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Module 15 Gas turbine engine: 50 practice questions

Real EASA paper: Category A 60 Q / 75 min · B1.1 92 Q / 115 min · B1.3 92 Q / 115 min. Three options, 75% to pass. Answer key on the last page. From part66exam.com · independent study aid, not an official exam or a past paper.

1. In a turbofan engine, the bypass (fan) air:
1. A. is compressed by the fan and leaves without passing through the combustor
2. B. passes through the core compressor and is burned with the fuel
3. C. is ducted inside the core solely to cool the turbine blades
2. A pitch lock on a turboprop propeller engages when:
1. A. oil pressure is lost or overspeed occurs
2. B. the aircraft is on the ground
3. C. the propeller is feathered
3. As a turbojet's forward speed increases at constant rpm, net thrust initially:
1. A. increases steadily because of the increased mass flow
2. B. decreases, because ram drag rises faster than the ram pressure recovery
3. C. remains constant because the rpm is held constant
4. In a pneumatic (gas-filled) fire detector with two switches, what does the integrity switch detect?
1. A. A rapid rise in nacelle temperature
2. B. Loss of gas pressure in the sensing tube
3. C. Current flowing through a eutectic salt
5. A thrust bearing that has failed on a turbine engine rotor may allow:
1. A. the rotor to turn faster than normal
2. B. axial rotor movement and blade rubs
3. C. oil pressure to increase
6. Air leaving the diffuser vanes of a centrifugal compressor enters the combustion chambers through outlet elbows fitted with turning vanes that:
