AMT Powerplant Theory and Maintenance Questions and Answers 1 — Questions and Answers
Question 1: During a ground run of a gas turbine engine, a technician observes that the engine lights up, but the RPM fails to accelerate to idle, stabilizing at a lower-than-normal speed. The EGT is observed to be rising, but may not have exceeded limits yet. This condition is known as a:
- Hung start (Correct answer)
- Hot start
- Wet start
- False start
Correct answer: Hung start
A hung start occurs when the engine ignites but fails to accelerate to a self-sustaining idle speed. This is often caused by insufficient airflow from the starter to assist the compressor, causing the engine to 'hang' at a lower RPM. A hot start involves EGT exceeding limits, a wet start is a failure to ignite, and a false start is when the starter disengages prematurely.
Question 2: What is the primary function of the diffuser section in a centrifugal flow gas turbine engine?
- To increase the velocity and decrease the pressure of the air.
- To mix the fuel and air for combustion.
- To decrease the velocity and increase the static pressure of the air. (Correct answer)
- To straighten the airflow before it enters the turbine section.
Correct answer: To decrease the velocity and increase the static pressure of the air.
The diffuser is a divergent duct located after the compressor. Its purpose is to slow down the high-velocity air leaving the compressor and convert its kinetic energy (velocity) into potential energy (static pressure). This high-pressure, low-velocity air is ideal for efficient combustion.
Question 3: A technician is troubleshooting a reciprocating engine that is consistently backfiring through the induction system during idle. Which of the following is the most likely cause?
- An excessively rich idle mixture.
- A stuck-open exhaust valve.
- An excessively lean idle mixture. (Correct answer)
- Incorrectly gapped spark plugs.
Correct answer: An excessively lean idle mixture.
An excessively lean fuel/air mixture burns very slowly. The combustion can linger in the cylinder until the intake valve opens for the next cycle, at which point it ignites the fresh fuel/air charge in the intake manifold, causing a backfire. An overly rich mixture is more likely to cause afterfiring in the exhaust.
Question 4: For a turbocharged reciprocating engine, what is meant by the term 'critical altitude'?
- The maximum altitude at which the engine can produce 50% of its rated sea-level power.
- The highest altitude the aircraft is certified to fly.
- The maximum altitude at which a turbocharger can maintain a rated sea-level manifold pressure. (Correct answer)
- The altitude at which the turbocharger waste gate is fully open.
Correct answer: The maximum altitude at which a turbocharger can maintain a rated sea-level manifold pressure.
Critical altitude is the maximum density altitude at which a turbocharged engine can maintain its rated sea-level manifold pressure. Above this altitude, the turbocharger can no longer compensate for the decreasing air density, and engine power will begin to decrease.
Question 5: A maintenance technician inspects a magnetic chip detector removed from a gas turbine engine's oil system and finds several small, metallic particles. This discovery is a primary indicator of what condition?
- Oil contamination from an external source.
- Normal operation and requires no action.
- Wear or failure of internal ferrous components like gears or bearings. (Correct answer)
- Coking and carbon buildup in the lubrication system.
Correct answer: Wear or failure of internal ferrous components like gears or bearings.
Magnetic chip detectors are placed in the oil system specifically to attract and capture ferrous (iron-based) metal particles. The presence of these particles indicates that components such as bearings, gears, or shafts are wearing down or failing, releasing metal into the lubrication system.
Question 6: What is the primary purpose of the small vent holes found on aircraft magneto housings?
- To provide cooling airflow over the internal components.
- To allow for the draining of corrosive gases and moisture. (Correct answer)
- To act as an inspection port for viewing breaker point alignment.
- To equalize pressure with the crankcase.
Correct answer: To allow for the draining of corrosive gases and moisture.
During normal operation, the electrical arcing inside a magneto produces ozone and other corrosive gases. These gases, combined with any moisture that enters the housing, can damage internal components. The vent holes allow these harmful gases and moisture to escape, protecting the magneto's internal parts.
AMT Powerplant Theory and Maintenance Questions and Answers 1
During a ground run of a gas turbine engine, a technician observes that the engine lights up, but the RPM fails to accelerate to idle, stabilizing at a lower-than-normal speed.
The EGT is observed to be rising, but may not have exceeded limits yet.
This condition is known as a: