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CAA NZ Basic Turbine Knowledge (Helicopter) Exam study materials
CAA NZ Basic Turbine Knowledge (Helicopter) Exam study resources

CAA NZ Basic Turbine Knowledge (Helicopter) Exam practice questions

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Question 1Turbine Engine Fundamentals and Thermodynamics

In a turboshaft engine fitted to a helicopter, which component extracts energy from the gas stream to drive the main rotor system through the combining/reduction gearbox?

  1. The gas generator compressor
  2. The free power turbine Correct answer.
  3. The accessory gearbox
  4. The combustion chamber liner

Explanation

In a free-turbine turboshaft, the power turbine is mechanically independent of the gas generator and its shaft drives the main rotor through the reduction gearbox. The gas generator turbine drives only the compressor and accessories, so it does not deliver rotor power.

Question 2Turbine Engine Fundamentals and Thermodynamics

According to the Brayton cycle, thermal efficiency of an ideal turbine engine improves primarily when which parameter is increased?

  1. The pressure ratio across the compressor Correct answer.
  2. The ambient humidity
  3. The mass flow of fuel only
  4. The exhaust nozzle area

Explanation

Ideal Brayton cycle efficiency rises with pressure ratio, since a greater compression ratio raises the temperature at which heat is added relative to the compressor work. Fuel flow alone changes power output but not the ideal cycle efficiency at a fixed pressure ratio.

Question 3Turbine Engine Fundamentals and Thermodynamics

During the compression stroke in a centrifugal compressor, what happens to the air as it passes outward through the impeller?

  1. Its pressure falls while its temperature rises
  2. Its pressure rises while its temperature falls
  3. Its pressure and temperature both rise Correct answer.
  4. Its pressure and temperature both remain constant

Explanation

Work is done on the air by the impeller, so both static pressure and temperature increase along the flow path. The diffuser then converts high velocity into further static pressure rise with a modest temperature change.

Question 4Turbine Engine Fundamentals and Thermodynamics

Which statement best describes the purpose of the diffuser immediately downstream of a centrifugal compressor impeller?

  1. It cools the compressed air before combustion
  2. It converts high air velocity into static pressure Correct answer.
  3. It accelerates the air to supersonic speed
  4. It meters fuel into the combustion chamber

Explanation

The diffuser has an increasing flow area that slows the air, converting kinetic energy into static pressure with minimal loss. This pressure rise is essential to the overall compressor pressure ratio.

Question 5Turbine Engine Fundamentals and Thermodynamics

In a can-annular combustion chamber, what is the primary function of the swirler at the front of each flame tube?

  1. It measures the exhaust gas temperature
  2. It seals the combustion chamber from the turbine
  3. It mixes fuel with air and stabilises the flame Correct answer.
  4. It regulates fuel pump output pressure

Explanation

The swirler imparts a rotational motion to the incoming primary air, promoting thorough mixing with fuel and creating a recirculation zone that anchors the flame. This stabilises combustion and improves light-up characteristics.

Question 6Turbine Engine Fundamentals and Thermodynamics

Why are turbine nozzle guide vanes and turbine blades often hollow and fed with cooling air?

  1. To increase the turbine pressure ratio artificially
  2. To reduce the weight of the rotating assembly only
  3. To keep metal temperatures below the gas temperature so the turbine survives Correct answer.
  4. To prevent the turbine from rotating too fast

Explanation

Turbine inlet gas temperatures can exceed the melting point of the blade alloy, so internal cooling passages and film cooling keep the metal within safe limits. This allows higher gas temperatures and therefore greater power and efficiency.

Question 7Turbine Engine Fundamentals and Thermodynamics

What is the primary reason a turboshaft engine uses a free power turbine rather than a fixed shaft connecting the gas generator to the rotor system?

  1. It removes the requirement for a fuel control unit
  2. It eliminates the need for a reduction gearbox
  3. It allows the compressor to run without any turbine
  4. The rotor can be governed independently of gas generator speed Correct answer.

Explanation

A free power turbine is mechanically independent of the gas generator, so the rotor system can be governed at constant speed while the gas generator speed varies with load. This improves rotor speed control and reduces torque spikes into the rotor system.

Question 8Turbine Engine Fundamentals and Thermodynamics

Which sequence correctly describes the path of the working fluid through a simple turbojet or turboshaft gas generator?

