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CAA NZ AME Licence Exam - Turbine Engines (Subject 6) study materials
CAA NZ AME Licence Exam - Turbine Engines (Subject 6) study resources

CAA NZ AME Licence Exam - Turbine Engines (Subject 6) practice questions

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Question 1Fundamentals of Gas Turbine Theory

In a pure turbojet operating at a fixed rotational speed, which statement best describes the relationship between compressor delivery pressure and the pressure at which combustion gases enter the turbine?

  1. They are essentially equal because the combustor is treated as a constant-pressure heat addition process Correct answer.
  2. The turbine entry pressure is roughly half the compressor delivery pressure because of the large pressure drop across the burner
  3. The turbine entry pressure is significantly higher because combustion adds energy to the gas
  4. The turbine entry pressure depends only on exhaust nozzle area and is unrelated to compressor delivery pressure

Explanation

In the ideal Brayton cycle the combustor is modelled as constant-pressure heat addition, so the static pressure at turbine entry is close to compressor delivery pressure, with only a small pressure loss across the burner and ducting. The energy added by combustion raises temperature and volume, not pressure.

Question 2Fundamentals of Gas Turbine Theory

A gas turbine engine is described as having a pressure ratio of 12:1. What does this figure represent?

  1. The ratio of compressor delivery total pressure to compressor inlet total pressure Correct answer.
  2. The ratio of combustor exit temperature to compressor delivery temperature
  3. The ratio of turbine entry pressure to exhaust nozzle exit pressure
  4. The ratio of mass flow through the turbine to mass flow through the compressor

Explanation

Overall pressure ratio is the total pressure at compressor delivery divided by the total pressure at compressor inlet. It is a key cycle parameter because it strongly influences thermal efficiency and specific fuel consumption.

Question 3Fundamentals of Gas Turbine Theory

Why does a turboprop engine generally achieve better propulsive efficiency at low flight speeds than a pure turbojet of similar core power?

  1. It accelerates a large mass of air to a modest velocity, reducing wasted kinetic energy in the jet Correct answer.
  2. Its exhaust gas leaves at a much higher velocity than the turbojet exhaust
  3. It does not require a compressor, so all shaft power goes to the propeller
  4. It operates at a higher turbine entry temperature than any turbojet

Explanation

Propulsive efficiency improves when a large mass of air is accelerated to a small velocity increment, as with a propeller. A turbojet accelerates a smaller mass to a much higher jet velocity, so more kinetic energy is left in the exhaust at low speed.

Question 4Fundamentals of Gas Turbine Theory

During steady-state operation, the turbine must extract approximately which proportion of the energy added in the cycle for a pure turbojet?

  1. Enough to drive the compressor and accessories, with the remainder left as jet kinetic energy Correct answer.
  2. All of the energy added in the combustor
  3. Only enough to drive the accessories and fuel pumps
  4. Exactly half, because the exhaust and turbine work are always equal

Explanation

In a turbojet the turbine extracts just enough work to drive the compressor and accessories. The remaining enthalpy in the gas is expanded through the nozzle to produce thrust, so turbine work is a large but not total fraction of the energy added.

Question 5Fundamentals of Gas Turbine Theory

Which change occurs to the axial velocity of the gas as it passes through a convergent exhaust nozzle of a turbojet?

  1. Axial velocity remains constant while total pressure falls
  2. Axial velocity increases while static pressure and temperature fall Correct answer.
  3. Axial velocity increases while static pressure rises
  4. Axial velocity decreases while static pressure rises

Explanation

In a convergent nozzle the flow accelerates, so static pressure and static temperature fall as enthalpy is converted into kinetic energy. Total pressure stays nearly constant apart from friction losses.

Question 6Fundamentals of Gas Turbine Theory

An engine's compressor is fitted with variable inlet guide vanes. What is their primary purpose?

  1. To cool the compressor casing and reduce thermal growth
  2. To alter the inlet air angle so early compressor stages operate at acceptable incidence at off-design speeds Correct answer.
  3. To increase the compressor pressure ratio at design speed only
  4. To measure mass airflow for the fuel control unit

Explanation

Variable inlet guide vanes change the direction of the incoming air so the first rotor stages see an acceptable angle of attack at low and transient speeds, improving surge margin and starting. They do not increase design pressure ratio.

Question 7Fundamentals of Gas Turbine Theory

What does the term 'surge margin' describe in an axial compressor?

  1. The difference between turbine entry and compressor delivery temperature
  2. The clearance between rotor blade tips and the casing
  3. The pressure rise available before the compressor stalls or surges at a given operating point Correct answer.
  4. The quantity of air bled from the compressor for cabin pressurisation

Explanation

Surge margin is the margin between the current operating point and the compressor's surge or stall line on its characteristic map. A larger margin means the engine can tolerate more disturbance before unstable flow develops.

Question 8Fundamentals of Gas Turbine Theory

Why is a small amount of air often bled from the compressor for turbine cooling rather than using only air that has passed through the combustor?

  1. Bleed air is required to drive the starter motor during starting
  2. Compressor air contains more oxygen and improves combustion efficiency
  3. Bleed air has a higher pressure than combustor gas and seals the turbine bearings
  4. Compressor air is cooler than combustor gas and can absorb heat from turbine components without exceeding their metal temperature limits Correct answer.

Explanation

Turbine cooling air must be much cooler than the gas it protects, and compressor delivery air is far cooler than combustor exit gas. It is ducted to internal passages to keep blade metal temperatures below material limits.

Question 9Fundamentals of Gas Turbine Theory

What is the main reason for using a can-annular (cannular) combustor in many large turbofan engines?

  1. It allows combustion to occur at constant volume rather than constant pressure
  2. It eliminates the need for any fuel nozzles
  3. It removes the requirement for dilution air holes
  4. It combines the structural simplicity and testability of separate cans with the compactness and uniform exit temperature of an annular chamber Correct answer.

Explanation

A cannular combustor uses individual cans inside a common annular casing, giving the ruggedness and ease of development of can designs with the compact, uniform outlet profile of an annular chamber. Fuel nozzles and dilution holes are still required.

Question 10Fundamentals of Gas Turbine Theory

In a free-turbine turboshaft engine, what mechanically connects the gas generator to the output shaft?

  1. A direct mechanical coupling through the gas generator shaft
  2. A reduction gearbox driven from the compressor front
  3. Nothing; the power turbine is aerodynamically coupled to the gas generator exhaust Correct answer.
  4. A hydraulic clutch engaged by the fuel control unit

Explanation

In a free-turbine layout the power turbine is not mechanically linked to the gas generator; it is driven only by the gas flow. This allows the output shaft speed to vary independently of gas generator speed, which suits helicopter rotors and similar loads.

Question 11Fundamentals of Gas Turbine Theory

An engine is fitted with a convergent-divergent exhaust nozzle. Under what condition does the divergent section produce additional thrust?

  1. When the aircraft is stationary on the ground
  2. When the exhaust gas temperature is below the turbine outlet temperature
  3. When the nozzle throat is choked and the flow is supersonic in the divergent section Correct answer.
  4. When the flow is subsonic throughout the nozzle

Explanation

The divergent section only contributes useful expansion when the throat is choked and flow becomes supersonic, allowing further pressure energy to be converted to velocity. If the flow is subsonic, the divergent section acts as a diffuser and reduces velocity.

Question 12Fundamentals of Gas Turbine Theory

As compressor pressure ratio is increased at constant turbine entry temperature, what is the effect on the ideal thermal efficiency of the Brayton cycle?

  1. Thermal efficiency decreases because more work is needed to drive the compressor
  2. Thermal efficiency falls to zero once the pressure ratio exceeds 10:1
  3. Thermal efficiency is unchanged because it depends only on turbine entry temperature
  4. Thermal efficiency increases, approaching the Carnot limit for the same temperature limits Correct answer.

Explanation

Ideal Brayton thermal efficiency rises with pressure ratio for fixed temperature limits, approaching the Carnot value as pressure ratio increases. The compressor work also rises, but the net effect on ideal efficiency is an increase.

