Questions Flashcards
1
Q
1.1 What are the basic sections of a turbine engine?
A
The basic sections of a turbine engine are the intake, the compressor, the combustion chamber,the turbine and the exhaust section
2
Q
1.2 What is the basic description of the functioning of a turbine engine?
A
Air is drawn (or rammed), via the intake, into a multi-stage compressor, fuel is added and ignited in the combustor, and the air is expanded through the turbine stages before being expelled out the back of the engine.
3
Q
1.3 How is Newton’s second law mathematically expressed?
A
Force = mass x acceleration or F = Ma.
4
Q
1.4 What is Newton’s third law?
A
For every action there is an equal and opposite reaction.
5
Q
1.5 What is the practical application of Newton’s third law?
A
Jet thrust.
6
Q
1.6 How is thrust produced in a turbojet or turbofan engine?
A
By accelerating a mass of air through the engine.
7
Q
1.7 What is Charles’ law?
A
Charles’ law states that at constant pressure, the volume of a given mass of gas is directly proportional to its absolute temperature.
8
Q
1.8 What is Boyle’s law?
A
Boyle’s law states that at constant temperature, the pressure and the volume of a gas are inversely proportional.
9
Q
1.9 How does the temperature, density, pressure and velocity of a gas flow vary through a convergent duct at subsonic speed?
A
Temperature, density and pressure reduce and velocity increases.
10
Q
1.10 How does the temperature, density, pressure and velocity of a gas flow vary through a divergent duct at subsonic speed?
A
Temperature, density and pressure increase and velocity reduces.
11
Q
1.11 How does the temperature, density, pressure and velocity of a gas flow vary through a convergent duct at supersonic speed?
A
Temperature, density and pressure increase and velocity reduces.
12
Q
1.12 How does the temperature, density, pressure and velocity of a gas flow vary through a divergent duct at supersonic speed?
A
Temperature, density and pressure reduce and velocity increases.
13
Q
1.13 Where does subsonic flow decrease in velocity and increase in pressure?
A
Through a divergent duct.
14
Q
1.14 Where does subsonic flow increase in velocity and decrease in pressure?
A
Through a convergent duct.
15
Q
1.15 Where does supersonic flow decrease in velocity and increase in pressure?
A
Through a convergent duct.
16
Q
1.16 Where does supersonic flow increase in velocity and decrease in pressure?
A
Through a divergent duct.
17
Q
1.17 What property is exhibited by accelerating gas flow?
A
That the total energy is constant.
18
Q
1.18 Where in a turbofan engine is the pressure highest?
A
In the diffuser.
19
Q
1.19 Where in a turbojet engine is the gas velocity highest?
A
Exiting the exhaust nozzle.
20
Q
1.20 Where in a turbine engine is the gas temperature highest?
A
At the flame in the combustion chamber.
21
Q
1.21 How does gas pressure vary as it passes through the combustion chamber?
A
It remains nearly constant, reducing very slightly due to construction inefficiencies.
22
Q
1.22 How does gas temperature and velocity vary as it passes through the combustion chamber?
A
Temperature and velocity both increase.
23
Q
1.23 How does gas pressure, temperature and velocity vary as it passes through the turbine section?
A
Pressure and temperature progressively reduce, and velocity increases through the nozzle guide vanes and stators and reduces through the turbine blades.
24
Q
1.24 Why is the Bravton Cycle in a turbine engine referred to as an open or continuous cycle?
A
Because intake, compression combustion and exhaust are all occurring at the same time.
25
2.1 What are the main types of turbine engine?
Turboshaft, turboprop, turbojet and turbofan.
26
2.2 What is the definition of the bypass ratio of a turbofan engine?
The ratio of the mass of the air bypassing the engine core, to the mass of the air passing through the engine core.
27
2.3 What is the main similarity between a turbine engine and a reciprocating engine?
Both the turbine and reciprocating engines are internal combustion.
28
2.4 What is the main advantage of a turbine engine over a reciprocating engine of the same power
output?
The turbine engine has a much higher power to weight ratio than a reciprocating engine of similar power output.
29
2.5 How does the combustion cycle of a turbine engine compare with that of a reciprocating engine?
A turbine engine combustion takes place at a near constant pressure, while in a reciprocating engine combustion takes place at near constant volume.
30
2.6 What is the functional difference between a turboprop and a turboshaft?
A turboprop engine has an output shaft which drives a gearbox for a propeller while a turboshaft engine has an output shaft which drives a gearbox for something other than a propeller, e.g. an helicopter's main rotor gearbox.
31
2.7 How is a free power turbine characterised?
It is mechanically completely independent of the other spools in the engine.
32
2.8 What are the advantages of a free turbine propulsion system?
Easier to start.The spools can run at their optimum speeds, independent of each other. Design flexibility of layout.The ability to stop the propeller or helicopter rotors while the engine is still running.
33
2.9 Why does a free turbine turboprop engine require a smaller starter motor than direct-drive and
compound turboprops?
Because only the high pressure turbine and compressor need to be rotated on start.