  1. Intake, combustion chamber, compressor, turbine, exhaust
  2. Intake, compressor, turbine, combustion chamber, exhaust
  3. Intake, compressor, combustion chamber, turbine, exhaust Correct answer.
  4. Intake, turbine, compressor, combustion chamber, exhaust

Explanation

Air is drawn in, compressed, heated by combustion at approximately constant pressure, expanded through the turbine to extract work, and then exhausted. Placing combustion before the turbine is essential because the turbine needs high-energy gas to drive the compressor.

Question 9Turbine Engine Fundamentals and Thermodynamics

A helicopter turboshaft engine is fitted with a centrifugal compressor of pressure ratio 8. Assuming ideal isentropic compression from an ambient temperature of 15 degrees Celsius, which value is closest to the compressor delivery temperature? (Use T2 = T1 x pressure ratio^0.286, with temperatures in kelvin.)

  1. Approximately 520 degrees Celsius
  2. Approximately 400 degrees Celsius
  3. Approximately 120 degrees Celsius
  4. Approximately 250 degrees Celsius Correct answer.

Explanation

Convert 15 degrees Celsius to 288 K and multiply by 8^0.286, which is about 1.81, giving roughly 522 K or about 249 degrees Celsius. This illustrates the substantial compression heating that must be accounted for in cycle analysis.

Question 10Turbine Engine Fundamentals and Thermodynamics

In a free-turbine turboshaft engine, an increase in collective pitch at constant governor setting will initially cause which response?

  1. Rotor load falls, power turbine speed rises, and the fuel control reduces fuel flow
  2. The power turbine decouples from the rotor and stops driving it
  3. Rotor load rises, power turbine speed falls slightly, and the fuel control increases fuel flow to restore rotor rpm Correct answer.
  4. Gas generator speed falls while rotor rpm remains exactly constant with no fuel change

Explanation

Increased collective pitch increases rotor blade angle of attack and therefore torque demand. The governor senses the resulting rotor rpm droop and increases fuel flow, raising gas generator speed and power turbine output until rpm is restored.

Question 11Turbine Engine Fundamentals and Thermodynamics

During starting of a turboshaft engine, why is the starter motor kept engaged until the engine reaches a self-sustaining speed?

  1. The starter cools the turbine blades during light-up
  2. The starter prevents the combustion chamber from lighting
  3. The gas generator cannot accelerate on its own until turbine power exceeds compressor and accessory loads Correct answer.
  4. The starter maintains rotor rpm at flight idle

Explanation

At low rpm the turbine produces less power than the compressor and accessories absorb, so external cranking is needed to accelerate the engine. Once combustion raises turbine power above the absorbed load, the engine becomes self-sustaining and the starter disengages.

Question 12Turbine Engine Fundamentals and Thermodynamics

What is the effect of compressor bleed air extraction on engine performance during high-power operation?

  1. It reduces available power and increases specific fuel consumption Correct answer.
  2. It decreases compressor pressure ratio but increases mass flow
  3. It increases turbine power output and decreases fuel consumption
  4. It has no effect on power or fuel consumption

Explanation

Bleed air is air that has already been compressed but is not passed through combustion and the turbine, so the work used to compress it is lost to the power-producing cycle. The result is reduced shaft power and higher specific fuel consumption for a given output.

Question 13Turbine Engine Fundamentals and Thermodynamics

A twin-spool gas generator is used in some turboshaft engines. What is the main advantage of this arrangement compared with a single-spool compressor?

  1. It eliminates the need for any turbine cooling
  2. It allows the power turbine to drive the compressor directly
  3. Each compressor stage runs nearer its optimum speed, improving surge margin and efficiency Correct answer.
  4. It removes the need for a combustion chamber

Explanation

In a twin-spool arrangement the low-pressure and high-pressure compressors rotate at different speeds, each matched to its own aerodynamic optimum. This widens the stable operating range and improves efficiency across the engine's operating envelope.

Question 14Turbine Engine Fundamentals and Thermodynamics

Which condition most directly indicates that a compressor is operating close to its surge line?

  1. A steady increase in compressor delivery temperature
  2. A slow decrease in fuel flow at constant collective
  3. A gradual rise in oil temperature
  4. Rapid fluctuations in compressor delivery pressure with accompanying rumble or bang Correct answer.