Question 13Fundamentals of Gas Turbine Theory

What is the function of the diffuser section at the inlet of a combustion chamber?

  1. To mix fuel with air before the compressor
  2. To accelerate the air to supersonic speed before combustion
  3. To reduce the air velocity and recover static pressure so flame stability is maintained Correct answer.
  4. To cool the turbine nozzle guide vanes

Explanation

Air leaving the compressor is fast enough to blow out a flame, so the diffuser slows it and raises static pressure. This creates conditions where stable combustion can occur with acceptable pressure loss.

Question 14Fundamentals of Gas Turbine Theory

A twin-spool axial compressor is used in a turbofan. What is the principal aerodynamic advantage of splitting the compression into two independently rotating spools?

  1. It guarantees a constant pressure ratio regardless of throttle setting
  2. Each spool can run at a speed suited to its blade size, avoiding stall in the low-pressure stages and excessive tip speed in the high-pressure stages Correct answer.
  3. It removes the need for any stator vanes
  4. It allows the high-pressure compressor to be driven by the low-pressure turbine

Explanation

Long, single-spool compressors face a mismatch: the large front blades need low speed while the small rear blades need high speed. Separate spools let each run near its optimum, improving surge margin and efficiency across the operating range.

Question 15Fundamentals of Gas Turbine Theory

Which statement correctly describes the behaviour of the gas as it passes through a turbine nozzle guide vane?

  1. Pressure rises and velocity falls to diffuse the flow before the rotor
  2. Velocity falls to zero so the rotor is driven purely by static pressure
  3. Pressure and velocity both remain constant while temperature alone falls
  4. Pressure falls and velocity increases, with the flow turned to enter the rotor at the correct angle Correct answer.

Explanation

A nozzle guide vane is a convergent passage that expands the gas, dropping static pressure and raising velocity while turning it to the design angle. The rotor then extracts work from this high-velocity flow.

Question 16Fundamentals of Gas Turbine Theory

An engine's exhaust gas temperature (EGT) is observed to rise while thrust remains constant. Which combination of factors could explain this?

  1. A decrease in compressor efficiency requiring more fuel to maintain the same thrust Correct answer.
  2. An increase in compressor efficiency with unchanged fuel flow
  3. A reduction in bleed air extraction with no change in fuel scheduling
  4. A decrease in turbine entry temperature caused by the fuel control unit

Explanation

If compressor efficiency falls, more fuel must be burned to hold the same thrust, raising EGT. Deterioration of compressor or turbine efficiency is a common cause of a high EGT at constant power.

Question 17Fundamentals of Gas Turbine Theory

What is the purpose of the dilution holes in a combustion chamber liner?

  1. To provide cooling air only to the combustor casing
  2. To admit fuel into the primary zone
  3. To introduce secondary air that lowers the gas temperature to a level the turbine can accept and shapes the outlet profile Correct answer.
  4. To increase the pressure of the gas before it reaches the turbine

Explanation

Dilution air enters downstream of the primary combustion zone to reduce gas temperature to a value compatible with turbine materials and to produce a uniform exit temperature profile. It does not add fuel or raise pressure.

Question 18Fundamentals of Gas Turbine Theory

Which factor most directly limits the maximum turbine entry temperature in a modern high-bypass turbofan?

  1. The melting point of the compressor casing material
  2. The capacity of the fuel pumps
  3. The creep and oxidation resistance of the turbine blade and nozzle guide vane materials, together with the effectiveness of cooling Correct answer.
  4. The pressure ratio of the fan

Explanation

Turbine entry temperature is limited by the high-temperature capability of the turbine components. Blade alloys, thermal barrier coatings and cooling design determine how hot the gas can be while keeping metal temperatures within creep and oxidation limits.

Question 19Fundamentals of Gas Turbine Theory

In a turbofan engine, what happens to the bypass ratio if the fan diameter is increased while core mass flow is held constant?

  1. The bypass ratio stays the same because it depends only on core speed
  2. The bypass ratio becomes undefined because the fan is no longer driven by the turbine
  3. The bypass ratio increases Correct answer.
  4. The bypass ratio decreases

Explanation

Bypass ratio is bypass mass flow divided by core mass flow. Enlarging the fan increases the bypass flow for a given core flow, so the ratio rises, which usually improves propulsive efficiency at typical subsonic cruise speeds.