34
2.10 Which part of a turboprop engine usually powers the accessory gearbox?
The high pressure compressor (N2) section.
35
2.11 What drives the main drive gearbox in a free turbine turboprop or turboshaft?
The low pressure turbine(s).
36
2.12 What is the meaning of the term spool?
A spool is an independent compressor, shaft, turbine combination.
37
2.13 What is a compressor/turbine combination called?
A spool.
38
2.14 Why do turbine engines have multiple spools?
So that each can operate closer to their design rpm.
39
2.15 In a twin spool turbine engine, which are the high pressure and low pressure stages of the
compressor and turbine?
The first stage(s) of the compressor and the last stage(s) of the turbine are low pressure, whilethe last stage(s) of the compressor and the first stage(s) of the turbine are high pressure
40
2.16 Within an engine's spools, what drives the intermediate pressure compressor?
The intermediate pressure turbine.
41
2.17 Within an engine's spools, what drives the low pressure compressor?
The low pressure turbine.
42
2.18 What is the description of a turbofan engine?
A fan/low pressure compressor supplying supercharged (pressurised) air to the engine core and to the bypass duct.A ducted propeller, suitable for powering high speed aircraft.
43
2.19 What is the 'core' of a turbofan engine comprised of?
The compressor, combustion chamber, turbine and exhaust sections.
44
2.20 What is the bypass ratio if 20% of the intake air passes through the engine core?
4:1
45
2.21 In terms of bypass ratio, how does a pure turbojet compare to a turbofan?
A pure turbojet has no bypass air.
46
2.22 What percentage of bypass air passes through the compressor of a turbofan engine?
0%
47
2.23 Why is a turbofan engine more efficient than a turbojet engine?
Because the turbofan accelerates a relatively larger amount of air a relatively lower amount.
48
2.24 Why are turbofan engines more resistant to FOD ingestion than pure turbojet engines?
Because the fan blades are bigger and stronger than the first stage compressor stages of theturbojet, and because most of the air bypasses the engine
49
2.25 What are the advantages of a twin spool over a triple spool bypass engine?
Lower cost.Easier maintenance
50
2.26 What is the advantages of a triple spool over a twin spool bypass engine?
Shorter and lighter.Better range/fuel consumption.
51
2.27 What is the name given to a turbine engine which has propeller blades mounted radially from
the low pressure turbine?
An aft fan turbojet.
52
2.28 What output is provided by an APU?
AC electrical power and bleed air.
53
2.29 What is the primary purpose of an APU?
To provide AC electrical power and pneumatic bleed on the ground, and in flight in the event of a main engine failure.
54
2.30 What section of an APU provides pneumatic bleed air for engine start?
The compressor.
55
3.1 What are the ideal characteristics of a turbine engine's intake?
To deliver the air to the compressor evenly, with no turbulence and at higher than ambient pressure.
56
3.2 What is the shape formed by the duct of an intake on a turbofan powered subsonic aircraft and why?
Divergent, to convert kinetic energy into pressure energy.
57
3.3 What is RAM effect?
The pressurisation of the air entering the intake caused by the aircraft's forward speed.
58
3.4 What is the most common type of intake on turbofan engines?
Pitot type.
59
3.5 What is the disadvantage of using a divergent intake duct?
Separation of the boundary layer on the duct walls.
60
3.6 What are the disadvantages of a long intake duct?
Too much drag, with a resulting loss of efficiency.
61
3.7 What is the primary disadvantage of divided entrance intakes?
The potential for interference to the flow when the aircraft yaws.
62
3.8 Why do some turboprop and turboshaft engines have shrouds or screens over their intakes?
To reduce the possibility of ingesting birds, dust, sand and other FOD.
63
3.9 How do supersonic air intakes decelerate the air to subsonic speeds prior to compressor entry?
By creating shock waves through which the airflow passes, thereby increasing its pressure,density and temperature and decreasing its velocity.
64
4.1 What is the configuration/description of a centrifugal compressor?
Short in length, spoke like design, easy to manufacture, with high compression per stage.
65
4.2 What are the advantages of a centrifugal compressor over an axial flow compressor?
Cheaper and easier to manufacture.More robust.Better able to handle FOD ingestion. Less prone to stalling and surging. Relatively lower in weight.
66
4.3 What is an impellor?
The internal rotating component of a centrifugal compressor, which takes inlet/intake air at its centre and compresses it by centrifugal force. Sometimes known as the rotor.
67
4.4 What is the purpose of the diffuser?
To decrease the air's velocity and to increase its pressure.
68
4.5 Where does the air pressure increase as it passes through a centrifugal compressor?
Air pressure increases progressively through the impeller and increases rapidly through the diffuser.
69
4.6 How does velocity and pressure vary through a centrifugal compressor?
Velocity and pressure both increase through the impeller, however through the diffuser the velocity decreases and the pressure increases.
70
4.7 What is the configuration/description of an axial flow compressor?
Multiple stages of rotating aerofoil blades and fixed vanes (stators), with a large mass flow capacity with high efficiency.
71
4.8 What are the advantages of an axial flow compressor over a centrifugal compressor of the same frontal diameter?
Higher compressor pressure ratios and greater mass airflow.
72
4.9 What are the advantages of an axial flow compressor over a centrifugal compressor?
A lower frontal area for a given mass airflow, which allows for a smaller diameter engine.
Achieves higher compression ratios more efficiently.
A lower pressure rise per stage.
A much lower airflow exit velocity compared to its inlet velocity.
73
4.10 What is the definition of the compressor pressure ratio?
The ratio of the compressor outlet pressure to the compressor inlet pressure.
74
4.11 What is the typical total compressor pressure ratio of a single stage centrifugal compressor?
Up to about 7.5:1
75
4.12 What is the typical total compressor pressure ratio of a double stage centrifugal compressor?
Up to about 15:1
76
4.13 What is the typical total pressure ratio of a modern axial flow compressor?
20:1, but up to 30:1 in some cases.
77
4.14 How does the temperature, pressure and velocity of the air change as it passes through each stage of an axial flow compressor?
Temperature and pressure progressively increase and the axial velocity increases across the rotors and decreases across the stators, but overall velocity decreases, especially when considering the effect of the diffuser.
78
4.15 What is the function of the inlet guide vanes?
The inlet guide vanes are designed to direct the airflow onto the first stage rotors at an appropriate angle of attack.
79
4.16 What is the function of variable inlet guide vanes?
To change angle depending on flow characteristics, directing the airflow onto the first stage rotors at the optimal angle of attack, thereby reducing the chance of a compressor stall during operation off design (at low) rpm.
80
4.17 What is the function of the variable stators in an axial flow compressor?
The variable stators are designed to direct the airflow onto the subsequent stage of rotors at theoptimum angle of attack, thereby increasing the compressor pressure ratio.
81
4.18 What is the primary function of each row of stator blades?
To convert the kinetic energy of the airflow into pressure energy.
82
4.19 What is the secondary function of the stator blades?
To stop swirl in the air thereby maintaining axial flow as it passes through the compressor.
83
4.20 What are the function of the rotors and stators in an axial flow compressor?
The rotors accelerate the airflow and the stators decelerate the flow to increase the air's pressure.
84
4.21 What shape does an axial flow compressor form towards the combustion chamber?
The annulus area of an axial flow compressor reduces towards the high pressure end, forming a convergent duct.
85
4.22 Why does the air annulus area decrease towards the high pressure end of an axial flow compressor?
To allow for the reduction in the volume of the air as its pressure progressively increases and to maintain the axial velocity of the air constant as the density increases.
86
4.23 Why is there only a small pressure increase across each stage of an axial flow compressor?
Because the aerofoil blades are, by necessity, quite small and consequently only produce a small amount of lift at each stage.
87
4.24 What effect does the diffuser have on the air leaving the compressor?
A diffuser reduces the velocity and increases the pressure of the airflow.
88
4.25 What is likely to result from unstable airflow through a compressor?
A compressor stall.
89
4.26 What determines the angle of attack of a compressor blade at a given blade angle at any given time?
The rpm of the compressor and the TAS of the airflow.
90
4.27 What is the condition that leads to compressor stalls?
The axial velocity of the air flow is too low relative to the rpm.
91
4.28 What are the effects of a dirty or FOD damaged compressor?
Lower compression ratio/output and/or a compressor which is more prone to stalling.
92
4.29 What are the indications of a compressor stall?
A surging/popping noise.A rise in gas temperature (E.g. EGT/ITT) Fluctuations in rpm.
93
4.30 What is the principle effect/purpose of the bleed valves/bands in the compressor?
To offload the compressors (LP and HP) and increase the mass airflow through the early stages of the compressor, during acceleration from low engine speed thereby reducing the possibility of compressor stall and surge at low rpm.
94
4.31 What is the function of the bleed valves/bands in the compressor?
To increase the mass airflow through the early stages of the compressor at low rpm.
95
4.32 What positions are the surge/stall bleed valves or bleed bands during engine acceleration?
Open at low and increasing rpm, progressively closing as rpm increases to design rpm.
96
4.33 Do turbine engines use the same compressor bleed outlets for surge/stall protections as they do
for environmental control systems?
No.
97
4.34 What is the normal source of bleed air at high and moderate (climb and cruise) power settings?
The low pressure compressor.
98
4.35 What is the normal source of bleed air at low (descent) power settings?
Both the low and the high pressure compressors in a two spool engine, and all three of the low intermediate and high pressure compressors in a three spool engine.
99
4.36 What are the effects of bleeding air off the compressor for systems such as anti-icing?
Gas temperature increases, thrust reduces and SFC increases.
100
4.37 What is the main disadvantage of bleed significant amounts of air from a turbine engine's compressor?
The mass flow of air through the compressor reduces and consequently the efficiency of the engine reduces.
101
5.1 Why is the velocity of the airflow slowed significantly as it enters the combustion chamber?
To reduce the velocity to below the flame propagation speed and thereby reduce the possibility of a flame out.
