Part-66 Module 15 questions cover the gas turbine engine from first principles to the hangar floor: why the engine produces thrust, how each section works, how fuel, oil, air and ignition systems support it, and how it is monitored, run and stored. Turbine aeroplane and helicopter engineers all sit it, at two depths. This page sets out the paper sizes, works through all 22 sub-modules with the traps examiners use, includes one worked performance calculation and links free timed practice.
Part-66 Prep is an independent study aid. The syllabus is Appendix I and the exam standard Appendix II to Part-66 in Regulation (EU) No 1321/2014 and, for UK licences, the retained UK Part-66. For background reading, the FAA's public-domain Aviation Maintenance Technician Handbook, Powerplant (FAA-H-8083-32B) explains turbine engines well, though it uses US framing and its rules are not Part-66 rules. The Part-66 modules guide shows the full module list.
Part-66 Module 15 paper size by category
| Category | Aircraft | EASA questions / time | UK CAA questions / time | Marks needed |
|---|---|---|---|---|
| A1 | Turbine aeroplanes | 60 / 75 min | 60 / 75 min | 45 |
| A3 | Turbine helicopters | 60 / 75 min | 60 / 75 min | 45 |
| B1.1 | Turbine aeroplanes | 92 / 115 min | 92 / 115 min | 69 |
| B1.3 | Turbine helicopters | 92 / 115 min | 92 / 115 min | 69 |
Module 15 is the same size in both systems. The format is standard: three options, one correct, about 75 seconds a question, a 75% pass mark, no negative marking and no essays.
Under EASA rules a module exam over 90 minutes may be split into two partial exams, each with a number of questions that is a multiple of four and each passed at 75% on its own, so the 115-minute B1 paper qualifies. The retained UK text has no general partial-exam rule; ask the CAA or your Part-147 organisation how they run it.
Key fact: B1.1 and B1.3 sit the same Module 15 paper. The engine theory is the same for aeroplanes and helicopters; turboshaft engines simply get their own sub-module.
Knowledge levels: A versus B1
Appendix I sets B1 at knowledge level 2 (general working knowledge) on almost everything in Module 15 and level 3 (detailed knowledge) on engine monitoring and ground operation. Category A is at level 1 (familiarisation) on most sub-modules and level 2 on inlets and turbines. Category A does not sit engine performance, bearings and seals, alternate turbine constructions or storage and preservation at all, which is why the A paper is a third shorter.
What Module 15 covers, sub-module by sub-module
Figures in brackets are EASA's suggested question numbers for the B1 paper from AMC1 to Appendix II. Setters may vary them, but they show where the marks are.
15.1 Fundamentals (about 4 questions)
Potential and kinetic energy, Newton's laws of motion and the Brayton cycle; the relationship between force, work, power, energy, velocity and acceleration; and the construction and operation of turbojet, turbofan, turboshaft and turboprop engines. Our free Module 15 fundamentals note covers the cycle and engine types.
15.2 Engine performance (about 6 questions, B1 only)
Gross and net thrust, choked nozzle thrust, thrust distribution, resultant thrust, thrust horsepower and equivalent shaft horsepower, specific fuel consumption, engine efficiencies, bypass ratio and engine pressure ratio, pressure, temperature and velocity of the gas flow, and engine ratings, static thrust and the effects of speed, altitude and hot climate. This is where calculations live. The engine performance note has worked examples.
A short example of the method. An engine passes 50 kg of air per second. The jet leaves the nozzle at 600 m/s while the aircraft flies at 100 m/s. Net thrust is mass flow times the change in velocity: 50 times (600 minus 100), which is 50 times 500, or 25,000 newtons (25 kN). Gross thrust ignores the aircraft's speed: 50 times 600 is 30 kN. The procedural-error options come from exactly that confusion, or from adding the speeds instead of subtracting them. On the ground, with no forward speed, gross and net thrust are equal.
