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When a combustion chamber drain valve opens

Aviation Mechanic Powerplant (AMP) practice question 11 of 20 on Engine Fuel and Fuel Metering Systems. ACS element AM.III.I.K16 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONA combustion chamber drain valve allows accumulated fuel to drain at what point?

  1. AWhenever the engine is running at idle speed
  2. BWhen fuel manifold pressure is highest
  3. CWhen burner pressure nears atmospheric
Show the answer and explanation

Correct answer: C. When burner pressure nears atmospheric

  • A — incorrect The valve is closed during engine operation, because combustion chamber pressure overcomes the spring and seats it even at low power.
  • B — incorrect High manifold or burner pressure is exactly what holds the valve closed; draining occurs only after that pressure decays.
  • C — correct A small spring holds the valve off its seat, so the valve drains fuel whenever pressure within the manifold or burners has fallen to near atmospheric pressure.

BACKGROUND

Drain valves are used for draining fuel from the engine components where accumulated fuel is most likely to cause problems, such as a fire hazard in the combustion chamber or gum deposits left in manifolds and nozzles after evaporation. The combustion chamber drain valve drains fuel that collects after each shutdown and fuel that may have accumulated during a false start. A small spring holds the valve off its seat until combustion chamber pressure during operation overcomes the spring and closes it, so the valve is closed while the engine runs. After the pressure within the manifold or burners falls to near atmospheric, the valve reopens and the trapped fuel drains. It is imperative that this valve be in good working condition, because otherwise a hot start during the next starting attempt, or an afterfire after shutdown, is likely to occur.

STUDY TIP

Anchor the logic on the spring: it wants the valve open, and only running pressure holds it shut. That is why a valve that fails to drain shows up later as a hot start on the next start attempt or as an afterfire after shutdown.

KEY POINTS

  • A small spring holds the combustion chamber drain valve off its seat.
  • Operating burner pressure overcomes the spring, so the valve is closed while the engine runs.
  • Fuel drains once manifold or burner pressure falls to near atmospheric after shutdown.

SOURCES

  • amp-powerplant/ch02.txt "Combustion Drain Valves"
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MORE ON ENGINE FUEL AND FUEL METERING SYSTEMS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.