102
5.2 What is the purpose of burning fuel in a combustion chamber?
To increase the volume (velocity) of the gases.
103
5.3 How does the velocity of the gas vary through the combustion chamber?
The velocity is reasonably constant through the burner, but it increases significantly through the nozzle guide vanes as it exits the combustion chamber.
104
5.4 What are the basic combustion chamber design configurations?
Can, can-annular and annular.
105
5.5 What is the configuration/description of a can combustion chamber?
Combustors external to the engine, and most easily serviced.
106
5.6 What is the configuration/description of a can-annular combustion chamber?
Multiple, separate interconnected chambers, encircling the turbine.An efficient combination of the other two designs.
107
5.7 What is the configuration/description of an annular combustion chamber?
A segmented chamber made up of partitions and baffles, mounted co-axially about the engine axis, receiving primary air through a cylindrical shroud. (Dual ring system).
108
5.8 What is the function of the inter connectors between the chambers of multiple chamber combustion sections?
To equalise operating pressures and to allow combustion propagation between chambers.
109
5.9 What are the two types of can-annular or annular combustion chamber?
Through flow and reverse flow.
110
5.10 What does reverse flow mean with respect to combustion chamber design?
If a combustion chamber is described as reverse flow, the direction of the air/gas flow is changed through 180 o, at least once but normally twice as it progresses through the engine core.
111
5.11 Why is reverse flow used in combustion chamber design?
To reduce the length of the engine.
112
5.12 What are the advantages of an annular combustion chamber?
Less cooling air required.Improved combustion efficiency. No flame propagation problems. Compact, i.e. maximum utilisation of available space, with minimum length and diameter.Less weight.Lower cost.
113
5.13 What is a disadvantages of an annular combustion chamber?
Less structural integrity.
114
5.14 Why is far more air than is needed for combustion fed into the combustion chamber?
To supply air for cooling.
115
5.15 What is the purpose of the combustion chamber liner and its associated metering and air flow distribution devices?
To separate and meter the air flow for cooling and combustion and to create a region of low velocity recirculation of the air in the primary zone.
116
5.16 What proportion of the air passing from the compressor/diffuser into the inner combustor is used for combustion?
Conventionally about 25% of the air passing from the compressor/diffuser into the innercombustor is used for combustion, however as turbine engine design becomes more refined this proportion is increasing
117
6.1 What is the function of a turbine nozzle guide vane assembly (nozzle diaphragm)?
To accelerate the gas flow and to guide it onto the first stage turbine blades at the optimum angle.
118
6.2 Why does the diameter of the turbine discs increase progressively through the various stages?
To extract the maximum energy out of the gas stream as its energy reduces by the expansion through the earlier turbine stages.
119
6.3 What type of turbine blades are generally used in modern turbine engines?
Modern turbine engines utilise combined impulse/reaction blades.
120
6.4 Why is there a limit to the maximum gas temperature?
To overheat damage to the turbine section, primarily the nozzle guide vanes and the first stage turbines.
121
6.5 Which part of a turbine engine is most likely to sustain damage as the result of a hot start?
The nozzle guide vanes and first stage turbine.
122
6.6 What factor limits the output of the turbine section?
The temperature bearing capacity of the nozzle guide vanes.
123
6.7 How are turbine section components cooled?
By passing relatively cooler air over and or through them.
124
6.8 How does the pressure, velocity and temperature of the gas change as it passes through the turbine section?
Pressure progressively reduces. Velocity increases significantly through the nozzle guide vanes and fluctuates on a reducing trend through the turbine stages but is increased overall. Temperature progressively reduces.
125
6.9 What are the purposes of adding a shroud to the tips of some turbine blades?
To prevent gas leakage past the blade tips.To increase rigidity and reduce vibration.
126
6.10 What is turbine creep?
The gradual, progressive and permanent lengthening (elongation) of turbine blades.
127
6.11 What causes creep?
Creep is caused the combined effects of temperature and centrifugal stresses.
128
6.12 What is tertiary creep?
The final stage of creep development before turbine blade failure, where it is potentially detrimental to continue to operate the engine.
129
7.1 What is the shape of the duct formed by the cone and the wall of the exhaust section (propelling or outlet nozzle)?
Convergent.
130
7.2 Why are convergent-divergent nozzles incorporated into exhaust system design?
Used in transonic and supersonic aircraft where exhaust flow is supersonic. Shockwave at thethroat causes further acceleration of the flow and creates pressure thrust.To maximise cruise thrust and reduce engine noise.
131
7.3 What component of the exhaust system is designed to straighten the gas flow exiting the engine?
The rear turbine exhaust cone support struts.
132
7.4 What is the purpose of thrust reversal?
To assist the brakes to stop the aircraft during landing or RTO, thereby reducing stopping distance.
133
7.5 Up to what angle does the reverse thrust system change the exhaust gas flow?
Up to about 135 degrees.
134
7.6 What are the three types of thrust reverser?
Thrust reversers can be of either the clamshell, the bucket or the coldstream/cascade types.
135
7.7 What is the function of the blocker doors on a cold stream thrust reverser system?
To divert the bypass air outward and forward.