Specific fuel consumption (SFC) is the other performance figure to master. For a jet it is fuel flow divided by thrust; for a turboprop or turboshaft it is fuel flow divided by shaft power. Lower is better. Using the example above, if the engine burns 0.5 kg of fuel per second at 25 kN net thrust, the thrust SFC is 0.5 divided by 25, or 0.02 kg per second per kN, which is 72 kg per hour per kN. The distractors come from forgetting to convert seconds to hours, or dividing thrust by fuel flow.
Bypass ratio is the mass of air that goes around the core divided by the mass that goes through it. A ratio of 5:1 means five kilograms of fan air for every kilogram through the core. High bypass engines move a large mass of air slowly, which is more propulsively efficient and quieter at subsonic speeds than accelerating a small mass very fast.
Exam tip: Thrust falls as forward speed rises (ram drag grows) and as air density falls with altitude or heat. If an option says a hot day increases take-off thrust, it is wrong.
15.3 Inlet (about 4 questions)
Compressor inlet ducts, the effects of different inlet configurations, and ice protection. The inlet's job is to deliver smooth, slowed-down air to the compressor face across the whole flight envelope, so expect questions on divergent subsonic ducts and on what distortion does to the compressor.
15.4 Compressors (about 7 questions)
Axial and centrifugal types, construction, fan balancing, operation, causes and effects of compressor stall and surge, airflow control methods (bleed valves, variable inlet guide vanes, variable stator vanes, rotating stator blades) and compressor ratio. One of the heaviest sub-modules. The inlet and compressors note explains each control method, and the compressors drill (Pro) gives focused practice.
Common trap: in an axial compressor the air passage gets smaller towards the back, because the air is being compressed, not because it is speeding up. Axial velocity stays roughly constant through the stages.
15.5 and 15.6 Combustion and turbine sections (about 8 questions)
Combustion chamber types (can, can-annular, annular), primary and secondary airflow, flame stabilisation. Then turbines: nozzle guide vanes, blade types (impulse, reaction, impulse-reaction), attachment to the disc, blade stress and creep, and the reasons turbine blades are cooled. Know why the turbine inlet temperature is the limiting factor for engine life.
15.7 Exhaust (about 4 questions)
Convergent, divergent and variable area nozzles, engine noise reduction and thrust reversers. Know the difference between clamshell, bucket and cascade reverser types and the safety interlocks that stop them deploying in flight.
15.8 to 15.10 Bearings, seals, lubricants, fuels and lubrication (about 11 questions)
Bearing and seal types, the properties and specifications of turbine oils and fuels, fuel additives and safety precautions, then lubrication system operation and components: pressure and scavenge pumps, filters, coolers, breathers and chip detectors.
15.11 Fuel systems (about 5 questions)
Engine control and fuel metering systems including full authority digital engine control (FADEC), system layout and components. Understand the inputs a FADEC uses and what happens if one channel fails.
15.12 and 15.13 Air systems, starting and ignition (about 7 questions)
Internal engine air distribution, anti-ice and cooling air, then starter systems (electric, air turbine), ignition components and safety when working on high-energy ignition units. Starting sequences, hung starts and hot starts are popular questions.
15.14 Engine indication systems (about 7 questions)
Exhaust gas and interstage turbine temperature, engine thrust indication (EPR or fan speed), oil pressure and temperature, fuel pressure and flow, engine speed, vibration measurement and indication, torque and power. Know which parameter is the primary thrust setting on different engine types.
15.15 to 15.17 Alternate constructions, turboprops and turboshafts (about 10 questions)
Alternate turbine constructions such as geared fans and propfans (level 1), then turboprops (gas-coupled and free turbine, reduction gears, integrated engine and propeller controls, overspeed safety devices) and turboshafts (arrangements, drive systems, reduction gearing, couplings, control systems). B1.3 candidates should pay particular attention to turboshafts.
15.18 to 15.20 APUs, installation and fire protection (about 9 questions)
Purpose, operation and protective systems of auxiliary power units; firewalls, cowls, acoustic panels, engine mounts, anti-vibration mounts, hoses, pipes, feeders, connectors, wiring looms, control cables and rods, lifting points and drains; then fire detection and extinguishing systems on the engine.