1. A. accelerate the air to sonic velocity for burning
2. B. mix the fuel into the air before the liner
3. C. turn the flow from radial to axial with low loss
7. The turbine shaft is usually made of:
1. A. aluminium alloy
2. B. alloy steel
3. C. magnesium alloy
8. Extracting large amounts of customer bleed air from an engine causes:
1. A. higher EGT and fuel flow for a given thrust
2. B. lower EGT for the same thrust setting
3. C. an increase in thrust at the same N1
9. Oil quantity in a turbine oil tank is typically indicated by:
1. A. a tank quantity transmitter
2. B. an oil temperature thermocouple
3. C. the N2 tachometer generator
10. Fuel manifold drain (dump) valves operate at shutdown to:
1. A. pressurise the manifolds for restart
2. B. return fuel to the aircraft tanks at start
3. C. drain residual fuel from the manifolds
11. Before starting an engine after maintenance in the inlet, the technician must:
1. A. run the engine at high power to clear it
2. B. account for all tools and inspect the inlet
3. C. spray water into the inlet
12. An APU fire on the ground commonly results in:
1. A. the APU running faster to clear the fire
2. B. no action until the crew notice it
3. C. automatic shutdown of the APU
13. Before refuelling with Jet B instead of Jet A-1, a technician must check that:
1. A. the tanks are empty of all Jet A-1
2. B. Jet B is an approved fuel for the aircraft
3. C. the engine is running at idle
14. Why must the inlet duct of a high-bypass turbofan have a sufficiently straight section ahead of the fan?
1. A. A straight duct gives the nacelle the lowest weight
2. B. A straight duct raises the air to supersonic speed
3. C. The very high airflow needs smooth, even flow at the fan face
15. The two basic categories of thrust reverser are:
1. A. hydraulic and electric actuation
2. B. mechanical and aerodynamic blockage
3. C. fixed and retractable types
16. The reduction gearing of a turboshaft engine reduces the:
1. A. power turbine speed for the output
2. B. torque delivered to the rotor
3. C. gas generator speed at idle
17. Combustion stability ('flame holding') depends mainly on:
1. A. a high axial gas velocity through the primary zone
2. B. a recirculation zone of low axial velocity in the primary zone
3. C. a very rich overall mixture of about 5:1
18. How is a turbine ignition system functionally checked on the ground?
1. A. By firing an igniter held outside the engine
2. B. By measuring EGT rise with ignition only
3. C. By listening for the igniters snapping with fuel off
19. The breather system of a turbine engine connects:
1. A. the fuel tanks to the oil tank
2. B. the exhaust to the compressor inlet
3. C. the bearing chambers, gearbox and oil tank
20. In an intercooled gas turbine, cooling the air between compressor stages mainly:
1. A. raises the turbine entry temperature
2. B. increases the exhaust gas temperature
3. C. reduces the work needed for compression
21. After depreservation and installation, a turbine engine should receive:
1. A. no further checks at all
2. B. only an oil and filter change
3. C. a ground run and leak check
22. Engine control cables or rods pass through the firewall via:
1. A. fireproof seals or bulkhead fittings
2. B. open holes for easy movement
3. C. rubber grommets only
23. In the ideal Brayton cycle of an open-cycle gas turbine, where is heat rejected?
1. A. In the combustor, at constant volume
2. B. In the compressor, at constant temperature
3. C. In the atmosphere after exhaust, at constant pressure
24. A turboprop engine is most efficient in the speed range of about:
1. A. Mach 0.9 to Mach 1.2
2. B. 300 to 600 km/h
3. C. above Mach 2.0
25. A turbojet's thrust specific fuel consumption is 0.065 kg/(N·h) at 40 kN of thrust. What is its fuel flow?
1. A. 3 kg/h
2. B. 260 kg/h
3. C. 2600 kg/h
26. A pneumatic (gas-filled) fire detector alarms when:
1. A. the sensing tube cools rapidly
2. B. heat raises the tube gas pressure
3. C. light strikes the sensor window
27. Frosting on a bearing race indicates:
1. A. early surface distress
2. B. a correctly run-in bearing
3. C. a new, unused bearing
28. Which condition is most likely to cause an axial compressor stall?
1. A. Rapid acceleration with too much fuel for the rpm
2. B. Steady cruise at the design rpm and airflow
3. C. Stable ground idle with the bleed valves open
29. In an impulse turbine, the gas:
1. A. is accelerated mainly between the rotor blades
2. B. is accelerated in the nozzles and turned by bucket-shaped blades
3. C. passes through with no change of direction
30. A precooler in an engine bleed system uses:
1. A. fan air to cool the bleed air
2. B. fuel to heat the bleed air
3. C. oil to cool the bleed air
31. Thermocouple leads for EGT systems must NOT be shortened or lengthened because:
1. A. the leads carry fuel
2. B. circuit resistance is calibrated
3. C. the leads are pressurised
32. The main engine-driven fuel pump on most turbofans consists of:
1. A. a centrifugal LP and a gear HP stage
2. B. a single variable-stroke piston pump
3. C. a hand pump and an electric vane pump
33. An engine shows a gradual loss of EGT margin across many flights. This information is used to:
1. A. plan engine removal before limits are reached
2. B. increase the take-off EGT limit
3. C. change the engine's thrust rating immediately
34. An APU overspeed is prevented by:
1. A. the main engine EEC
2. B. the pilot's thrust lever
3. C. an automatic overspeed shutdown
35. Which additive is used to control microbial growth in fuel tanks?
1. A. Antioxidant
2. B. Metal deactivator
3. C. Biocide
36. A test-cell bellmouth inlet is fitted with probes. What do they measure?
1. A. Intake temperature and total and static pressure
2. B. Fan blade tip clearance and vibration
3. C. Exhaust gas temperature and thrust
37. A clamshell or bucket thrust reverser is classed as:
1. A. mechanical blockage
2. B. aerodynamic blockage
3. C. cascade-type blockage
38. An engine 'torque split' warning on a twin turboshaft helicopter indicates:
1. A. an engine fire
2. B. a low fuel quantity