136
7.8 What safeguards are designed into the thrust reverser system to prevent inadvertent deployment in flight?
Mechanical, hydraulic and/or electrical interlocks and isolation valves, as well as air/ground sensor switches.
137
7.9 What actions should the pilots take if a thrust reverser unlocked light comes on in flight.
Be prepared to shut the engine down immediately if yaw, significant loss of airspeed or buffet is detected (indicating thrust reverser deployment).
138
7.10 In which type of engine is compressor and turbine noise the biggest issue?
In a high bypass turbofan.
139
8.1 How does the volatility of AVTUR compare to that of AVGAS?
The volatility of AVTUR is lower.
140
8.2 How does the calorific value of AVTUR compare to that of AVGAS?
The calorific value of AVTUR is higher.
141
8.3 How does the specific gravity of AVTUR compare to that of AVGAS?
The specific gravity of AVTUR is higher.
142
8.4 How does AVTUR's propensity for absorbing water compare to that of AVGAS?
AVTUR has a higher propensity for absorbing water. AVTUR holds significantly more water in suspension.
143
8.5 How does the flashpoint of AVIUR compare to that of AVGAS?
The flashpoint of AVTUR is higher.
144
8.6 What safety devices are used to prevent inadvertent selection of the wrong fuel?
Different colour dyes added to the fuel.The AVTUR delivery nozzle is too large to fit into an AVGAS tank.
145
8.7 By what names is turbine fuel known?
AVTUR; Jet A Jet A1; JP4 / JP5; or F40 / F35.
146
8.8 What is the difference between Jet A / Jet A1 and JP4 / JP5?
In the United States, Jet A is often called JP4 and Jet A1 is often called JP5.
147
8.9 Are turbine fuels required to have lubricating properties
Yes.
148
8.10 If alternative/emergency fuels are approved for use in an aircraft, in which document would this be specified?
In the aircraft flight manual.
149
8.11 Why is water contamination more of a problem in AVTUR than in AVGAS?
Water contamination is more of a problem in AVTUR than in AVGAS because AVTUR can hold significantly more water in suspension than can AVGAS.
150
8.12 How is water tested for in turbine fuel?
A chemical water check is carried out, where a tester changes colour if an excessive amount of water is present in the fuel sample.
151
8.13 What are the common additives put into AVTUR?
Anti-icing and anti-microbiocidal additives are often added to AVTUR.
152
8.14 What is the ideal air to fuel ratio for combustion in a turbine engine?
The ideal (stochiometric) air to fuel ratio is 15:1.
153
8.15 When refuelling, why is the fuel's density measured and recorded?
To enable an accurate fuel weight to be calculated for the volume loaded.
154
8.16 Why is knowledge of fuel's specific gravity (SG) relevant when refuelling an aircraft?
Loading is in volume (litres or gallons), and to convert volume to a weight (mass) the SG is required.
155
8.17 What is the specific gravity (SG) of Jet Al at 15C?
Approximately 0.8 kgs per litre (0.8 grams per millilitre)
156
8.18 Does the temperature of the fuel affect its specific gravity?
Yes. The relationship is inversely proportional. That is, if the temperature increases the specific gravity reduces.
157
8.19 Does the specific gravity of a fuel affect the range of an aircraft?
Yes. Higher specific gravity means greater range for a given volume of fuel.
158
8.20 Does the temperature of the fuel affect its calorific value?
No.
159
8.21 Before refuelling (pressure) how should the aircraft and tanker/bowser be bonded?
The aircraft and tanker should first be bonded to earth then to each other.
160
8.22 When an aircraft is parked for a prolonged period, how is condensation formation in the fuel tank minimised?
By leaving the tanks as full as possible?
161
8.23 Why are fuel temperatures monitored in cruise flight?
Because if the fuel is allowed to get too cold, ice may form in the fuel and the fuel may freeze(become waxy).
162
8.24 Why should turbine fuel not be allowed to drop below specified minimum temperature limits?
To reduce the possibility of ice crystals forming in the fuel or of the fuel freezing (becoming waxy).
163
8.25 What is the freezing point of Jet A1 (JP5)?
The freezing point of Jet Al (JP5) is -47°C.
164
8.26 What is the freezing point of Jet A (JP4)?
The freezing point of Jet A (JP4) is -40°C.
165
8.27 Why is a fuel heater (heat exchanger) part of a turbine engine's fuel system?
To prevent ice crystal formation in the fuel.
166
8.28 What do fuel heaters/heat exchangers use to raise the temperature of fuel?
Usually engine or gearbox oil, sometimes hydraulic fluid and occasionally bleed air.
167
8.29 What would be the function of a hydraulic fluid (or engine oil) heat exchanger located in a fuel tank?
To cool the hydraulic fluid/engine oil and to warm the fuel.
168
8.30 Why is AVTUR given an upper temperature limit?
To ensure the fuel does not boil (vapour release/lock).
169
8.31 What is the function of the fuel boost (auxiliary) pumps?
To provide fuel under pressure to the inlet side of the main fuel pump.