Question 20Fundamentals of Gas Turbine Theory

A turbojet is operating with a choked exhaust nozzle. What does 'choked' mean in this context?

  1. The exhaust gas temperature has reached its maximum limit
  2. The compressor has stalled and airflow has reversed
  3. The nozzle has become blocked with carbon deposits
  4. The nozzle throat has reached sonic velocity and further downstream pressure reduction cannot increase the mass flow Correct answer.

Explanation

When the nozzle throat reaches Mach 1, the mass flow is fixed by upstream conditions and cannot be increased by lowering the downstream pressure further. This condition is normal at high power settings.

Question 21Engine Construction and Materials

In a turbojet engine, which material is most commonly used for the compressor rotor blades in the low-pressure stages because of its good strength-to-weight ratio and corrosion resistance?

  1. Aluminium bronze
  2. Nickel-based superalloy
  3. Titanium alloy Correct answer.
  4. Carbon steel

Explanation

Low-pressure compressor blades are typically made from titanium alloys, which offer high specific strength and good corrosion resistance at the moderate temperatures found in the front stages. Nickel superalloys are reserved for hotter rear stages and turbine sections.

Question 22Engine Construction and Materials

The combustion chamber liner of a gas turbine engine is most commonly manufactured from which material?

  1. Titanium alloy forging
  2. Copper brazing alloy
  3. Heat-resistant nickel alloy sheet Correct answer.
  4. Aluminium casting

Explanation

Combustion liners operate at very high temperatures and are made from heat-resistant nickel-based alloys, often formed from sheet and protected by thermal barrier coatings. Titanium and aluminium cannot survive the liner temperatures.

Question 23Engine Construction and Materials

Which type of turbine blade attachment is designed to allow controlled relative movement that dampens vibration while transmitting centrifugal loads to the disc?

  1. Fir-tree root Correct answer.
  2. Welded root
  3. Threaded root
  4. Brazed root

Explanation

Fir-tree (and dovetail) roots permit slight relative movement between blade and disc that provides friction damping of vibration while transferring centrifugal load. Welded, threaded and brazed roots are not used for turbine blade attachment.

Question 24Engine Construction and Materials

What is the primary purpose of a thermal barrier coating applied to turbine blades and nozzle guide vanes?

  1. To increase the mechanical strength of the blade
  2. To improve the aerodynamic profile of the aerofoil
  3. To reduce heat transfer into the base metal Correct answer.
  4. To provide electrical insulation for sensors

Explanation

A ceramic thermal barrier coating lowers the temperature of the underlying metal, allowing higher gas temperatures or longer component life. It does not carry structural load or provide electrical insulation.

Question 25Engine Construction and Materials

Which material property is most important when selecting the alloy for a turbine disc that must resist burst at high rotational speed?

  1. High thermal expansion coefficient
  2. High electrical conductivity
  3. Low density alone
  4. High tensile and creep strength at temperature Correct answer.

Explanation

A turbine disc must withstand large centrifugal stresses at elevated temperature, so high tensile strength and creep resistance are the governing properties. Electrical conductivity and thermal expansion are not primary selection criteria.

Question 26Engine Construction and Materials

In a gas turbine engine, what is the function of the compressor front bearing housing?

  1. To compress the incoming air before the first stage
  2. To cool the turbine blades
  3. To support the front of the rotor and house the front bearing Correct answer.
  4. To meter fuel into the combustor

Explanation

The front bearing housing provides structural support for the front bearing and the compressor rotor, and often contains the inlet guide vane mounting. It does not perform compression or combustion functions.

Question 27Engine Construction and Materials

Which of the following best describes the purpose of the stator vanes in an axial compressor?

  1. To cool the compressor casing
  2. To increase the rotational speed of the airflow
  3. To diffuse the air and redirect it into the next rotor stage Correct answer.
  4. To support the rear bearing

Explanation

Stator vanes convert kinetic energy into pressure by diffusion and turn the flow to the correct angle for the following rotor stage. They do not drive rotation or cool the casing.