15.21 and 15.22 Monitoring, ground operation and storage (about 10 questions)
Starting and ground run-up procedures, interpreting engine power output and parameters, trend monitoring (including oil analysis, vibration and boroscope inspection), inspection of the engine and components against criteria, compressor washing and cleaning, and foreign object damage. Then preservation and depreservation of the engine and its accessories and systems. This is the only sub-module at level 3 for B1, so expect practical questions on what an engineer does on a ground run and what a trend tells you.
A revision order for Module 15
The sub-module numbers follow the gas path and then the systems, and that order works for revision too:
- Theory first (15.1 and 15.2). The cycle, Newton's laws and the performance terms. Everything later refers back to them.
- The gas path (15.3 to 15.7). Inlet, compressor, combustion, turbine, exhaust. Sketch a cross-section of a twin-spool turbofan and label pressure, temperature and velocity changes through each section. If you can draw that graph from memory, a whole family of questions becomes easy.
- What keeps it turning (15.8 to 15.13). Bearings, oil, fuel, air, starting and ignition.
- What the crew and engineer see (15.14). Indications, linked back to the gas path you drew.
- Variants (15.15 to 15.18). Turboprops, turboshafts and APUs, which are the same core with a different output.
- The installed engine (15.19 to 15.22). Installation, fire protection, ground running, monitoring and storage. Draw on hangar experience here if you have it.
Give the last fortnight to full timed papers, and keep a list of every wrong answer by sub-module so your final revision targets the weak areas instead of the whole syllabus.
Which categories sit Module 15, and what it pairs with
Module 15 sits alongside the airframe module of each turbine category:
- B1.1 and A1: Module 11A (turbine aeroplanes) and Module 17A propeller, because turboprops are turbine aeroplanes.
- B1.3 and A3: Module 12 helicopters, with no propeller module.
Module 15 draws on Module 2 physics (gas laws, Newton) and Module 8 aerodynamics (aerofoils, which is what compressor and turbine blades are). The Part-66 B1 licence guide shows the full module set for each subcategory.
Practising Module 15 on Part-66 Prep
Our Module 15 bank holds 941 original three-option questions, 138 of them free, spread across the 22 sub-modules in line with the AMC weighting. Performance questions use calculation templates, so thrust and fuel consumption figures change between papers and you have to work each one.
Free, without an account:
- B1.1 Module 15 paper 1, 92 questions
- B1.3 Module 15 paper 1, 92 questions
- Category A Module 15 paper 1, 60 questions
- a 20-question quick practice with instant explanations
- the printable Module 15 sheet, 50 questions with an answer key
Neither EASA nor the CAA publishes past papers; these are exam-style papers written for the app. Papers 2 to 10, the sub-module drills and the other 11 Module 15 notes come with Pro.
Common reasons people lose marks in Module 15
- Gross versus net thrust. The single most common calculation slip.
- Stall versus surge. Know the difference in cause, extent and symptoms.
- Spool numbering. In a two- or three-spool engine, know which turbine drives which compressor (N1, N2, N3).
- Reverser and starter safety. Questions on interlocks and safety precautions are easy marks if you know the procedure and costly if you guess.
- Indication parameters. Mixing up EGT, ITT and turbine inlet temperature, or which parameter sets thrust on a given engine type.
Your study plan
- Take the free B1.1 diagnostic to check your foundation modules.
- Read the free fundamentals note, then go section by section through the engine from inlet to exhaust before the systems.
- Practise performance calculations by hand until gross and net thrust are automatic.
- Use the free quick practice, then sit paper 1 under timed conditions.
- Book when you score 85% or more on three different papers. If you are B1.1, Module 11A is a natural next step, since it covers the aircraft systems the engine feeds.
20 free Part-66 Module 15 Gas turbine engine practice questions with answers
Try each one, then open the answer. For the timed experience use the free exam-style paper or the printable version.