170
8.32 Where are the fuel boost (auxiliary) pumps usually located?
In the fuel tanks.
171
8.33 What type of pump is usually used for the fuel boost pumps?
Electrically driven gear or impeller type pumps.
172
8.34 What is the effect of a fuel tank's boost (auxiliary) pump(s) failing?
The main pump sucks the fuel out of the tanks.
173
8.35 What is the function of the fuel jettison (dump) system?
To reduce the fuel quantity and therefore aircraft weight in the event of an emergency landing.
174
8.36 During fuel jettisoning, what feature protects against the inadvertent dumping of all of the fuel?
Automatic jettison system shutoff and standpipes in the tanks.
175
8.37 What is the function of the fuel filter bypass valve?
To ensure that fuel flow is not impeded if the filter becomes blocked.
176
8.38 What is the function of a fuel flow meter system?
To measure the rate fuel is being used by the engine.
177
8.39 Where is the fuel flow transmitter located in the fuel system?
At the outlet of the fuel control/metering unit.
178
8.40 What is the purpose of the bypass system in the high pressure filter of a turbine engines fuel system?
To open and allow continued fuel flow if the filter becomes blocked.
179
8.41 What limiting functions does the FCU perform?
Limits the turbine outlet temperature and fan speeds.
180
8.42 What is the basic function of an electronic engine control (EEC)?
To schedule (trim/fine tune/adjust) the fuel flow to the engine under all conditions.
181
8.43 What method is used to reduce combustion temperature without reducing mass flow of the gases?
To trim the fuel/air ratio as lean as possible.
182
8.44 What is the function of the air shroud on an atomiser type (duplex) fuel spray nozzle?
To cool the nozzle and to reduce the formation of carbon deposits around the tip of the nozzle.
183
8.45 What is the function of the dump valve on engine shutdown?
To allow fuel in the manifold to drain away.
184
9.1 What is the primary function of a turbine engine's lubrication system?
The primary function of a turbine engine's lubrication system is to prevent metal to metal contact.
185
9.2 What are the secondary functions of a turbine engine's lubrication system?
The secondary functions of a turbine engine's lubrication system are cooling, cleaning, sealing and the reduction of corrosion.
186
9.3 What type of oil is typically used in turbine engines?
Turbine engines usually use synthetic oils.
187
9.4 What type of oil system is typically used in modern turbine engines, the PRV systems or the full flow system?
The full flow system.
188
9.5 Compared to a reciprocating engine, is the oil consumption on a turbine engine high or low?
Low.
189
9.6 What type of oil pump is usually used in turbine engines?
Turbine engines usually utilise gear type oil pumps.
190
9.7 Do oil pump driveshafts incorporate a weak shear point, as driveshafts in fuel pumps might?
No.
191
9.8 What is the function of a bypass valve?
To open and allow oil to pass, in the event that a filter or heat exchanger becomes blocked.
192
9.9 What is the function of magnetic plugs/chip detectors in turbine engine lubrication systems?
To remove ferrous metal debris and warn of impending gearbox component (gears/bearings)failure
193
9.10 How are bearing chambers normally cooled?
By lubricating oil
194
9.11 How are modern turbine blades cooled?
Modern turbine blades are cooled by passing relatively cooler air through and over them.
195
9.12 How do engine designers ensure cooling air does not flow in the reverse direction?
By ensuring a positive pressure gradient between the compressor air and the pressure of thecombustion gases
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9.13 What are the functions of air seals in a turbine engine?
To control cooling airflow by pressurising labyrinth and carbon oil seals.To cool hot section areas.
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9.14 Do hydraulic seals allow a controlled flow of air across the seal?
No.
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9.15 What type of seal comprises a metal ring housed in a close fitting static groove?
A ring seal.
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10.1 What is the function of a turbine engine starting system?
To uniformly accelerate the engine from rest to an rpm at which the gas flow through the turbine section provides sufficient torque to drive the compressor at idle rpm.
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10.2 What is the most common type of starter used on large modern turbofan engines?
An air turbine starter.
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10.3 What are the sources of bleed air to the air turbine starter?
An APU, another engine or from a ground compressed air source.
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10.4 When/how is an air turbine starter disengaged?
Automatically by a starter clutch at a pre-determined rpm.
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10.5 Why is a uniform rate of acceleration required during engine start?
To uniformly accelerate the engine from rest to an rpm at which the gas flow through the turbine section provides sufficient torque to drive the compressor at idle rpm, thereby avoiding a hung start.
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10.6 What is the most positive indication of light-up (light-off) during a turbine engine's start cycle?
The most positive indication of light-up (light-off) during a turbine engine's start cycle is an increase in the turbine temperature, however it is measured and displayed - EGT, ITT etc.
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10.7 What is 'self-sustaining' rpm?
The rpm at which the starter has cut-out and the turbine is able to accelerate on its own.
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10.8 What actions should be taken if the turbine or exhaust gas temperature is rising rapidly to exceed, or nearly exceed the specified limits?