Question 28Engine Construction and Materials

A turbine blade in a high-pressure stage shows a creep-induced elongation after service. Which material property is most directly relevant to this degradation mode?

  1. Ductility at room temperature
  2. Fatigue limit
  3. Hardness
  4. Creep rupture strength Correct answer.

Explanation

Creep is time-dependent deformation under sustained load at high temperature, so creep rupture strength governs the blade's resistance to elongation. Room-temperature ductility and hardness do not directly predict creep behaviour.

Question 29Engine Construction and Materials

Why are compressor casings often manufactured as a split (horizontally divided) construction?

  1. To improve the compressor pressure ratio
  2. To reduce the cost of the casing material
  3. To allow removal of the rotor and inspection without disturbing the whole engine Correct answer.
  4. To allow the casing to expand freely at high temperature

Explanation

A horizontally split casing permits the upper half to be removed so the rotor and stator can be inspected or removed without dismantling the entire engine. It is a maintenance-access feature, not a performance or cost feature.

Question 30Engine Construction and Materials

In a can-annular combustion chamber, what is the primary function of the transition piece (transition duct)?

  1. To support the rear bearing
  2. To cool the compressor casing
  3. To mix fuel with air before ignition
  4. To guide the combustion gases from the flame tube to the turbine nozzle guide vanes Correct answer.

Explanation

The transition piece directs the hot gases from each flame tube to the annular turbine inlet, smoothing the flow and matching the combustor outlets to the nozzle guide vanes. Fuel-air mixing occurs in the flame tube itself.

Question 31Engine Construction and Materials

A turbine disc is found to have a crack initiating at a bolt hole. Which feature of the design most directly influences this type of failure?

  1. The fuel nozzle spray pattern
  2. The compressor pressure ratio
  3. Stress concentration at the hole Correct answer.
  4. The thermal barrier coating thickness

Explanation

Bolt holes act as stress raisers, concentrating stress and promoting fatigue crack initiation. Coating thickness, pressure ratio and spray pattern do not directly cause cracks at bolt holes.

Question 32Engine Construction and Materials

Which of the following is a typical reason for using a nickel-based superalloy rather than a cobalt-based alloy for turbine nozzle guide vanes?

  1. Lower melting point
  2. Better resistance to thermal fatigue and oxidation at high temperature Correct answer.
  3. Greater machinability
  4. Higher thermal conductivity

Explanation

Nickel-based superalloys offer excellent high-temperature strength, oxidation resistance and thermal fatigue resistance, which suits nozzle guide vanes exposed to the hottest gas. Cobalt alloys are used where hot corrosion resistance or specific properties are required.

Question 33Engine Construction and Materials

What is the purpose of the labyrinth (knife-edge) seal on a turbine rotor?

  1. To cool the turbine disc
  2. To measure rotor speed
  3. To provide structural support for the rotor
  4. To minimise leakage of gas between rotating and stationary components Correct answer.

Explanation

Labyrinth seals create a tortuous path that restricts leakage of gas across a pressure boundary between rotating and stationary parts. They do not carry structural load or measure speed.

Question 34Engine Construction and Materials

During inspection, a compressor blade is found to have a nick on its leading edge. Why is this significant?

  1. It can act as a stress raiser and initiate fatigue cracking Correct answer.
  2. It improves the aerodynamic efficiency
  3. It increases the compressor mass flow
  4. It reduces the compressor pressure ratio directly

Explanation

A nick creates a local stress concentration that can initiate fatigue cracks under cyclic loading, so blending or replacement is required. It does not improve efficiency or increase mass flow.

Question 35Engine Construction and Materials

Which bearing type is typically used to carry the main thrust loads of a gas turbine rotor?

  1. Ball bearing Correct answer.
  2. Roller bearing
  3. Plain sleeve bearing
  4. Needle bearing

Explanation

Ball bearings are used as thrust (location) bearings because they can carry axial loads in both directions. Roller bearings carry radial loads but not significant thrust.

