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Sight glass bubbles in a vapor cycle system

Aviation Mechanic Airframe (AMA) practice question 11 of 15 on Environmental Systems. ACS element AM.II.G.K6 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONBubbles are visible in the receiver dryer sight glass of a vapor cycle air conditioning system. What does this indicate?

  1. AThe system needs more refrigerant.
  2. BThe system is overcharged with oil.
  3. CThe expansion valve is stuck open.
Show the answer and explanation

Correct answer: A. The system needs more refrigerant.

  • A — correct When the charge is low, vapor in the receiver dryer is drawn up the stand tube and appears as bubbles in the sight glass, indicating refrigerant must be added.
  • B — incorrect Oil quantity is checked at the compressor crankcase, and an oil condition is not what the sight glass reports.
  • C — incorrect An expansion valve fault shows up as poor cooling or ice formation, not as bubbles in the receiver dryer sight glass.

BACKGROUND

The receiver dryer is the reservoir of the vapor cycle system, located downstream of the condenser and upstream of the expansion valve. Liquid refrigerant entering it passes through filters and a desiccant that removes foreign particles and captures water, because water forms acids and can freeze at the expansion valve orifice. A stand tube runs to the bottom of the unit so that liquid rather than vapor is withdrawn and sent to the expansion valve. A sight glass at the top of the stand tube lets the technician see the refrigerant: liquid flows there when the charge is adequate. If the system is low, vapor present in the receiver dryer is sucked up the stand tube and bubbles become visible.

STUDY TIP

Remember that a system needing refrigerant added should also be suspected of leaking, so pair a sight glass indication with a leak check. Electronic leak detectors held near fittings and hose connections find the very small losses that cause this.

KEY POINTS

  • Bubbles in the sight glass mean the system is low on refrigerant.
  • The receiver dryer stores refrigerant and removes particles and water with filters and desiccant.
  • A system that needs refrigerant added should be suspected of having a leak.

SOURCES

  • sources/ama-airframe/ch16.txt "Receiver Dryer"
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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.