Powerplant Theory and Maintenance Flashcards
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Read the first 6 Powerplant Theory and Maintenance flashcards as text
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:
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.
What is the primary function of the diffuser section in a centrifugal flow gas turbine engine?
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.
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?
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.
For a turbocharged reciprocating engine, what is meant by the term 'critical altitude'?
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.
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?
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.
What is the primary purpose of the small vent holes found on aircraft magneto housings?
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.