ACS Powerplant Systems & Engine Overhaul 3 — Questions and Answers
Question 1: What is the function of an intercooler in a turbocharged aircraft engine installation?
- Cool compressed induction air between the turbocharger compressor and the engine intake (Correct answer)
- Cool engine oil after it passes through the turbocharger bearing
- Reduce exhaust gas temperature before entering the turbocharger turbine
- Transfer heat from the engine core to the cabin heating system
Correct answer: Cool compressed induction air between the turbocharger compressor and the engine intake
An intercooler (also called aftercooler) cools the hot compressed air from the turbocharger before it enters the engine, increasing air density and preventing detonation.
Question 2: Which FAA document provides the official time between overhaul (TBO) limits for an aircraft engine?
- The engine manufacturer's Type Certificate Data Sheet or service manual (Correct answer)
- FAA Advisory Circular AC 43.13-1B
- The aircraft's Airworthiness Directive database
- FAR Part 91 maintenance regulations
Correct answer: The engine manufacturer's Type Certificate Data Sheet or service manual
TBO limits for certified engines are established by the manufacturer and listed in the Type Certificate Data Sheet or the engine service/overhaul manual, not in FARs.
Question 3: During engine assembly, what is the correct procedure for installing a new oil seal on a crankshaft?
- Lubricate the seal lip with clean engine oil and press it in squarely to the specified depth (Correct answer)
- Install the seal dry to maximize friction and prevent leakage
- Heat the seal to expand it before sliding it onto the crankshaft
- Apply RTV sealant to the seal lip before installation
Correct answer: Lubricate the seal lip with clean engine oil and press it in squarely to the specified depth
Oil seals should be lubricated on the sealing lip with clean engine oil to prevent lip damage during installation and initial start-up, and pressed squarely to the correct depth.
Question 4: What condition does a 'hung start' describe in a turbine engine?
- The engine lights off and runs but stabilizes at an RPM below idle due to insufficient power from the starter (Correct answer)
- The starter motor fails to disengage after the engine reaches self-sustaining speed
- The engine achieves start but then surges repeatedly above and below idle
- Fuel flow is interrupted during the start sequence before light-off occurs
Correct answer: The engine lights off and runs but stabilizes at an RPM below idle due to insufficient power from the starter
A hung start occurs when the turbine engine lights and reaches a self-sustaining condition but RPM stabilizes below normal idle because the starter cannot supply enough energy to accelerate the engine further.
Question 5: In reciprocating engine ignition timing, what does 'BTDC' mean and why is it used?
- Before Top Dead Center; spark is fired early so combustion pressure peaks as the piston passes TDC (Correct answer)
- Below Top Dead Center; the piston fires at its lowest point to maximize power stroke length
- Between Two Dead Centers; timing between compression and power strokes
- Behind Top Dead Center; alternate term for retarded ignition timing
Correct answer: Before Top Dead Center; spark is fired early so combustion pressure peaks as the piston passes TDC
BTDC means Before Top Dead Center; the spark fires early because combustion takes time, allowing peak pressure to build just after TDC for maximum mechanical advantage on the piston.
Question 6: What is the purpose of a propeller reduction gearbox in a turboprop engine?
- Reduce turbine shaft RPM to a speed appropriate for efficient propeller operation (Correct answer)
- Increase shaft torque while decreasing rotational speed to drive accessories
- Redirect thrust from the gas generator to the propeller shaft
- Provide a mechanical connection between the N1 and N2 spools
Correct answer: Reduce turbine shaft RPM to a speed appropriate for efficient propeller operation
A reduction gearbox steps down the high RPM of the turbine shaft to the much lower RPM range (typically 1,000–2,000 RPM) at which a propeller operates most efficiently.
Question 7: Which of the following best describes 'detonation' in an aircraft reciprocating engine?
- Uncontrolled, explosive combustion of the fuel-air charge instead of a controlled flame front (Correct answer)
- A normal phase of the combustion process occurring after ignition
- Pre-ignition caused by a hot spot in the cylinder firing before the spark plug
- Backfiring through the induction system due to a late ignition timing
Correct answer: Uncontrolled, explosive combustion of the fuel-air charge instead of a controlled flame front
Detonation is the spontaneous, explosive auto-ignition of the end-gas portion of the fuel-air charge ahead of the flame front, producing damaging pressure spikes and characteristic knocking.
ACS Powerplant Systems & Engine Overhaul 3
What is the function of an intercooler in a turbocharged aircraft engine installation?