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Slow-opening oxygen shutoff valves

Aviation Mechanic Airframe (AMA) practice question 8 of 15 on Environmental Systems. ACS element AM.II.G.R3 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONWhy are the shutoff valves used in high-pressure gaseous oxygen systems designed to open slowly?

  1. ATo limit the velocity of the oxygen.
  2. BTo prevent the regulator from freezing.
  3. CTo reduce wear on the valve seat.
Show the answer and explanation

Correct answer: A. To limit the velocity of the oxygen.

  • A — correct Slow-opening valves keep high-pressure oxygen from rushing into a low-pressure area, where adiabatic compression against an obstruction can raise temperature past the ignition point.
  • B — incorrect The concern addressed by slow opening is ignition from adiabatic compression, not icing of the regulator.
  • C — incorrect Seat wear is a maintenance consideration but is not the reason the design requires slow opening.

BACKGROUND

Five types of valves are common in high-pressure gaseous oxygen systems: filler, check, shutoff, pressure reducer and pressure relief. They function as in any other system with one exception, which is that oxygen shutoff valves are specifically designed to open slowly. The ignition point of any substance is lower in pure oxygen than in air, and when high-pressure oxygen rushes into a low-pressure area its velocity can approach the speed of sound. If it meets an obstruction such as a valve seat, an elbow or a piece of contaminant, the oxygen compresses so quickly that no heat escapes, a process called adiabatic compression, and the resulting temperature can exceed the ignition point of the surrounding material. Even stainless steel can ignite under high pressure and temperature in 100 percent oxygen, so technicians should always open oxygen valves slowly.

STUDY TIP

Carry the slow-opening habit over to every oxygen valve you touch, including filler and cylinder valves during servicing. Pair it with the rule that all oxygen equipment must be free of grease and oil.

KEY POINTS

  • Oxygen shutoff valves open slowly to reduce oxygen velocity.
  • Rapid flow against an obstruction causes adiabatic compression and high temperature.
  • Even stainless steel can ignite in high-pressure pure oxygen.

SOURCES

  • sources/ama-airframe/ch16.txt "Oxygen Plumbing & Valves"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.