1. In a turbofan engine, the bypass (fan) air:
- A. is compressed by the fan and leaves without passing through the combustor
- B. passes through the core compressor and is burned with the fuel
- C. is ducted inside the core solely to cool the turbine blades
Show answer
A. is compressed by the fan and leaves without passing through the combustor
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 on a turboprop propeller engages when:
- A. oil pressure is lost or overspeed occurs
- B. the aircraft is on the ground
- C. the propeller is feathered
Show answer
A. oil pressure is lost or overspeed occurs
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. As a turbojet's forward speed increases at constant rpm, net thrust initially:
- A. increases steadily because of the increased mass flow
- B. decreases, because ram drag rises faster than the ram pressure recovery
- C. remains constant because the rpm is held constant
Show answer
B. decreases, because ram drag rises faster than the ram pressure recovery
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. In a pneumatic (gas-filled) fire detector with two switches, what does the integrity switch detect?
- A. A rapid rise in nacelle temperature
- B. Loss of gas pressure in the sensing tube
- C. Current flowing through a eutectic salt
Show answer
B. Loss of gas pressure in the sensing tube
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. A thrust bearing that has failed on a turbine engine rotor may allow:
- A. the rotor to turn faster than normal
- B. axial rotor movement and blade rubs
- C. oil pressure to increase
Show answer
B. axial rotor movement and blade rubs
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. Air leaving the diffuser vanes of a centrifugal compressor enters the combustion chambers through outlet elbows fitted with turning vanes that:
- A. accelerate the air to sonic velocity for burning
- B. mix the fuel into the air before the liner
- C. turn the flow from radial to axial with low loss
Show answer
C. turn the flow from radial to axial with low loss
The elbow turning vanes guide the air through the 90° turn into the combustor with minimal turbulence and pressure loss.
7. The turbine shaft is usually made of:
- A. aluminium alloy
- B. alloy steel
- C. magnesium alloy
Show answer
B. alloy steel
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. Extracting large amounts of customer bleed air from an engine causes:
- A. higher EGT and fuel flow for a given thrust
- B. lower EGT for the same thrust setting
- C. an increase in thrust at the same N1
Show answer
A. higher EGT and fuel flow for a given thrust
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. Oil quantity in a turbine oil tank is typically indicated by:
- A. a tank quantity transmitter
- B. an oil temperature thermocouple
- C. the N2 tachometer generator
Show answer
A. a tank quantity transmitter
A tank quantity transmitter (float/reed or capacitance) sends the level to EICAS; a sight glass is also used for servicing.
10. Fuel manifold drain (dump) valves operate at shutdown to:
- A. pressurise the manifolds for restart
- B. return fuel to the aircraft tanks at start
- C. drain residual fuel from the manifolds
Show answer
C. drain residual fuel from the manifolds
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. Before starting an engine after maintenance in the inlet, the technician must:
- A. run the engine at high power to clear it
- B. account for all tools and inspect the inlet
- C. spray water into the inlet
Show answer
B. account for all tools and inspect the inlet
A tool check against the list and a detailed inlet inspection for nuts, bolts, lockwire or tools prevent FOD on start.
12. An APU fire on the ground commonly results in:
- A. the APU running faster to clear the fire
- B. no action until the crew notice it
- C. automatic shutdown of the APU
Show answer
C. automatic shutdown of the APU
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. Before refuelling with Jet B instead of Jet A-1, a technician must check that:
- A. the tanks are empty of all Jet A-1
- B. Jet B is an approved fuel for the aircraft
- C. the engine is running at idle
Show answer
B. Jet B is an approved fuel for the aircraft
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. Why must the inlet duct of a high-bypass turbofan have a sufficiently straight section ahead of the fan?
- A. A straight duct gives the nacelle the lowest weight
- B. A straight duct raises the air to supersonic speed
- C. The very high airflow needs smooth, even flow at the fan face
Show answer
C. The very high airflow needs smooth, even flow at the fan face
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. The two basic categories of thrust reverser are:
- A. hydraulic and electric actuation
- B. mechanical and aerodynamic blockage
- C. fixed and retractable types
Show answer
B. mechanical and aerodynamic blockage
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. The reduction gearing of a turboshaft engine reduces the:
- A. power turbine speed for the output
- B. torque delivered to the rotor
- C. gas generator speed at idle
Show answer
A. power turbine speed for the output
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. Combustion stability ('flame holding') depends mainly on:
- A. a high axial gas velocity through the primary zone
- B. a recirculation zone of low axial velocity in the primary zone
- C. a very rich overall mixture of about 5:1
Show answer
B. a recirculation zone of low axial velocity in the primary zone
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. How is a turbine ignition system functionally checked on the ground?
