AMT Powerplant Theory and Maintenance 2 — Questions and Answers
Question 1: What is the purpose of the intercooler in a turbocharged reciprocating engine?
- To cool the oil before it enters the engine
- To reduce the temperature of compressed induction air (Correct answer)
- To cool exhaust gases before they enter the turbocharger
- To regulate the temperature of the magneto
Correct answer: To reduce the temperature of compressed induction air
An intercooler reduces the temperature of induction air after compression by the turbocharger, increasing air density for better fuel-air mixture.
Question 2: A reciprocating engine is said to be 'detonating' when:
- The fuel-air mixture ignites too late in the power stroke
- The fuel-air mixture explodes instantaneously rather than burning progressively (Correct answer)
- The magneto fires both plugs simultaneously
- Excessive oil enters the combustion chamber
Correct answer: The fuel-air mixture explodes instantaneously rather than burning progressively
Detonation occurs when the fuel-air mixture explodes instantaneously instead of burning progressively, causing severe pressure spikes and potential engine damage.
Question 3: Which turbine engine component is most susceptible to thermal fatigue and oxidation?
- Compressor inlet guide vanes
- High-pressure turbine blades (Correct answer)
- Accessory gearbox housing
- Fan blade leading edges
Correct answer: High-pressure turbine blades
High-pressure turbine blades operate at the highest temperatures in the engine and are therefore most susceptible to thermal fatigue and oxidation.
Question 4: When performing a differential compression test on a reciprocating engine cylinder, a reading of 72/80 psi is obtained. What does this indicate?
- The cylinder is serviceable; the leakage is within acceptable limits (Correct answer)
- The cylinder is unserviceable and must be removed immediately
- The piston rings are completely failed
- The intake valve is stuck open
Correct answer: The cylinder is serviceable; the leakage is within acceptable limits
A 72/80 reading (90% efficiency) is within the typically accepted limit of 75–80% of regulated pressure, indicating the cylinder is serviceable.
Question 5: In a gas turbine engine, the primary function of the diffuser section is to:
- Accelerate airflow into the combustion chamber
- Convert air velocity into static pressure (Correct answer)
- Mix fuel with compressed air
- Direct exhaust gases to the turbine
Correct answer: Convert air velocity into static pressure
The diffuser section converts high-velocity compressor discharge air into high-static-pressure, lower-velocity airflow before combustion.
Question 6: Which type of oil system is commonly used in dry-sump aircraft engines?
- A single pump circulates oil from a sump built into the crankcase
- A pressure pump and scavenge pump transfer oil to and from an external tank (Correct answer)
- Oil is gravity-fed from a header tank with no pump required
- A centrifugal pump draws oil directly from the accessory gearbox
Correct answer: A pressure pump and scavenge pump transfer oil to and from an external tank
Dry-sump systems use a pressure pump to deliver oil and one or more scavenge pumps to return it to an external oil tank, keeping the crankcase free of oil.
Question 7: What is the role of the fuel control unit (FCU) on a turbine engine?
- To ignite the fuel-air mixture during start
- To meter fuel flow based on power lever position and engine parameters (Correct answer)
- To pressurize the fuel before it enters the combustion liner
- To filter fuel contaminants before they reach the fuel nozzles
Correct answer: To meter fuel flow based on power lever position and engine parameters
The FCU meters the correct quantity of fuel to the combustion chamber based on power lever position, engine speed, and inlet conditions.
AMT Powerplant Theory and Maintenance 2
What is the purpose of the intercooler in a turbocharged reciprocating engine?