Explanation

Surge is an unstable aerodynamic condition in which the airflow separates and can reverse momentarily, producing pressure oscillations and characteristic bangs. Steady temperature rise or oil temperature changes are not the defining indicators of surge.

Question 15Turbine Engine Fundamentals and Thermodynamics

In the combustion chamber, what is the purpose of the dilution or secondary air zone?

  1. It cools the compressor casing
  2. It provides the initial ignition of the fuel-air mixture
  3. It lowers the gas temperature to a level acceptable to the turbine Correct answer.
  4. It increases the flame temperature to improve efficiency

Explanation

The dilution zone admits air downstream of the primary combustion zone to mix with the hot gas and reduce its temperature to a value the turbine nozzle guide vanes and blades can tolerate. Only a portion of the air is used in the primary combustion zone.

Question 16Turbine Engine Fundamentals and Thermodynamics

Why does a turbine engine generally produce less shaft power on a hot day at high altitude compared with a cold day at sea level?

  1. Lower air density reduces mass flow and hence available power Correct answer.
  2. Fuel has a lower calorific value at altitude
  3. The compressor pressure ratio is fixed and cannot change
  4. The turbine expands the gas less because of higher ambient pressure

Explanation

Power output depends strongly on mass flow of air through the engine, which falls as air density decreases with altitude and high temperature. Reduced mass flow limits the fuel that can be burned and the work extracted by the turbine.

Question 17Turbine Engine Fundamentals and Thermodynamics

What is the function of the exhaust diffuser in a turboshaft engine installation?

  1. It filters debris from the intake air
  2. It recovers residual velocity pressure and directs exhaust gas away from the airframe Correct answer.
  3. It provides cooling air to the combustion chamber
  4. It drives the power turbine by accelerating the exhaust

Explanation

The exhaust diffuser slows the gas and converts remaining kinetic energy into static pressure, reducing back pressure on the turbine and directing the flow safely clear of the airframe. In a turboshaft, residual exhaust thrust is not the primary power source.

Question 18Turbine Engine Fundamentals and Thermodynamics

A turboshaft engine shows a higher than normal exhaust gas temperature for a given torque, together with a lower than normal compressor delivery pressure. Which interpretation is most consistent with these indications?

  1. The engine is operating exactly at its design point
  2. The exhaust gas temperature probe is reading low because of a wiring fault
  3. The compressor is degraded, so more fuel is needed to achieve the demanded power Correct answer.
  4. The turbine is producing excess power and the governor is limiting fuel

Explanation

Loss of compressor efficiency reduces the pressure ratio and mass flow achieved for a given gas generator speed, so more fuel must be burned and higher EGT results for the same torque. This pattern points to compressor fouling or damage rather than a healthy engine.

Question 19Turbine Engine Fundamentals and Thermodynamics

Which statement correctly describes the relationship between turbine work and compressor work in a steady-state gas generator?

  1. The gas generator turbine extracts just enough work to drive the compressor and accessories Correct answer.
  2. The gas generator turbine work is independent of compressor pressure ratio
  3. The gas generator turbine extracts work only when the power turbine is stationary
  4. The gas generator turbine extracts less work than the compressor absorbs

Explanation

In steady state the gas generator turbine must supply exactly the work demanded by the compressor and its accessories, with any remaining energy passing to the power turbine or exhaust. If turbine work were less than compressor demand, the engine would decelerate.

Question 20Turbine Engine Fundamentals and Thermodynamics

Which material property is most important for turbine blades that operate in the hot section?

  1. High creep resistance at elevated temperature Correct answer.
  2. High ductility at room temperature
  3. Low density at cryogenic temperature
  4. High electrical conductivity at ambient temperature

Explanation

Turbine blades operate under sustained centrifugal stress at high temperature, so resistance to creep, oxidation and thermal fatigue is critical. Nickel-based superalloys with directional solidification or single-crystal structures are used for this reason.

Question 21Turbine Engine Fundamentals and Thermodynamics

How does a turboshaft engine's fuel control typically prevent an over-temperature condition during rapid acceleration?

  1. It limits fuel flow according to a schedule that respects turbine temperature limits Correct answer.
  2. It disengages the power turbine from the rotor
  3. It opens the bleed valves fully to increase turbine cooling
  4. It reduces compressor speed by applying a brake

Explanation

The fuel control schedules acceleration fuel flow so that turbine inlet temperature and compressor surge margins are respected during transients. Excess fuel during acceleration would over-temperature the turbine and could provoke a surge.

Question 22Turbine Engine Fundamentals and Thermodynamics

Considering the ideal Brayton cycle, which combination best explains why turbine inlet temperatures are pushed as high as material limits allow?

  1. Higher turbine inlet temperature reduces the compressor pressure ratio needed for any given power
  2. Higher turbine inlet temperature increases specific work output and cycle efficiency Correct answer.
  3. Higher turbine inlet temperature has no effect on specific fuel consumption
  4. Higher turbine inlet temperature decreases the mass flow required for a given power and reduces efficiency

Explanation

Raising turbine inlet temperature increases the enthalpy drop available across the turbine, so specific work and thermal efficiency improve, provided the turbine can survive the temperature. This is why cooling technology and blade materials are so significant.

Question 23Turbine Engine Fundamentals and Thermodynamics

A turboshaft engine is operating at a high-density altitude with the governor maintaining rotor rpm. Which change would most likely be observed compared with sea-level operation at the same torque setting?

  1. Higher gas generator speed and higher exhaust gas temperature to produce the same torque Correct answer.
  2. Lower gas generator speed and lower exhaust gas temperature
  3. The power turbine would overspeed because of reduced air density
  4. The same gas generator speed with no change in exhaust gas temperature

Explanation

At high density altitude the reduced mass flow means each unit of air does less work, so the gas generator must run faster and hotter to deliver the same torque. This is why performance charts include density altitude corrections and why torque limits are altitude-dependent.

Question 24Turbine Engine Fundamentals and Thermodynamics

In a free-turbine turboshaft, why can gas generator speed change substantially while rotor rpm remains constant?

  1. The compressor is driven by the power turbine rather than the gas generator turbine
  2. The reduction gearbox varies its ratio continuously to hold rotor rpm
  3. The gas generator and power turbine share a common shaft with a slipping clutch
  4. The power turbine is mechanically independent, so only its speed is tied to rotor rpm Correct answer.

Explanation

Because the power turbine and gas generator are on separate shafts, the gas generator can accelerate or decelerate to meet power demand while the power turbine and rotor remain governed at constant speed. A fixed-ratio reduction gearbox then converts power turbine speed to rotor speed.

Question 25Turbine Engine Fundamentals and Thermodynamics

Which thermodynamic reasoning best explains why intercooling between compressor stages can improve specific power output in a complex cycle?

  1. Cooling the air between stages eliminates the need for a diffuser
  2. Cooling the air between stages reduces the work of subsequent compression and lowers delivery temperature for a given final pressure Correct answer.
  3. Cooling the air between stages increases the heat added in the combustion chamber for the same fuel flow
  4. Cooling the air between stages raises the turbine inlet temperature limit

Explanation

Intercooling reduces the specific volume of the air entering the next compressor stage, lowering the work required to reach the final pressure and reducing delivery temperature. The net benefit depends on the cycle arrangement and the cooling penalty, which is why intercooling is more common in industrial and marine applications.

Question 26Turbine Engine Fundamentals and Thermodynamics

In a simple turboshaft engine, which component extracts energy from the hot gases to drive the compressor?

  1. The free power turbine
  2. The combustion chamber
  3. The exhaust diffuser
  4. The compressor turbine Correct answer.

Explanation

The compressor turbine, also called the gas generator turbine, is mechanically linked to the compressor and extracts just enough energy from the gas stream to drive it. The free power turbine is a separate stage that extracts the remaining energy to drive the output shaft. The combustion chamber adds energy, and the exhaust diffuser recovers pressure, but neither drives the compressor.

Question 27Turbine Engine Fundamentals and Thermodynamics

Which statement best describes the function of the combustor in a turbine engine?

  1. It increases the pressure of the air before it enters the turbine.
  2. It cools the turbine inlet gases to protect the turbine blades.
  3. It converts the kinetic energy of the exhaust into shaft power.
  4. It adds heat energy to the air at approximately constant pressure. Correct answer.

