Airframe Mechanic Certification Powerplant Systems & Maintenance 5 — Questions and Answers
Question 1: What is the purpose of a fuel-air ratio enrichment system (or power enrichment valve) in a carburetor?
- Lean the mixture at cruise to save fuel
- Enrich the mixture at full throttle to prevent detonation and provide cooling (Correct answer)
- Maintain stoichiometric mixture at all power settings
- Prevent vapor lock during high-altitude operation
Correct answer: Enrich the mixture at full throttle to prevent detonation and provide cooling
The power enrichment system richens the mixture at high power settings to prevent detonation and provide additional cooling through excess fuel.
Question 2: What color is Jet-A turbine fuel, and what is its approximate flash point?
- Blue, 25°F (-4°C)
- Colorless to straw-colored, approximately 100°F (38°C) (Correct answer)
- Red, 72°F (22°C)
- Green, 150°F (66°C)
Correct answer: Colorless to straw-colored, approximately 100°F (38°C)
Jet-A is colorless to straw-colored with a minimum flash point of approximately 100°F (38°C), making it less volatile than avgas.
Question 3: What is the primary cause of hot section deterioration in a turbine engine?
- High compressor inlet temperatures
- Thermal cycling and oxidation from extreme combustion temperatures (Correct answer)
- Oil contamination of the turbine blades
- Vibration from compressor imbalance
Correct answer: Thermal cycling and oxidation from extreme combustion temperatures
Combustion temperatures far exceeding blade material limits cause oxidation, thermal fatigue cracking, and erosion in the turbine hot section.
Question 4: In a constant-speed propeller system, if the propeller control is moved to the INCREASE RPM position, the governor:
- Sends high-pressure oil to the propeller to increase pitch
- Releases oil from the propeller dome allowing springs/counterweights to decrease pitch (Correct answer)
- Directly commands the engine to increase fuel flow
- Moves the blades to feather position momentarily
Correct answer: Releases oil from the propeller dome allowing springs/counterweights to decrease pitch
Moving toward higher RPM causes the governor to port oil away from the propeller piston, allowing centrifugal twisting moment or springs to decrease blade pitch and increase RPM.
Question 5: A turbine engine hung start is defined as:
- An engine that fails to light off during start sequence
- An engine that lights off but does not accelerate to idle RPM (Correct answer)
- An engine that exceeds temperature limits during start
- An engine that starts normally but immediately shuts down
Correct answer: An engine that lights off but does not accelerate to idle RPM
A hung start occurs when the engine ignites and sustains combustion but fails to accelerate to self-sustaining idle speed, requiring an abort.
Question 6: Which condition would cause high oil temperature and low oil pressure simultaneously in a reciprocating engine?
- Clogged oil filter
- Low oil quantity (Correct answer)
- Stuck-open oil cooler thermostat
- Faulty oil temperature gauge only
Correct answer: Low oil quantity
Low oil quantity means less oil is available to absorb heat and maintain pressure, causing both high temperature and low pressure simultaneously.
Question 7: When checking ignition timing on a reciprocating engine using a timing light, the timing mark should align at:
- Top Dead Center (TDC) for all engines
- The manufacturer-specified number of degrees Before Top Dead Center (BTDC) (Correct answer)
- Bottom Dead Center (BDC) for accurate timing
- Any position as long as both magnetos fire simultaneously
Correct answer: The manufacturer-specified number of degrees Before Top Dead Center (BTDC)
Ignition timing is set to the manufacturer-specified degrees BTDC to allow combustion to reach peak pressure just after TDC for optimal power.