3. C. unequal load sharing
39. Swirl vanes around the fuel nozzle in the combustor dome are used to:
1. A. increase the axial velocity of the flame to move it downstream
2. B. cool the fuel before it is injected so it does not vaporise
3. C. improve mixing and reduce the axial air velocity to stabilise the flame
40. A tailwind during engine start is most likely to cause:
1. A. a faster, cooler start
2. B. a hot start
3. C. a starter clutch slip
41. An oil cooler relief/bypass valve protects the cooler against:
1. A. loss of fuel pressure at idle
2. B. rupture when the oil is cold
3. C. high exhaust gas temperature
42. Distributed propulsion concepts use:
1. A. one very large engine only
2. B. rocket boosters for take-off
3. C. many small propulsors on the airframe
43. A humidity indicator in an engine container has turned pink. This indicates:
1. A. normal, dry conditions inside
2. B. high humidity inside the container
3. C. the engine is ready to be run
44. A 'bootstrap' kit on some transport aircraft is used to:
1. A. start the engine without a starter
2. B. test the engine fire bottles
3. C. hoist the engine from the wing structure
45. In a geared turbofan engine, the reduction gearbox is located between:
1. A. the fan and the low-pressure spool
2. B. the high-pressure compressor and the high-pressure turbine
3. C. the accessory gearbox and the starter
46. Turboprop engine power is often expressed as:
1. A. equivalent shaft horsepower
2. B. static thrust in lbf only
3. C. engine pressure ratio
47. With a choked convergent nozzle, total thrust is:
1. A. momentum thrust plus pressure thrust
2. B. momentum thrust only
3. C. pressure thrust minus ram drag only
48. A red disc missing from the fuselage indicator shows that the fire bottle:
1. A. discharged due to overheat
2. B. was fired by the crew
3. C. is fully charged
49. The most critical bearing to lubricate in a gas turbine engine is the:
1. A. compressor front bearing
2. B. turbine bearing
3. C. accessory gearbox bearing
50. Which is an advantage of a centrifugal compressor compared with an axial compressor?
1. A. Higher pressure rise per stage and robustness against FOD
2. B. Smaller frontal area for a given airflow
3. C. Higher peak efficiency at high pressure ratios

Answer key

1. A — The fan accelerates a large mass of air; the portion that flows around the core (the bypass flow) produces most of the thrust on a high-bypass engine without being burned. The remainder enters the core compressor.
2. A — Pitch lock ratchets engage if pitch-change oil pressure is lost or rpm exceeds a set value, preventing the blades moving toward fine pitch and overspeeding.
3. B — Ram drag increases immediately with speed, so net thrust falls at first. At higher speed, ram compression increases mass flow and partly recovers thrust, so the curve dips then rises again.
4. B — The alarm switch closes when heat raises the gas pressure. The integrity switch is held closed by the normal fill pressure; if the tube leaks and loses pressure, it opens and gives a fault indication.
5. B — Loss of axial location allows the rotor to move fore and aft, causing blades to rub the casing or stator vanes, with severe damage.
6. C — The elbow turning vanes guide the air through the 90° turn into the combustor with minimal turbulence and pressure loss.
7. B — The turbine shaft must absorb high torque loads, so it is made from alloy steel; it is bolted or welded to the disc and splined to the compressor.
8. A — Bleed air has been compressed using turbine work but does not produce thrust. To maintain thrust the engine must burn more fuel, raising EGT and reducing stall margin.
9. A — A tank quantity transmitter (float/reed or capacitance) sends the level to EICAS; a sight glass is also used for servicing.
10. C — Residual fuel left in manifolds and nozzles would carbonise (coke) or cause post-shutdown fire; the drain valve empties it to a drain collector or tank.
11. B — A tool check against the list and a detailed inlet inspection for nuts, bolts, lockwire or tools prevent FOD on start.
12. C — On many aircraft an APU fire on the ground triggers automatic shutdown, fuel shut-off and sometimes automatic extinguisher discharge, plus external warning horns.
13. B — Only fuels listed in the approved data may be used. Jet B has different volatility and flash point, so its use may carry operating restrictions.
14. C — On engines with low flow rates, a turned (curved) inlet allows a smaller nacelle with less drag. Turbofans pass so much air that the duct needs a sufficiently straight section to deliver smooth, even airflow to the fan.
15. B — Mechanical blockage places clamshell doors or buckets behind the nozzle to block and reverse the gas; aerodynamic blockage uses blocker doors and cascade vanes to turn the flow within the nacelle.
16. A — Power turbines run at tens of thousands of rpm; engine reduction gearing (and then the main rotor gearbox) reduces speed and increases torque for the rotor.
17. B — Swirlers and air jets create a toroidal recirculation zone where hot gases return upstream, continuously igniting fresh mixture. Too high an axial velocity or too lean a mixture causes flame-out.
18. C — With fuel off and ignition selected per the manual, a technician listens at the exhaust or tailpipe for the regular snap of each igniter. Removed igniters must never be fired because the energy is lethal.
19. C — All bearing compartments, the oil tank and accessory gearbox are vented together so their pressures stay balanced; the air passes through the deoiler before going overboard.
20. C — Compressing cooler air takes less work. Intercooling therefore reduces compressor work and allows higher overall pressure ratios, though the heat removed must be made up with fuel unless recuperation is also used.
21. C — A ground run confirms performance and leak-free operation before release to service.
22. A — Penetrations in the firewall must maintain its fire integrity, using fireproof seals, grommets or bulkhead fittings.