Explanation

In the Brayton cycle, the combustor adds heat energy to the compressed air at approximately constant pressure. The pressure rise occurs in the compressor, and work extraction occurs in the turbines. Cooling air is used for turbine blades, but that is not the primary function of the combustor.

Question 28Turbine Engine Fundamentals and Thermodynamics

A turboshaft engine is operating at a constant rotor rpm and the pilot increases collective pitch. What immediate change occurs in the engine?

  1. The compressor pressure ratio decreases to reduce power.
  2. The exhaust gas temperature decreases due to higher airflow.
  3. The gas generator speed increases to produce more torque. Correct answer.
  4. The power turbine speed increases to meet the higher load.

Explanation

Increasing collective pitch increases the power required by the rotor. The governor senses the resulting rotor rpm droop and increases fuel flow, which raises gas generator speed and torque to maintain rotor rpm. The power turbine speed remains essentially constant because it is governed to match rotor rpm.

Question 29Turbine Engine Fundamentals and Thermodynamics

Why does a turbine engine produce less shaft power on a hot day compared with a cold day at the same altitude and gas generator speed?

  1. The turbine blades expand more in the heat, increasing clearances and reducing efficiency.
  2. The fuel control limits fuel flow to prevent over-temperature, reducing power.
  3. Hot air increases the compressor efficiency, but the turbine cannot handle the higher temperature.
  4. Hot air is less dense, so the compressor mass flow and pressure ratio are reduced. Correct answer.

Explanation

Hot air has lower density, so for a given compressor speed and volume flow the mass flow is reduced. Lower mass flow means less work available from the turbine, hence less shaft power. The other options either misstate the effect of temperature on compressor efficiency or describe secondary effects that are not the primary cause.

Question 30Turbine Engine Fundamentals and Thermodynamics

In a free-turbine turboshaft engine, which statement best explains why the gas generator speed can change while the rotor rpm remains constant?

  1. The free power turbine is mechanically linked to the gas generator, so they always rotate together.
  2. The rotor rpm is maintained by a separate electric motor, not by the engine.
  3. The free power turbine is aerodynamically coupled to the gas generator, allowing independent speeds. Correct answer.
  4. The gas generator speed is fixed by the fuel control and does not vary with load.

Explanation

In a free-turbine design, the power turbine is not mechanically connected to the gas generator; it is driven by the gas stream. Therefore, the gas generator can change speed to meet power demands while the power turbine and rotor remain at governed rpm. The other options incorrectly describe mechanical linkage or fixed gas generator speed.

Question 31Engine Construction, Compressors, Combustion and Turbines

In a single-spool turbojet engine, which component directly drives the compressor through the main shaft?

  1. The accessory gearbox
  2. The turbine Correct answer.
  3. The combustion chamber
  4. The exhaust nozzle

Explanation

The turbine extracts energy from the hot gas stream and delivers shaft power to the compressor via the main shaft, which is the defining mechanical link in a single-spool engine. The combustion chamber adds heat but produces no shaft work itself, and the nozzle converts remaining pressure into thrust.

Question 32Engine Construction, Compressors, Combustion and Turbines

The primary purpose of the compressor in a gas turbine engine is to:

  1. Mix fuel with air to form a combustible mixture
  2. Convert rotational energy into thrust directly
  3. Cool the combustion gases before they reach the turbine
  4. Raise the pressure of the air before combustion Correct answer.

Explanation

The compressor raises air pressure so that combustion occurs at high pressure, greatly increasing the work available to the turbine. Fuel metering and mixing occur in the combustion section, and thrust is generated by the exhaust and turbine work extraction.

Question 33Engine Construction, Compressors, Combustion and Turbines

Which statement best describes the function of the diffuser at the compressor outlet?

  1. It cools the turbine blades
  2. It converts air velocity into static pressure Correct answer.
  3. It increases air velocity to improve combustion
  4. It meters fuel into the airstream

Explanation

The diffuser slows the high-velocity compressor discharge air, converting kinetic energy into static pressure and providing stable, low-speed flow to the combustion chamber. It does not add energy or meter fuel.