Question 36Engine Construction and Materials

What is the function of the compressor variable inlet guide vanes (VIGVs)?

  1. To support the front bearing
  2. To modulate airflow into the compressor to prevent surge and improve efficiency Correct answer.
  3. To increase the compressor rotational speed
  4. To cool the compressor blades

Explanation

Variable inlet guide vanes change the angle of the incoming airflow to the first stage, helping to avoid surge at low speeds and improving efficiency across the operating range. They do not drive rotation or cool blades.

Question 37Engine Construction and Materials

Which of the following best explains why turbine blades are often hollow and provided with internal cooling passages?

  1. To reduce the blade weight only
  2. To allow cooling air to flow through the blade and reduce metal temperature Correct answer.
  3. To increase the blade stiffness
  4. To simplify manufacturing

Explanation

Hollow blades with internal passages allow cooling air to be passed through the aerofoil, reducing metal temperature and permitting higher gas temperatures. Weight reduction is a secondary benefit, not the primary purpose.

Question 38Engine Construction and Materials

A turbine blade tip clearance is found to be excessive. Which effect is most likely?

  1. Increased rotor speed
  2. Increased compressor pressure ratio
  3. Reduced turbine inlet temperature
  4. Reduced engine efficiency due to tip leakage Correct answer.

Explanation

Excessive tip clearance allows gas to leak past the blade tips without doing work, reducing turbine efficiency. It does not increase pressure ratio or reduce turbine inlet temperature.

Question 39Engine Construction and Materials

Which manufacturing process is commonly used to produce single-crystal turbine blades for high-pressure stages?

  1. Directional solidification casting Correct answer.
  2. Forging
  3. Powder metallurgy pressing
  4. Electroplating

Explanation

Single-crystal blades are produced by directional solidification casting, which controls grain growth to eliminate grain boundaries that would otherwise reduce creep strength. Forging and powder metallurgy are not used for single-crystal blades.

Question 40Engine Construction and Materials

In a gas turbine engine, which component is most likely to be manufactured from a cobalt-based alloy?

  1. Combustion outer casing
  2. Fan blades
  3. Compressor front rotor blades
  4. Turbine nozzle guide vanes in a hot corrosion environment Correct answer.

Explanation

Cobalt-based alloys are selected for hot-section components such as nozzle guide vanes where resistance to hot corrosion and thermal fatigue is important. Fan and compressor blades use titanium or composite materials.

Question 41Engine Construction and Materials

A turbine disc alloy is being assessed for service. Which combination of properties best indicates suitability for a high-pressure turbine disc?

  1. High thermal conductivity and low density
  2. High electrical resistivity and low hardness
  3. High thermal expansion and low ductility
  4. High yield strength, good creep resistance and adequate fracture toughness Correct answer.

Explanation

A high-pressure turbine disc must carry large centrifugal loads at temperature, so high yield strength, creep resistance and fracture toughness are the key properties. Thermal conductivity and expansion are not primary selection criteria.

Question 42Engine Construction and Materials

Why are compressor blades often given a protective coating or surface treatment?

  1. To increase the compressor pressure ratio
  2. To reduce the blade weight
  3. To improve resistance to erosion and corrosion from the ingested air Correct answer.
  4. To allow the blade to operate at turbine temperatures

Explanation

Compressor blades are coated or treated to resist erosion and corrosion caused by airborne particles and moisture. Coatings do not increase pressure ratio or allow operation at turbine temperatures.

Question 43Engine Construction and Materials

Which of the following best explains why the compressor and turbine rotor assemblies are often dynamically balanced as a complete assembly?

  1. To increase the rotor speed
  2. To minimise vibration by ensuring the centre of mass aligns with the axis of rotation Correct answer.
  3. To improve the compressor pressure ratio
  4. To reduce the number of bearings required

Explanation

Dynamic balancing aligns the centre of mass with the rotational axis, minimising vibration and bearing loads. It does not alter speed, bearing count or pressure ratio.

Question 44Engine Construction and Materials

Which material is typically used for the fan blades of a high-bypass turbofan engine because of its high strength-to-weight ratio and resistance to foreign object damage?

