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Vacuum gauges moving before deice system is turned on

Aviation Mechanic Airframe (AMA) practice question 1 of 9 on Ice and Rain Control Systems. ACS element AM.II.L.K6 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONDuring an operational check of a pneumatic deice boot system, the vacuum-operated instruments begin to operate before the deicing system is switched on. What does this indicate?

  1. AA check valve failed to close
  2. BThe vacuum regulator is set too high
  3. CThe air pump is worn beyond limits
Show the answer and explanation

Correct answer: A. A check valve failed to close

  • A — correct Movement of the vacuum-operated instrument pointers before the system is switched on shows that one or more check valves have failed to close and reverse flow is occurring through the instruments.
  • B — incorrect Regulator setting affects the suction level supplied to the gyros, not reverse flow through the instruments before system operation.
  • C — incorrect A worn pump shows up as low output capacity, such as slow boot inflation, rather than instrument movement before the system is turned on.

BACKGROUND

A deice boot system can be checked by running the engines or by using an external air source, and most systems have a test plug so the check can be made without running the engines. When an external source is used, the air pressure must not exceed the test pressure established for the system. Before turning the deicing system on, the technician observes the vacuum-operated instruments; if any of the gauges begin to operate, one or more check valves have failed to close and reverse flow through the instruments is occurring. That difficulty must be corrected before the test continues. With the system on, the pressure gauge fluctuates as the deicer tubes inflate and deflate while the vacuum gauge should hold a relatively steady reading, and an outside observer verifies the inflation sequence and timing against the maintenance manual.

STUDY TIP

Learn the order of the operational check: watch the vacuum instruments first with the system off, then check pressure and suction indications, then verify sequence and timing with an observer outside.

KEY POINTS

  • Check valves prevent backflow in the shared gyro and deice pneumatic system.
  • Instrument pointers moving with the system off means a check valve did not close.
  • With the system on, pressure fluctuates while suction stays relatively steady.

SOURCES

  • sources/ama-airframe/ch15.txt — "Operational Checks"
Practice the Aviation Mechanic Airframe (AMA) test →Timed 100-question exam, by ACS area, or review your misses. Free.
MORE ON ICE AND RAIN CONTROL SYSTEMS

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.