If a hot start is experienced, the fuel flow to engine should be shut off immediately and the engine should be dry motored to reduce the turbine or exhaust gas temperature.
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10.9 What is a hot start?
A hot start is a start or attempted start where the turbine or exhaust gas temperature is rising rapidly to exceed, or nearly exceed the specified limits.
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10.10 What are the potential causes of a hot start?
Excessive fuel flow / excessive fuel to air ratio.A starter that is slow to accelerate the compressor/turbine-spool. A starter that cuts out too early.Low battery voltage.
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10.11 What actions are usually recommended in the event of a hot start?
Shut off the fuel supply and continue to dry motor the engine.
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10.12 What is the main indication of a hung start?
Very slow or no increase in rpm.
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10.13 What actions are usually recommended in the event of a hung start?
Shut off the fuel supply, discontinue the start cycle and continue to dry motor the engine.
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10.14 What are the indications of a compressor stall during start?
The indications of a compressor stall during start are the same as for any compressor stall, i.e. a popping or banging noise, fluctuating rpm indications and rapidly increasing turbine or exhaust gas temperature.
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10.15 What actions are usually recommended in the event that an engine stalls during start?
Shut off the fuel supply and continue to dry motor the engine.
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10.16 What are the indications of a wet start?
Torching out of the exhaust section.
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10.17 What actions are usually recommended in the event of a wet start?
Shut off the fuel supply and continue to dry motor the engine.
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10.18 What are the normal indications of an engine power loss/flame-out in a turboprop in the cruise?
Torque, rpm, temperature, and fuel flow decrease.
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10.19 What are the potential causes of a turbine engine flame-out?
Fuel exhaustion.Compressor stall.Flight at extremely high altitude. Severe turbulence.Severe icing.High speed manoeuvres.
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10.20 What is the purpose of the high energy component of the ignition system?
For in-flight relight/starting.
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10.21 What is the purpose of the low energy component of the ignition system?
For continuous energising of the igniter system.
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10.22 What output is usually available from the ignition system when continuous is selected?
Low output of 3 to 6 joules.
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10.23 Why are some aircraft (helicopters and agricultural a/c) left running during short stops?
Because the stop/start cycles are where most damage/wear is done to turbine engines.
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11.1 What are the instruments (gauges) which indicate the thrust output of a turbine engine?
Engine Pressure Ratio (EPR) gauge - turbojet/turbofan. Fan speed (Ni) gauge - turbojet/turbofanTorque gauge - turboprop.
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11. 2 What is the primary engine power gauge in a turboprop?
The torque gauge.
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11.3 What is the function of the torque gauge (torquemeter) in a turboprop engined aircraft?
To measure the power shaft output.
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11.4 Does a turbofan engine have a torque gauge?
No.
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11.5 What is the Engine Pressure Ratio (EPR)?
The ratio of exhaust gas pressure (P7) to compressor intake/inlet pressure (P1 or P2).
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11.6 In a multi-spool engine, what does N1 refer to?
The rpm of the low pressure compressor and/or fan.
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11.7 What is N1 on a turbofan?
Fan rpm.
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11.8 How is a turbine engine's rpm typically measured?
A generator, driven off the accessory gearbox, which drives an independent three phase ACelectrical system and a synchronous motor.
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11.9 What is the basic operating principle of an Exhaust Gas Temperature (EGT) indicating system?
A set of thermocouples arranged in parallel.
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11.10 What is ITT?
Interstage turbine temperature.
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11.11 What is EGT?
Exhaust gas temperature.
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12.1 What components of a turbine engine contribute to forward acting propulsive forces?
The compressor, diffuser, combustion chamber and exhaust section all contribute to forward acting propulsive forces.
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12.2 What components of a turbine engine contribute to rearward acting forces?
The turbine section and the propelling nozzle (if fitted) contribute to rearward acting forces.
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12.3 What factors will increase the thrust/torque output of a turbine engine?
An increase in air intake pressure/density (mass).An increase in fuel flow to the combustors (acceleration).
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12.4 How does RAM effect vary a turbine engine's thrust output?
Initially as airspeed increases RAM drag reduces thrust, but at higher airspeeds RAM rise (RAM effect) overcomes this and thrust increases.
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12.5 What is the result of RAM effect?
The kinetic energy of the intake air is converted into pressure (potential) energy, there by increasing the mass flow of air into the engine.
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12.6 Why does an increase in fuel flow increase thrust output?
Because the temperature of the combustion gases increases leading directly to an increase in turbine rpm and exhaust gas velocity.
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12.7 What is the effect of increasing the thrust levers of a turbofan aircraft on takeoff?
Fuel flow increases causing an increase in the EGT and engine rpms (and EPR if fitted).
240
12.8 What is the effect of air temperature on static (gross) thrust?
Increased ambient air temperature reduces static (gross) thrust.
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12.9 What is the effect of air pressure on static (gross) thrust?
Increased air pressure increases static (gross) thrust.
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12.10 How can the thrust output performance of a turbine engine be increased?
By increasing the maximum operating temperature, thereby increasing the maximum rpm.