- A. By firing an igniter held outside the engine
- B. By measuring EGT rise with ignition only
- C. By listening for the igniters snapping with fuel off
Show answer
C. By listening for the igniters snapping with fuel off
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. The breather system of a turbine engine connects:
- A. the fuel tanks to the oil tank
- B. the exhaust to the compressor inlet
- C. the bearing chambers, gearbox and oil tank
Show answer
C. the bearing chambers, gearbox and oil tank
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. In an intercooled gas turbine, cooling the air between compressor stages mainly:
- A. raises the turbine entry temperature
- B. increases the exhaust gas temperature
- C. reduces the work needed for compression
Show answer
C. reduces the work needed for compression
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.
Frequently asked questions
+How many questions are in Part-66 Module 15?
B1.1 and B1.3 sit 92 questions in 115 minutes. A1 and A3 sit 60 questions in 75 minutes. The sizes are identical under EASA and UK CAA rules, with a 75% pass mark.
+Who needs Module 15?
Anyone whose licence covers turbine-powered aircraft: A1 and B1.1 for turbine aeroplanes, and A3 and B1.3 for turbine helicopters. Piston categories sit Module 16 instead, and B2 covers engines from the avionics side in Module 14.
+What is in Module 15 gas turbine engine?
Twenty-two sub-modules: fundamentals, performance, inlets, compressors, combustion, turbines, exhausts, bearings and seals, lubricants and fuels, lubrication, fuel systems, air systems, starting and ignition, indication, alternate constructions, turboprops, turboshafts, APUs, installation, fire protection, monitoring and ground operation, and storage.
+How does a gas turbine engine work (Brayton cycle)?
Air is drawn in and compressed, fuel is burned in it at roughly constant pressure, and the hot gas expands through the turbine, which drives the compressor, and then out of the exhaust. The four stages are induction, compression, combustion and exhaust, happening continuously rather than in separate strokes.
+What is compressor stall and surge?
A stall happens when the airflow over compressor blades breaks away because the angle of attack is too high, usually on a few stages. Surge is a more severe breakdown of flow through the whole compressor, sometimes with flow reversal and a loud bang. Bleed valves, variable stator vanes and multi-spool designs help prevent both.
+Can Module 15 be split into partial exams?
Under EASA rules, yes: the B1 paper is 115 minutes, over the 90-minute threshold, so it may be taken as two partial exams if the exam organisation offers it. The A paper at 75 minutes is sat in one go. UK rules have no general partial-exam provision.
+How long should I study for Module 15?
There is no official figure. Candidates who work on turbine engines often need a few weeks of focused revision; those without engine experience commonly spend two months or more. Book when you score well on several full timed papers.
Study offline on your phone too
The Part-66 Prep: EASA & UK CAA app has the same questions plus a module board, essay coach, units drill and an attempt tracker for resit dates.
Keep reading
- Part-66 Modules 1 to 17: The Complete GuideAll 17 Part-66 modules explained: what each covers, which categories sit it, EASA and UK CAA question counts and times, and free practice papers.
- Part-66 Module 11 Questions: Aeroplane Structures and SystemsPart-66 Module 11 questions for 11A, 11B and 11C: paper sizes per category, partial exams, every ATA sub-module explained and free timed papers.
- Part-66 Module 12 Helicopter Questions and Practice PaperPart-66 Module 12 helicopter questions with a free timed paper: 128 questions for B1.3, 100 for A3/A4, every sub-module and the rotor aerodynamics traps.
- Part-66 Module 17 Propeller QuestionsPart-66 Module 17 propeller: 17A and 17B paper sizes under EASA and UK CAA rules, every sub-module from blade theory to balancing, and free practice.
- The Part-66 B1 Licence: Modules, Exams and ExperienceThe Part-66 B1 licence explained: B1.1, B1.2 and B1.3 privileges, modules and paper sizes under EASA and UK CAA rules, experience, costs and timescales.