DGCA Technical General: Gas Turbines — Systems & Operation Questions
146 exam-style Gas Turbines — Systems & Operation questions for the DGCA Technical General paper (CPL · ATPL · PPL). The first 12 are free below with answers and explanations — sign up free to practise all of them with progress tracking.
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Q1
A centrifugal breather is used on a gas turbine engine:
- A To circulate the oil smoothly.
- B To minimise oil loss.
- C To emulsify the oil and air mixture for greater viscosity.
- D To allow oxidisation of the oil.
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Correct answer: B. To minimise oil loss.
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Q2
What is the effect on EGT and EPR if a bleed valve is opened?
- A Increase, increase
- B Decrease, decrease
- C Decrease, increase
- D Increase, decrease
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Correct answer: D. Increase, decrease
Opening a bleed valve extracts compressed air, so less mass flow passes through the turbine and the engine pressure ratio falls; to maintain thrust the fuel system holds or increases fuel flow into the reduced airflow, raising the exhaust gas temperature. The result is EGT up, EPR down.
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Q3
Where is torque measured in a turboprop engine
- A Accessory gearbox
- B Reduction gearbox
- C At the turbine
- D At the constant speed unit oil pump
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Correct answer: B. Reduction gearbox
Turboprop torque is measured at the reduction gearbox, where a torquemeter senses the reaction of the gear train, typically as oil pressure proportional to the torque being transmitted to the propeller. This is the point where the drive to the propeller can be directly sensed.
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Q4
A relight is :
- A The initiation of the after burning system.
- B What occurs when the engine drain valve is stuck.
- C The action of the restarting a flamed out engine usually while airborne.
- D mid air engine restart
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Correct answer: C. The action of the restarting a flamed out engine usually while airborne.
A relight is the restarting of an engine that has flamed out, normally carried out while airborne within the engine's relight envelope of speed and altitude. Ignition is reintroduced with fuel flowing, using windmilling rpm or starter assistance to sustain rotation until combustion is re-established.
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Q5
EPR is a
- A Ratio between ambient pressure and exhaust pressure.
- B Ratio between ambient pressure and fan pressure.
- C Ratio between intake pressure and compressor delivery pressure.
- D Ratio between exhaust pressure and intake pressure.
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Correct answer: D. Ratio between exhaust pressure and intake pressure.
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Q6
What is engine pressure ratio?
- A The ratio of turbine outlet pressure to compressor inlet pressure
- B The ratio of turbine inlet pressure to compressor inlet pressure
- C Turbine outlet pressure x compressor outlet pressure
- D Compressor inlet pressure divided by turbine outlet pressure
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Correct answer: A. The ratio of turbine outlet pressure to compressor inlet pressure
Engine pressure ratio (EPR) is the ratio of the turbine (jet-pipe) outlet total pressure to the compressor inlet total pressure. It is used as a direct measure of the thrust being produced by the engine.
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Q7
The term "Self Sustaining Speed" means that:
- A The speed from which the engine can accelerate to full power within 5 seconds
- B The engine will run independently of external help
- C The speed from which the engine can accelerate to idle without the help of the starter motor.
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Correct answer: C. The speed from which the engine can accelerate to idle without the help of the starter motor.
Self-sustaining speed is the rpm from which a gas turbine can accelerate to idle without further help from the starter motor, because the turbine is by then extracting enough energy to drive the compressor and continue accelerating unaided. Below this speed, cutting the starter would cause the engine to hang or hot start.
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Q8
343.In a High Energy Igniter Unit, the choke:
- A Protects the unit from excessive voltages.
- B Prolongs the discharge to the plug
- C Protects the unit from excessive current.
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Correct answer: B. Prolongs the discharge to the plug
In a high-energy ignition unit the choke is an inductor that opposes the rapid collapse of current, so it extends the duration of the spark discharge at the igniter plug, giving the fuel/air mixture more time to light.
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Q9
When after burning the jet pipe will:
- A Close to allow for the decreased volume of gas.
- B Open to allow for the increased volume of gas.
- C Remain the same
- D Remain the same until the temperature reaches 1700C then open.
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Correct answer: B. Open to allow for the increased volume of gas.
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Q10
A re-light envelope:
- A Shows the flame stability limits.
- B Shows airspeed and altitude ilmitations for an in-flight restart.
- C Shows fuel/air mixture limitations for an in-flight restart
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Correct answer: B. Shows airspeed and altitude ilmitations for an in-flight restart.
The relight envelope defines the combinations of airspeed and altitude within which an in-flight restart can be expected to succeed, because windmilling RPM and combustion conditions deteriorate at high altitude and low airspeed.
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Q11
Use of reverse thrust below the recommended speed may cause:
- A Over stressing of the gear oleos.
- B Ingestion of the exhaust gases and foreign objects.
- C More fuel to be provided to the burners.
- D The T.G.T. limit to be exceeded, in which case the reverse thrust lever will return to the forward thrust position.
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Correct answer: B. Ingestion of the exhaust gases and foreign objects.
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Q12
How do you control power in a jet engine
- A By controlling the mixture ratio
- B By controlling the fuel flow
- C By controlling the airflow
- D By controlling the bleed valves
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Correct answer: A. By controlling the mixture ratio
Thrust in a gas-turbine engine is set solely by regulating the fuel flow to the combustion chambers; moving the thrust lever schedules more or less fuel, which changes the gas energy and hence the engine speed and thrust.
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