Question 34Engine Construction, Compressors, Combustion and Turbines

In an axial compressor, the function of the stator vanes is to:

  1. Diffuse the air and redirect it into the next rotor stage Correct answer.
  2. Add kinetic energy to the air
  3. Support the turbine discs
  4. Ignite the fuel/air mixture

Explanation

Stator vanes act as diffusers, slowing the air and converting velocity into pressure while turning the flow to the correct angle for the following rotor row. Rotors add kinetic energy; stators recover it as pressure.

Question 35Engine Construction, Compressors, Combustion and Turbines

Which compressor type is characterised by a series of diverging passages that turn the airflow through approximately 90 degrees?

  1. Axial compressor
  2. Can-annular combustor
  3. Centrifugal compressor Correct answer.
  4. Mixed-flow turbine

Explanation

A centrifugal compressor uses an impeller with diverging passages that turn the air radially outward, converting velocity into pressure. Axial compressors pass air essentially straight through, and combustors and turbines are different components.

Question 36Engine Construction, Compressors, Combustion and Turbines

A compressor stall in a centrifugal compressor is most likely to be indicated by:

  1. A gradual increase in compressor efficiency
  2. An increase in oil pressure only
  3. A steady decrease in fuel flow with no other change
  4. A rise in turbine gas temperature and a drop in engine rpm Correct answer.

Explanation

Stall disrupts smooth airflow, reducing mass flow and pressure ratio, which typically raises turbine gas temperature and causes rpm to fall or fluctuate. It is an abnormal condition, not a benign efficiency improvement.

Question 37Engine Construction, Compressors, Combustion and Turbines

The main advantage of a multi-stage axial compressor over a single-stage centrifugal compressor is:

  1. Greater resistance to foreign object damage
  2. Lower manufacturing cost
  3. A higher mass flow capability for a given frontal area Correct answer.
  4. Simpler construction with fewer parts

Explanation

Axial compressors handle large mass flows with a small frontal area, which suits high-thrust engines. They are, however, more complex and more susceptible to FOD than centrifugal designs.

Question 38Engine Construction, Compressors, Combustion and Turbines

In a combustion chamber, the primary air is used mainly to:

  1. Support initial combustion in the primary zone Correct answer.
  2. Seal the compressor bearings
  3. Cool the turbine nozzle guide vanes
  4. Dilute the exhaust gases before the turbine

Explanation

Primary air enters the primary zone and mixes with fuel to sustain a stable flame. Secondary and dilution air are used downstream for cooling and temperature control of the gases entering the turbine.

Question 39Engine Construction, Compressors, Combustion and Turbines

Which combustion chamber arrangement uses individual flame tubes mounted around the engine casing and interconnected by flame tubes?

  1. Reverse-flow annular type
  2. Annular type
  3. Can-annular type
  4. Can (tubular) type Correct answer.

Explanation

A can or tubular combustor has separate flame tubes arranged around the casing, connected by interconnectors so the flame can propagate between cans during starting. Annular types use a single continuous flame space.

Question 40Engine Construction, Compressors, Combustion and Turbines

The purpose of the dilution zone in a combustion chamber is to:

  1. Increase the flame temperature for better efficiency
  2. Reduce the gas temperature to a value acceptable to the turbine Correct answer.
  3. Provide the initial ignition source
  4. Direct cooling air onto the compressor casing

Explanation

Dilution air mixes with the hot combustion products to lower the gas temperature to a level the turbine nozzle guide vanes and blades can tolerate. It does not raise flame temperature or provide ignition.

Question 41Engine Construction, Compressors, Combustion and Turbines

A rich blowout in a combustion chamber is most likely to occur when:

  1. Fuel flow is excessive relative to the available air Correct answer.
  2. The turbine nozzle is partially blocked
  3. The igniter plug fails during start
  4. Air flow is excessive relative to the fuel flow

Explanation

A rich blowout occurs when the fuel/air ratio is too high, so the mixture cannot sustain combustion. A lean blowout is the opposite condition, where insufficient fuel relative to air extinguishes the flame.

Question 42Engine Construction, Compressors, Combustion and Turbines

In a turbine, the nozzle guide vanes are designed to:

  1. Accelerate and turn the gas onto the rotor blades Correct answer.
  2. Seal the combustion chamber outlet
  3. Extract work from the gas stream
  4. Support the turbine shaft bearings

Explanation

Nozzle guide vanes form converging passages that accelerate the hot gas and direct it at the correct angle onto the rotor blades. Work extraction occurs in the rotor, not the vanes.