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12.11 What limits the output of a turbine engine?
Temperature and rpm.
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12.12 Where is design rpm?
Slightly less than the maximum rpm.
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12.13 What is the effect of operating a turbine engine at design rpm?
SFC is minimised.
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12.14 How does an increase in altitude affect the maximum thrust output performance of a turbofan?
The maximum thrust output performance progressively reduces up to the tropopause, then because the ambient temperature remains constant, the rate of thrust reduction increases as the aircraft climbs through the tropopause.
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12.15 At a constant flight level, how does the maximum thrust output performance of a turbofan engine vary between 180 knots and 450 knots TAS?
The maximum thrust output performance initially reduces slightly due to RAM drag and thenrecovers to progressively increase due to RAM rise (RAM effect).
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12.16 How does an increased pressure altitude affect a turbofan engined aircraft's takeoff distancerequired (TODR)?
TODR increases.
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12.17 How does the use of bleed air services, such as anti-icing or air-conditioning, affect theperformance of a turbofan engine during a takeoff?
For a given set of takeoff performance requirements, the required EPR and/or Ni will beincreased and the EGT will be higher.
250
12.18 What is the effect of an increase in the relative humidity of the intake air on engine thrust?
Thrust will decrease because the density of the air will decrease.
251
12.19 How does thrust and fuel flow vary, at a constant engine rpm, if the air temperature increases?
Thrust and fuel flow will decrease because density reduces.
252
12.20 When is the thrust output from a turbojet or turbofan engine maximum?
The thrust output from a turbojet or turbofan engine is maximum at full power stationary on theground.
253
12.21 How much power is a turbojet or turbofan engine generating at maximum thrust?
At maximum thrust a turbojet or turbofan engine is generating no power at all, as it is stationary it is doing no work.
254
12.22 What is equivalent horsepower?
Shaft horsepower plus the effect of any residual jet thrust. Used in turboprops.
255
12.23 What is power?
Power is the rate of doing work, which effectively equates to thrust times lAS.
256
12.24 What is thrust horsepower (THP)?
A force, equating to mass x acceleration.
257
12.25 How is the power/thrust output of a turbine engine controlled?
The power/thrust output of a turbine engine is controlled by varying the fuel flow to the combustion chamber.
258
12.26 What is the normal power lever limit during a take-off in a turboprop aircraft?
Either the Ng; turbine temperature or torque limit, whichever occurs first.
259
12.27 How do thrust and fuel flow vary in the climb at a constant engine rpm and lAS?
Thrust/torque and fuel flow decrease.
260
12.28 How does N1 vary in the climb at constant thrust?
N1 increases.
261
12.29 Why does range increase at high (up to optimum) altitude?
Because the engine operates at its design rpm at the most efficient angle of attack.
262
12.30 What is specific fuel consumption (SFC)?
SFC is the fuel flow divided by the thrust output of an engine. That is, units of fuel used per unit of thrust per hour.
263
12.31 What does a low SFC mean?
A low SFC means that less fuel is being used for a given thrust output, so the engine's efficiency is high.
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12.32 What is the specific fuel consumption (SFC) if an engine in the cruise produces 5000lb of thrust and burns 2000lb of fuel over a 30 minute period?
2000 x 2 = 4000 lb/hr, divided by 5000 lb of thrust = a SFC of 0.8
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12.33 What is the effect on Specific Thrust and Specific Fuel Consumption (SFC) if the compression ratio increases?
Specific thrust increases and SFC decreases.
266
12.34 What are the two factors which determine a turbine engine's propulsive efficiency?
Temperature and rpm.
267
12.35 What type of turbine engine is preferred for operating at cruise speeds above about 350knots TAS, and why?
Turbofan engines, because turboprop engines lose efficiency at higher airspeeds.
268
12.36 Which type of turbine engine has the highest propulsive efficiency in the medium to highsubsonic speed ranges (>350kts)?
A high bypass turbofan.
269
12.37 Why can a ducted fan operate at high speed more efficiently than a propeller?
Because the ducting reduces tip losses and more blades means more power can be developed with a smaller diameter.
270
12.38 Why does the propulsive efficiency of a turboprop reduce with increasing airspeed?
Because of increasing compressibility effects on the propeller blades.
271
12.39 Why is a turboprop more efficient at low airspeed than a turbojet of equal thrust?
Because the turboprop accelerates a higher air mass to a lower velocity.
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12.40 Why can a turboprop not operate at the high airspeeds of a turbofan?
Because shockwaves form at the propeller blade tips at higher Mach numbers.
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12.41 What operating parameters best suit a turboprop engined aircraft?
Relatively shorter routes flown at relatively lower altitudes and airspeeds.
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12.42 What environmental conditions are most likely to require the use of thrust augmentation(water/methanol)?
High DA - i.e. high pressure altitude and high ambient temperature
275
12.43What is the effect of injecting water/methanol into the combustion chamber?
It cools the air, thereby increasing the weight (mass) of the gas flow through the engine.
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12.44 Why is methanol mixed with the injected water?
To resist the water freezing at altitude.
