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DGCA Technical General · 79 questions

Powerplant: DGCA Technical General Questions and Answers

79 practice questions on Powerplant with answers and explanations, plus the key concepts and formulas, from the TrueHeading question bank.

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About this topic

Use this page to revise Powerplant for the DGCA Technical General paper. Below are the key ideas first, then 12 practice questions with answers and explanations, picked from the 79 questions in the TrueHeading bank for this topic.

In short: Intake, compression, power, exhaust: “suck, squeeze, bang, blow”.

Open the full set in the student zone to answer every question, track your accuracy and take timed mock tests.

Key concepts: Powerplant

7 ideas to know cold.

Four-stroke piston cycle?

Intake, compression, power, exhaust: “suck, squeeze, bang, blow”.

  • Two crankshaft turns per cycle
  • Dual magnetos for safety and efficiency
  • Rich mixture cools; lean peak EGT gives best economy
1 Intake2 Compression3 Power4 Exhaustfour-stroke: suck, squeeze, bang, blow

Detonation, pre-ignition and fuel?

Detonation: uncontrolled explosive burn after normal ignition. Pre-ignition: fuel ignites before the spark.

  • Avgas 100LL is blue; avgas 100 green
  • Jet A-1: freezing point −47 °C or below
  • Low-octane fuel in a high-compression engine invites detonation

How does a gas turbine produce thrust?

It accelerates a mass of air rearwards (Brayton cycle: intake, compression, combustion, exhaust).

F=m˙ (Ve−V0)F=\dot m\,(V_e-V_0)
BPR=m˙bypassm˙coreBPR=\frac{\dot m_{bypass}}{\dot m_{core}}
  • Turbofans accelerate a large mass by a small amount: more efficient
  • Thrust falls with altitude as density falls

Compressor stall and surge?

Airflow separates from the compressor blades, pressure falls and may reverse (a bang, vibration, flame from the intake).

  • Caused by disturbed intake flow, rapid throttle movement or damage
  • Reduce power, check engine instruments

What are the four strokes of a piston engine?

Induction, compression, power, exhaust.

CR=Vs+VcVcCR=\dfrac{V_s+V_c}{V_c}
  • 2 crankshaft revolutions per cycle
  • Firing order selected to balance
  • Compression ratio = (swept + clearance)/clearance

Name the gas turbine engine sections and their order.

Intake → compressor → combustion chamber → turbine → exhaust.

  • Brayton cycle: constant-pressure combustion
  • Turbofan: bypass ratio
  • Turboprop: most power to propeller via gearbox

What are the causes of compressor stall?

Disturbed airflow (inlet distortion), excessive fuel flow, damaged blades, rapid throttle movement.

  • Symptoms: bangs, rising EGT, falling rpm
  • Action: close the thrust lever, check
  • Prevention: bleed valves, variable stator vanes

Practice questions: Powerplant

Tap “Show answer” after you have tried each one.

Q1A driving gear with 30 teeth turns at 2400 rpm and drives a gear with 40 teeth. The driven gear speed is:

  1. 1350 rpm
  2. 2880 rpm
  3. 2070 rpm
  4. 1800 rpm
Show answer

Answer: D. 1800 rpm

Speed ratio = 30/40; driven rpm = 2400 × 30/40 = 1800.

Q2A driving gear with 24 teeth turns at 1500 rpm and drives a gear with 60 teeth. The driven gear speed is:

  1. 600 rpm
  2. 840 rpm
  3. 450 rpm
  4. 540 rpm
Show answer

Answer: A. 600 rpm

Speed ratio = 24/60; driven rpm = 1500 × 24/60 = 600.

Q3A driving gear with 36 teeth turns at 1500 rpm and drives a gear with 40 teeth. The driven gear speed is:

  1. 1215 rpm
  2. 1350 rpm
  3. 1890 rpm
  4. 1012 rpm
Show answer

Answer: B. 1350 rpm

Speed ratio = 36/40; driven rpm = 1500 × 36/40 = 1350.

Q4A cylinder has a swept volume of 450 cc and a clearance volume of 50 cc. The compression ratio is:

  1. 12:1
  2. 10:1
  3. 8:1
  4. 9:1
Show answer

Answer: B. 10:1

CR = (swept + clearance) / clearance.

Q5A cylinder has a swept volume of 1000 cc and a clearance volume of 125 cc. The compression ratio is:

  1. 7:1
  2. 11:1
  3. 9:1
  4. 8:1
Show answer

Answer: C. 9:1

CR = (swept + clearance) / clearance.

Q6The ideal Otto cycle thermal efficiency (γ = 1.4) at a compression ratio of 9:1 is approximately:

  1. 67 %
  2. 58 %
  3. 49 %
  4. 29 %
Show answer

Answer: B. 58 %

η = 1 − r^(1−γ); higher compression ratio, higher efficiency (limited by detonation).

Q7A turbojet with mass flow 60 kg/s, jet velocity 500 m/s and flight speed 250 m/s (ignore pressure thrust) produces a net thrust of:

  1. 7.5 kN
  2. 45 kN
  3. 30 kN
  4. 15 kN
Show answer

Answer: D. 15 kN

Net thrust = ṁ (Vjet − Vflight).

Q8In a four-stroke piston engine the sequence of strokes is:

  1. power, induction, compression, exhaust
  2. induction, compression, power, exhaust
  3. compression, induction, exhaust, power
  4. exhaust, power, induction, compression
Show answer

Answer: B. induction, compression, power, exhaust

Intake, compression, power (expansion), exhaust.

Q9A rich mixture compared with the best-power mixture gives:

  1. no change
  2. lower fuel consumption
  3. lower EGT and cooler combustion
  4. higher EGT
Show answer

Answer: C. lower EGT and cooler combustion

The extra fuel absorbs heat and cools the cylinders.

Q10The compressor of a gas turbine engine:

  1. heats the fuel
  2. cools the exhaust
  3. increases the pressure of the incoming air
  4. reduces the pressure
Show answer

Answer: C. increases the pressure of the incoming air

Air is compressed before the combustion chamber.

Q11A high bypass turbofan produces most of its thrust from:

  1. the fan
  2. the turbine
  3. the core exhaust
  4. the afterburner
Show answer

Answer: A. the fan

The fan accelerates a large mass of air by a small amount.

Q12A hot start on a gas turbine engine is indicated by:

  1. EGT exceeding the starting limit
  2. low oil pressure
  3. low fuel flow
  4. high N2 only
Show answer

Answer: A. EGT exceeding the starting limit

The start should be aborted.

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More DGCA Technical General topics

Aerodynamics & Performance181 QFlight Controls & Stability38 QAirframe & Structures25 QPropellers32 QAircraft Systems116 QPitot Statics35 QBarometric Altimeters61 QRadio Altimeters37 QVertical Speed Indicators (VSI)32 QAirspeed Indicators24 QAirspeeds41 QMach55 QGyroscope52 QAttitude Indicators20 QTurn and Slip50 QINS, IRS and FMS32 QEFIS24 QAutoflight23 QFlight Director21 QEICAS / ECAM24 QEngine Instruments29 QTemperature Indicators34 QFuel Systems35 QCompass54 QWarning & Recording Systems33 QTCAS39 Q
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