Question 43Engine Construction, Compressors, Combustion and Turbines

The turbine in a gas turbine engine is required to operate at very high temperatures, so its blades are commonly:

  1. Operated at a lower temperature than the compressor
  2. Made from solid mild steel
  3. Coated with a thick layer of insulation
  4. Cooled internally by compressor bleed air Correct answer.

Explanation

Turbine blades are internally cooled using compressor bleed air and often carry thermal barrier coatings, allowing them to survive gas temperatures above the melting point of the blade material. They also operate at much higher temperatures than the compressor.

Question 44Engine Construction, Compressors, Combustion and Turbines

Which statement correctly describes the energy conversion in a turbine stage?

  1. The rotor adds energy to the gas to drive the compressor
  2. The nozzle converts pressure into velocity, and the rotor converts velocity into shaft work Correct answer.
  3. The nozzle extracts shaft work from the gas
  4. The nozzle converts velocity into pressure, and the rotor converts pressure into velocity

Explanation

In a turbine stage, the nozzle guide vanes expand the gas, converting pressure into velocity, and the rotor turns that velocity into shaft work. This is the reverse of a compressor stage, which uses shaft work to raise pressure.

Question 45Engine Construction, Compressors, Combustion and Turbines

A free power turbine in a turboshaft engine is one that:

  1. Rotates independently of the gas generator and drives the output shaft Correct answer.
  2. Drives the compressor through the main shaft
  3. Is directly coupled to the gas generator compressor
  4. Is used only during starting

Explanation

A free power turbine is mechanically independent of the gas generator and delivers power to the output shaft, often through a reduction gearbox. This arrangement allows the rotor system to operate independently of gas generator speed.

Question 46Engine Construction, Compressors, Combustion and Turbines

In a twin-spool axial compressor, the low-pressure and high-pressure compressors are driven by:

  1. The power turbine only
  2. Separate turbines on concentric shafts Correct answer.
  3. The accessory gearbox
  4. A single turbine on one shaft

Explanation

A twin-spool engine has two concentric shafts, each driven by its own turbine, allowing the low-pressure and high-pressure compressors to rotate at their optimised speeds. A single-spool engine uses one shaft and one turbine.

Question 47Engine Construction, Compressors, Combustion and Turbines

The main reason for using variable inlet guide vanes on a compressor is to:

  1. Reduce the number of compressor stages required
  2. Increase the maximum turbine temperature
  3. Eliminate the need for a diffuser
  4. Improve airflow matching at low speeds and reduce the risk of stall Correct answer.

Explanation

Variable inlet guide vanes alter the angle of air entering the first stage, improving matching between compressor stages and the engine's operating condition, which helps prevent stall at low speeds. They do not replace the diffuser or affect turbine temperature directly.

Question 48Engine Construction, Compressors, Combustion and Turbines

Which component converts the residual energy in the exhaust gas into a high-velocity jet in a turbojet engine?

  1. The combustion chamber
  2. The diffuser
  3. The accessory gearbox
  4. The exhaust nozzle Correct answer.

Explanation

The exhaust nozzle is a converging duct that accelerates the hot gas to a high velocity, producing thrust. The diffuser and combustor are upstream components that do not produce thrust.

Question 49Engine Construction, Compressors, Combustion and Turbines

Compressor surge differs from a simple stall in that surge:

  1. Always results in an immediate engine fire
  2. Is a mild local disturbance affecting only one blade row
  3. Involves a complete breakdown of flow through the compressor with flow reversal Correct answer.
  4. Occurs only during engine shutdown

Explanation

Surge is a more severe condition than stall, involving a complete breakdown of flow through the compressor with possible flow reversal and loud bangs. Stall may be localised to part of the compressor.

Question 50Engine Construction, Compressors, Combustion and Turbines

In a combustion chamber, the swirler at the fuel nozzle serves to:

  1. Cool the combustion chamber liner
  2. Increase the fuel pressure for injection
  3. Direct air into the turbine
  4. Create a region of recirculating flow to stabilise the flame Correct answer.

Explanation

The swirler imparts a rotational motion to the incoming air, creating a recirculation zone that anchors and stabilises the flame. Fuel pressure is set by the fuel system, and liner cooling uses separate cooling air.