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Refrigeration bypass valve function

Aviation Mechanic Airframe (AMA) practice question 12 of 15 on Environmental Systems. ACS element AM.II.G.K7 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONA temperature sensor in the water separator of an air cycle system opens the refrigeration bypass valve. What does opening this valve accomplish?

  1. AIt warms air entering the water separator.
  2. BIt increases ram air through the exchanger.
  3. CIt closes the pack valve to stop bleed air.
Show the answer and explanation

Correct answer: A. It warms air entering the water separator.

  • A — correct The valve bypasses warm air around the air cycle machine into the expansion ducting just upstream of the separator, keeping the air above freezing.
  • B — incorrect Ram airflow through the heat exchangers is regulated by ram air doors, not by the refrigeration bypass valve.
  • C — incorrect Closing the pack valve is how the bleed air supply is shut off, and that happens on an overheat or other abnormal condition.

BACKGROUND

Air leaving the air cycle machine turbine expands and cools to near freezing, cold enough to freeze the water in the water separator and block airflow. A temperature sensor in the separator therefore controls a refrigeration bypass valve, also called a temperature control valve, 35-degree valve or anti-ice valve. When it opens, it routes warm air around the air cycle machine and introduces it into the expansion ducting just upstream of the water separator, warming the air just enough to keep it from freezing. The water separator itself has no moving parts: foggy air is forced through a fiberglass sock that coalesces the mist into drops, which collect on the sides and drain out while dry air passes on. A bypass valve is built into the separator in case of blockage.

STUDY TIP

Learn the alternate names for this valve, since manufacturers' manuals rarely all use the same one. If a pack delivers little or no airflow, a frozen water separator caused by a failed bypass valve is a classic cause.

KEY POINTS

  • The refrigeration bypass valve keeps air entering the water separator above freezing.
  • A separator temperature sensor controls the valve.
  • It is also called a temperature control valve, 35-degree valve or anti-ice valve.

SOURCES

  • sources/ama-airframe/ch16.txt "Refrigeration Bypass Valve"
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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.