23. C — The Brayton cycle has adiabatic compression, heat addition at constant pressure (combustor), adiabatic expansion (turbine and nozzle) and heat rejection at constant pressure. In an open cycle the rejection happens as the exhaust mixes with the atmosphere.
24. B — Propeller efficiency falls at high speed as tip Mach number rises, so turboprops are most efficient at moderate speeds; the FAA handbook cites 180-350 mph.
25. C — 1 kN = 1 000 N, so fuel flow = TSFC × thrust = 0.065 × 40 × 1 000 = 2600 kg/h.
26. B — A helium-filled sensing tube connected to a responder: heating raises gas pressure until a switch closes. An integrity switch signals a fault if pressure is lost.
27. A — Frosting (fine grey surface pitting) shows the start of surface distress from marginal lubrication or contamination; manual limits decide acceptance.
28. A — Over-fuelling during acceleration raises combustor back-pressure faster than the compressor can respond, reducing airflow and increasing blade angle of attack. Other causes include inlet distortion, damaged blades, and operating off-design.
29. B — In an impulse stage, the pressure drop and acceleration occur in the nozzle vanes; the blades (buckets) only change the gas direction, and the reaction to that change drives the wheel.
30. A — The precooler is an air-to-air heat exchanger in the pylon that uses fan air (controlled by a fan air valve) to reduce bleed temperature before it enters the aircraft ducting.
31. B — Thermocouple indicators are calibrated for a specific circuit resistance; changing lead length alters resistance and the indication. Leads are supplied in matched lengths or with resistance trimming.
32. A — Fuel enters a low-pressure centrifugal boost stage, passes through the fuel-oil cooler and filter, then the positive-displacement gear-type high-pressure stage supplies the fuel metering unit.
33. A — EGT margin trends help plan shop visits before the engine can no longer achieve take-off thrust within limits.
34. C — The APU control monitors speed and shuts down the APU (closing the fuel valve) if overspeed is detected.
35. C — Biocides kill bacteria and fungi. Antioxidants prevent gum formation; metal deactivators neutralise the catalytic effect of dissolved metals such as copper.
36. A — The bellmouth is instrumented to measure intake air temperature and pressure (total and static) so that airflow and corrected performance can be calculated. Thrust is measured separately by the load cell on the moving part of the thrust stand.
37. A — Clamshell (or target/bucket) doors form a physical barrier in the exhaust stream, deflecting it forward at an angle. They are typically used with engines exhausting through a common nozzle.
38. C — Torque split shows that one engine is carrying more load; it may point to a control or engine fault.
39. C — Swirl gives high flame speed through good fuel-air mixing and a low axial velocity so the flame does not move too rapidly downstream; the recirculation zone anchors the flame.
40. B — A tailwind reduces airflow through the engine (and may blow exhaust forward), so EGT can exceed the start limit. Starts are made into wind where possible.
41. B — Cold, thick oil gives high pressure drop; the relief/bypass protects the thin-walled cooler from rupture.
42. C — Electric power allows many small fans to be placed to improve aerodynamic efficiency and reduce noise.
43. B — Pink indicates relative humidity well above 30 %, the level above which corrosion may occur. Desiccant must be renewed and the engine inspected for corrosion per the manual.
44. C — Bootstrap equipment attaches to the pylon/wing to hoist the engine, avoiding the need for large cranes.
45. A — A geared turbofan places a planetary reduction gearbox between the fan and the LP shaft, so the fan can turn slowly (limiting tip speed) while the LP compressor and LP turbine run faster and more efficiently with fewer stages.
46. A — ESHP combines shaft power delivered to the propeller with the equivalent power of residual jet thrust.
47. A — F = ṁ (Vj − Va) + Aj (Pj − Pam). Because the gas cannot exceed sonic speed at the throat, the unexpended pressure (Pj above ambient) acting over the nozzle area Aj produces the additional pressure thrust.
48. A — The red (thermal) indicator is blown out when excessive heat causes the bottle's safety relief to vent overboard.
49. B — The FAA handbook identifies the exhaust turbine bearing as the most critical lubricating point because of the high temperatures around it; air cooling may be used as well as oil.
50. A — Centrifugal compressors give a high pressure rise per stage (up to about 8:1 on some designs), are simple, cheap, rugged, efficient over a wide speed range and need little starting power.

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