

Question 37

If abradable materials was used in the manufacture of seals between two rotating shafts, there may be _______ generated between the fins and the abradable material due to flexing of the shaft. This would create ________ and the possibility of ________

Answer 37

friction; high temperature; shaft failure

Question 38

can be used to provide the seal without risking friction buildup.

Answer 38

inter-shaft hydraulic seal

Question 39

rotate close within an annulus of oil, any deflection of the shaft will cause the fin or fins to enter the oil, and the seal will be maintained without generating any undue friction of heat

Answer 39

fins of the inter-shaft hydraulic seal

Question 40

Air used to seal bearing chambers is taken from the intermediate stages of the compressor through ________ in the compressor rotor drum, and passed through communicating passages where it is required.

Answer 40

air transfer ports

Question 41

When cooling and sealing has been carried out, the air which has been doing the job has to be ________.

Answer 41

disposed of

Question 42

The high pressure air used for cooling is ejected into the ________. The low pressure air is fed out through its own dedicated ________.

Answer 42

exhaust system; vent pipe

Question 43

3 basic requirements which must be satisfied before a gas turbine engine can start:

Answer 43

1. Compressor/Turbine Assembly must be rotated to get air into the combustion chambers. 2. Fuel must be provided. 3. Ignition must be provided to start the air/fuel mixture burning.

Question 44

2 other requirements which satisfy servicing essentials:

Answer 44

- Motoring over the engine with no igniters operating or ‘blowout’ or ‘motoring over’ cycle. - Igniters operated independently of the start cycle.

Question 45

The two most common methods of rotating the high pressure compressor on modern civil aircraft are:

Answer 45

1. electric starter motor 2. air starter motor

Question 46

It was the original means of starting gas turbine engines, but it has fallen out of favor because of its weight.

Answer 46

electric starter motor

Question 47

where can the electric starter motor be fixed?

Answer 47

1. in front of the compressor casing 2. attached to the engine accessory gearbox

Question 48

It is the most popular starting system on large gas turbine engines. It is light, simple to use, and very economical.

Answer 48

air starter motor

Question 49

this method of obtaining engine rotation utilizes low pressure air as its power source

Answer 49

air starter motor

Question 50

where is an air starter motor attached?

Answer 50

accessory gearbox of the engine

Question 51

The sources of air available for engine start, in order of preference are:

Answer 51

1. Aircraft Auxiliary Power Unit (APU) 2. Ground Start Unit 3. Cross Bleed Start

Question 52

turns the reduction gear which rotates the engine high pressure compressor shaft through a ratchet clutch arrangement.

Answer 52

turbine rotor

Question 53

may also be included in the system which can be used to de-energize the air start valve.

Answer 53

flyweight cutout switch

Question 54

All gas turbine engines have two ________ which each feed a separate ________.

Answer 54

high energy igniter units; igniter plug

Question 55

how many high energy igniter units do gas turbines have?

Answer 55

2

Question 56

High energy ignition systems have an output of approximately

Answer 56

12 joules

Question 57

It may sometimes be necessary to help prevent engine flame out to have the ignition system selected in circumstances other than engine starting. Use of high energy ignition system on these occasions will cause the igniter plug to

Answer 57

erode, shortening its working life

Question 58

To minimize erosion of the igniter plugs, some aircraft engines are fitted with a ________ which includes a low energy continuous selection, as well as the high energy starting selection.

Answer 58

combination ignition system

Question 59

automatically deactivated at some point after self sustaining speed.

Answer 59

igniters

Question 60

If an aircraft stall is detected, the aircraft stall warning system will automatically select this

Answer 60

automatic ignition system

Question 61

works on the principle of charging up a very large capacitor and then discharging it across the face of an igniter plug.

Answer 61

high energy ignition unit

Question 62

this type of igniter plug work in similar to a piston engine spark plug but has a much bigger spark gap

Answer 62

older version

Question 63

The more modern version of the igniter plug is the ________.

Answer 63

surface discharge igniter plug