22 Turbine Engines Practice Questions & Answers

1. 1. In a gas turbine engine, the events of intake, compression, combustion, and exhaust occur

   • A.simultaneously in a single section that repeats them each cycle
   • B.continuously, but only in the sections aft of the diffuser case
   • C.continuously and simultaneously in separate sections of the engine✓ Answer
   • D.intermittently in the same section, one complete cycle at a time

   The gas turbine runs an open Brayton cycle in which each event has its own dedicated section, so all four happen at the same time and without interruption. A reciprocating engine performs the same four events intermittently, one after another, in the same cylinder.

   Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), turbine engine theory of operationReport a problem with this question

2. 2. At what point in a gas turbine engine does air pressure reach its highest value?

   • A.At the compressor inlet, where the air first enters the case
   • B.At the diffuser, in the divergent duct ahead of the burners✓ Answer
   • C.At the turbine nozzle vanes, just ahead of the first turbine wheel
   • D.In the primary zone of the combustion liner, at the flame front

   The diffuser is a divergent duct aft of the compressor: it slows the high-velocity discharge air and converts that velocity into static pressure, so pressure peaks there. Pressure then falls slightly across the burner and drops sharply through the turbine as energy is extracted.

   Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), diffuser and airflow through the engineReport a problem with this question

3. 3. On a high-bypass turbofan engine, approximately what share of total thrust is produced by the fan?

   • A.About 50 percent, shared evenly with the core exhaust flow
   • B.About 20 percent, with the core exhaust producing the rest
   • C.About 80 percent, with the core exhaust producing the rest✓ Answer
   • D.None at all; the fan only supercharges the core compressor

   A high-bypass fan accelerates a very large mass of air by a modest amount, which is the efficient way to make thrust at subsonic speed, and it accounts for roughly 80 percent of the engine's total thrust. That is also why fan air is the best source of reverse thrust on such engines.

   Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), turbofan enginesReport a problem with this question

4. 4. In a free-turbine turboprop engine, the power turbine is connected to the gas generator by

   • A.an air link only, with no mechanical connection between them✓ Answer
   • B.a reduction gearbox that ties both rotors to a common shaft
   • C.a fluid coupling that slips until self-sustaining speed is up
   • D.a splined coupling that drives the compressor at low speed

   In a free-turbine arrangement the gas generator drives only its own compressor, and a separate turbine downstream is driven by the gas stream alone. Because the propeller load is not mechanically tied to the compressor, the starter has far less mass to turn and gas generator speed can be set independently of propeller speed.

   Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), turboprop engines and free turbine arrangementsReport a problem with this question

5. 5. Compared with an axial-flow compressor handling the same airflow, a centrifugal-flow compressor has

   • A.a higher peak efficiency and an easier multi-stage layout
   • B.a higher pressure rise per stage but a larger frontal area✓ Answer
   • C.a lower pressure rise per stage but a smaller frontal area
   • D.a smaller frontal area and a lower starting power demand

   A centrifugal impeller throws air outward and can develop a large pressure rise in a single stage, and it is rugged and tolerant of damage, but the radial discharge makes the engine wide and makes staging awkward. The axial compressor is slim and reaches a higher peak efficiency, but needs many stages and more starting power.

   Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), centrifugal and axial flow compressorsReport a problem with this question

6. 6. Variable inlet guide vanes and compressor bleed valves are installed on a turbine engine mainly to

   • A.cool compressor discharge air before it reaches the burners
   • B.cut exhaust noise by smoothing the flow through the fan duct
   • C.raise the compressor pressure ratio during takeoff power
   • D.prevent compressor stall by matching airflow to rotor speed✓ Answer

   At low rotor speed the forward stages pump more air than the rear stages can pass, which drives up blade angle of attack. Variable vanes re-aim the air onto the rotor blades and bleed valves dump surplus air overboard, keeping airflow and rpm matched so the blades stay unstalled during starting and acceleration.

   Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), compressor stall control, variable vanes and bleed valvesReport a problem with this question

7. 7. A compressor stall in a turbine engine is the direct result of

   • A.an over-rich mixture that quenches the burner flame front
   • B.a loss of blade tip clearance at high compressor speed
   • C.an excessive angle of attack on the compressor blades✓ Answer
   • D.an excessive fuel flow that floods the combustion liners

   Compressor blades are airfoils, and they stall for the same reason a wing does: the angle at which the air meets the blade becomes too great. That happens when airflow and rotor speed no longer match, and the mechanic hears rumbling or backfiring with rising exhaust gas temperature and fluctuating rpm.

   Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), compressor stall and surgeReport a problem with this question

8. 8. In a turbine engine combustion chamber, the air that does not take part in burning is used to

   • A.seal the turbine bearing compartments against combustion gas
   • B.enrich the primary zone so the fuel-air ratio becomes burnable
   • C.spin the swirl vanes that anchor the flame front in one place
   • D.cool the gases and center the flame away from the liner walls✓ Answer

   Only about a quarter of the air entering the liner takes part in burning; the rest flows between the case and the liner and re-enters through downstream holes and louvers. That secondary air blankets the liner wall and drops gas temperature from roughly 3,500 degrees F to near 1,500 degrees F before the turbine.

   Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), combustion section primary and secondary airflowReport a problem with this question

9. 9. The turbine nozzle vanes ahead of the first-stage turbine wheel form a convergent passage in order to

   • A.raise gas velocity and aim the flow onto the turbine blades✓ Answer
   • B.lower gas temperature by expanding the flow ahead of the wheel
   • C.remove the swirl left in the gas by the last row of blades
   • D.raise gas pressure and slow the flow before the turbine blades

   A convergent duct trades pressure for velocity, so the nozzle diaphragm accelerates the hot gas and delivers it to the turbine blades at the most effective angle. Straightening residual swirl is the job of the exhaust cone struts downstream of the turbine, not of the nozzle vanes.

   Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), turbine nozzle diaphragmReport a problem with this question

10. 10. Turbine blades are sometimes shrouded at their tips in order to

    • A.carry cooling air out of the disk toward the blade tip
    • B.permit a thinner blade root at the fir-tree attachment
    • C.damp blade vibration and cut gas leakage past the blade tip✓ Answer
    • D.add mass to the blade so that it better resists creep

    Adjacent shrouds touch and lock the blades together as a ring, which damps vibration, and the shroud also forms a seal that keeps gas from spilling over the tip instead of doing work. The penalty is added weight at the largest radius, so shrouded blades must be made thinner elsewhere.

    Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), turbine blade constructionReport a problem with this question

11. 11. Permanent elongation of turbine blades in service, called growth or creep, is caused by

    • A.vibration set up by uneven flow from a worn fuel nozzle
    • B.repeated rubbing of the blade tips against the turbine case
    • C.corrosion pitting that thins the airfoil along its whole span
    • D.centrifugal load acting together with high gas temperature✓ Answer

    A turbine blade is under enormous centrifugal tension while running near the temperature limit of its alloy, and metal under sustained load at high temperature stretches slowly and permanently. Because the stretch is cumulative and irreversible, blade growth is a life-limiting condition rather than a repairable defect.

    Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), turbine blade growth and creepReport a problem with this question

12. 12. Stress rupture cracks in a turbine blade normally appear as

    • A.wavy variations in airfoil thickness along the trailing edge
    • B.shallow rounded pits grouped near the middle of the airfoil
    • C.long hairline cracks running parallel to the leading edge
    • D.minute hairline cracks at right angles to the leading edge✓ Answer

    Stress rupture shows up as fine cracks that start on the leading or trailing edge and run across it at a right angle, generally about a sixteenth of an inch or longer. Waviness or uneven airfoil thickness along the leading edge is the separate signature of over-temperature deformation.

    Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), turbine blade inspection and stress rupture cracksReport a problem with this question

13. 13. When blending damage out of a compressor blade, the blending should be done

    • A.parallel to the length of the blade, with generous radii✓ Answer
    • B.with a coarse rotary burr, leaving the surface as ground
    • C.across the blade at right angles to its length, then polished
    • D.as a sharp-bottomed notch confined to the damaged area

    Damage is removed with a fine file worked lengthwise, leaving a shallow depression with well-rounded edges that is then finished with a fine abrasive stone. Cross-blade scratches and sharp-cornered notches concentrate stress and can start the very crack the repair was meant to remove.

    Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), blending of compressor blades and vanesReport a problem with this question

14. 14. Which group of factors has the greatest effect on the thermal efficiency of a turbine engine?

    • A.Turbine inlet temperature, compression ratio, component efficiency✓ Answer
    • B.Compressor inlet temperature, fuel heating value, and bypass ratio
    • C.Engine pressure ratio, oil supply temperature, and cruise airspeed
    • D.Exhaust gas velocity, ambient humidity, and accessory gearbox load

    Thermal efficiency is how much of the fuel's heat becomes useful work, and it rises with the temperature at which heat is added and with how much the air is compressed first. Component efficiency matters because every bit of compressor or turbine loss is heat that never becomes shaft work or thrust.

    Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), turbine engine thermal efficiencyReport a problem with this question

15. 15. As outside air temperature rises with the engine held at a constant rpm, the thrust of a turbine engine will

    • A.stay the same, because the fuel control compensates for it fully
    • B.increase, because warm air passes through the compressor easier
    • C.decrease, because the exhaust nozzle chokes at a lower velocity
    • D.decrease, because the mass of air the engine pumps is reduced✓ Answer

    Thrust depends on the mass of air accelerated each second, and hot air is less dense, so the same rpm moves fewer pounds of air per second. The same reasoning explains why thrust falls with altitude and rises on a cold day, and why hot-day takeoff thrust must be limited.

    Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), factors affecting thrustReport a problem with this question

16. 16. Ram recovery for a turbojet engine describes the airspeed at which

    • A.the inlet duct turns ram velocity into heat at the compressor face
    • B.the thrust lost to rising forward speed is regained by ram pressure✓ Answer
    • C.the compressor regains rated speed once a bleed valve closes
    • D.the engine regains rated thrust once the anti-ice valve is closed

    Net thrust falls as the aircraft speeds up, because the incoming air already carries velocity that the engine no longer has to add. As speed keeps rising, ram pressure at the inlet raises airflow and pressure enough to offset that loss, and the point where thrust starts back up is ram recovery.

    Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), effect of airspeed on thrust and ram recoveryReport a problem with this question

17. 17. A turbine engine reaches its rated engine pressure ratio but runs hotter than normal at that setting. This most likely indicates that the engine

    • A.has a fuel control scheduling too lean during accelerations
    • B.has an idle speed adjustment set richer than the manual allows
    • C.has lost performance through compressor fouling or damage✓ Answer
    • D.is behaving normally for operation at a high ambient temperature

    High exhaust gas temperature for a given thrust parameter is the classic signature of a deteriorated or out-of-trim engine: it must burn more fuel to make the same pressure ratio. Contaminated or eroded compressor blades, lost tip clearance, bleed leaks and hot-section distress all produce that pattern.

    Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), causes of turbine engine performance lossReport a problem with this question

18. 18. A hung start on a turbine engine has occurred when the engine

    • A.lights off but will not accelerate to normal idle speed✓ Answer
    • B.fails to light off within the time allowed after fuel is on
    • C.exceeds its exhaust gas temperature limit during light-off
    • D.accelerates past idle and stabilizes above the rpm limit

    A hung start means light-off occurred but the engine stalled at some sub-idle speed, usually because the starter or the fuel schedule cannot supply enough energy to finish the acceleration. An over-temperature at light-off is a hot start, and no light-off at all calls for clearing the engine by motoring it with fuel and ignition off.

    Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), turbine engine starting, hot and hung startsReport a problem with this question

19. 19. Which marking material is acceptable on turbine engine parts that are directly exposed to the hot gas path?

    • A.A copper-based anti-seize paste in a thin stripe
    • B.A wax marking pencil used on a clean, dry surface
    • C.A soft graphite pencil, wiped off after inspection
    • D.A light application of layout dye, or ordinary chalk✓ Answer

    Layout dye used sparingly, or chalk, is permitted on parts in the gas path such as turbine blades, disks, vanes and combustion liners; a wax marking pencil is only for parts outside the gas path and never on a liner. Materials carrying carbon, lead, zinc or copper cause intergranular attack of hot-section alloys, so pencils and copper compounds are prohibited.

    Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), marking materials for turbine engine partsReport a problem with this question

20. 20. Extracting compressor bleed air for airframe services, with the power lever left where it is, will

    • A.raise the thrust available and lower compressor discharge pressure
    • B.lower the thrust available and lower exhaust gas temperature
    • C.lower the thrust available and raise exhaust gas temperature✓ Answer
    • D.leave thrust unchanged, since the fuel control resets the airflow

    Air taken from the compressor never reaches the turbine, so less mass flow is available to make thrust while the same fuel is burned in a smaller charge of air. The result is reduced thrust together with a higher exhaust gas temperature at the same power lever position.

    Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), compressor bleed air and its effect on engine performanceReport a problem with this question

21. 21. Fuel for an aircraft auxiliary power unit is normally supplied from

    • A.a small gravity reservoir refilled at each refueling stop
    • B.the airplane's main fuel tank system, through a boost pump✓ Answer
    • C.a dedicated auxiliary tank installed in the tail compartment
    • D.the engine-driven fuel pump on the nearest main powerplant

    The auxiliary power unit draws from the same main tanks as the engines, usually through a boost pump feeding its own fuel control, which avoids a separate tank and separate servicing. The unit is a small gas turbine supplying bleed air for main engine starting and air conditioning plus electrical power from its generator.

    Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), auxiliary power unitsReport a problem with this question

22. 22. Before performing a trim check on a turbine engine, the technician must obtain

    • A.ambient air temperature and field barometric pressure readings✓ Answer
    • B.pressure altitude and the forecast surface wind for the next hour
    • C.density altitude and the current dew point measured at the field
    • D.ambient air temperature and sea level barometric pressure values

    Trim curves are entered with the actual air conditions at the engine, so the mechanic reads ambient temperature and the field barometric pressure, not a sea level value, immediately before the run. The airplane is headed into the wind, and trimming is avoided in a tailwind, which causes hot gas reingestion, and in icing conditions.

    Source: FAA Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), turbine engine trimming proceduresReport a problem with this question

Practice questions based on the Aviation Mechanic Airman Certification Standards (FAA-S-ACS-1), 14 CFR parts 43, 65, and 91, and the FAA Aviation Maintenance Technician Handbook—Powerplant (FAA-H-8083-32). This site is not affiliated with or endorsed by the Federal Aviation Administration. This bank covers the POWERPLANT written test only; the General and Airframe written tests and the oral and practical tests are separate. Torque values, running clearances, compression limits, magneto timing angles, spark plug gaps, temperature and pressure limits, servicing quantities, and overhaul intervals always come from the manufacturer's current maintenance data, the type certificate data sheet, and the applicable airworthiness directives — never from a practice test. Confirm current eligibility and testing requirements with the FAA before you test. About the A&P mechanic certificate →