  1. Nickel-based superalloy
  2. Carbon steel
  3. Magnesium alloy
  4. Titanium alloy Correct answer.

Explanation

Fan blades are usually made from titanium alloys, which combine high specific strength with good resistance to foreign object damage. Nickel superalloys are reserved for hotter sections, and magnesium is too weak for fan blades.

Question 45Compressors and Airflow

In an axial compressor, what is the primary function of the stator vanes located between successive rotor stages?

  1. To seal the tip clearance between the rotor blades and casing
  2. To add tangential kinetic energy to the airflow
  3. To diffuse the air and redirect it into the next rotor stage at the correct angle Correct answer.
  4. To increase the axial velocity of the air through the stage

Explanation

Stator vanes act as diffusers: they slow the air leaving a rotor stage, converting some kinetic energy into static pressure, and they turn the flow so it enters the following rotor at the designed incidence angle. The rotor, not the stator, is the component that adds energy to the air.

Question 46Compressors and Airflow

Why are compressor rotor blades in the first stages of a large turbofan typically made from titanium alloys rather than aluminium alloys?

  1. Titanium retains strength at the higher temperatures and stresses of the front stages Correct answer.
  2. Titanium is easier to cast into complex cooled blade shapes
  3. Titanium has a higher thermal expansion coefficient, improving tip clearance
  4. Titanium has a lower density than aluminium, reducing centrifugal stress

Explanation

The front stages of an HP compressor run at the highest pressure and temperature and experience large centrifugal loads, so titanium alloys are chosen for their strength-to-weight ratio and temperature capability. Aluminium alloys lose strength at these temperatures and are used only in cooler, later stages or in low-pressure sections.

Question 47Compressors and Airflow

What is the effect of fouling on the performance of an axial compressor?

  1. Pressure ratio and efficiency fall while turbine entry temperature may rise for the same thrust Correct answer.
  2. Compressor delivery temperature falls at constant pressure ratio
  3. Surge margin increases because the blades become smoother
  4. Mass flow and pressure ratio increase while efficiency falls

Explanation

Dirt deposits roughen and thicken the blade profiles, reducing aerodynamic efficiency and pressure ratio and lowering mass flow, so the engine needs more fuel and higher turbine temperatures to hold thrust. Surge margin is reduced, not increased, because the stage matching changes adversely.

Question 48Compressors and Airflow

During a compressor wash, why is the engine usually cranked on the starter with ignition and fuel isolated?

  1. To distribute cleaning fluid through the gas path and expel it without combustion Correct answer.
  2. To check the operation of the igniter plugs under wet conditions
  3. To allow the compressor to reach self-sustaining speed for better cleaning
  4. To verify that the fuel nozzles are not blocked by cleaning fluid

Explanation

Cranking with fuel and ignition off drives the cleaning fluid through the compressor and out of the exhaust without any risk of combustion or hot-section damage. The engine never reaches self-sustaining speed during a wash because no fuel is being burned.

Question 49Compressors and Airflow

What is the main purpose of variable inlet guide vanes on an axial compressor?

  1. To cool the first-stage rotor blades during high-power operation
  2. To increase the maximum thrust available at take-off
  3. To schedule airflow angle into the first rotor stage so surge margin is maintained at low speeds Correct answer.
  4. To reduce the noise generated by the fan at high bypass ratios

Explanation

Variable IGVs and stators are scheduled to match the airflow angle to the rotor incidence across the speed range, which keeps the front stages away from surge at low corrected speeds. They do not add thrust; they protect the operating margin.

Question 50Compressors and Airflow

In a centrifugal compressor, what is the function of the diffuser that surrounds the impeller?

  1. To convert the high velocity leaving the impeller into static pressure Correct answer.
  2. To accelerate the air to supersonic velocity before combustion
  3. To direct cooling air onto the impeller bearings
  4. To prevent axial thrust by balancing the impeller

Explanation

The impeller adds energy largely as velocity, and the diffuser passages decelerate that air, converting kinetic energy into static pressure rise. Without the diffuser the centrifugal compressor could not achieve its pressure ratio efficiently.