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Suction gauge reading below minimum

Aviation Mechanic Airframe (AMA) practice question 3 of 20 on Aircraft Instrument Systems. ACS element AM.II.H.K8 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONThe attitude and heading indicators on an aircraft require 5 inches of mercury, with a stated minimum of 4.6 inches. The suction gauge reads 4.2 inches. What does this reading indicate?

  1. AThe gyros are spinning too slowly.
  2. BThe gyros are spinning too rapidly.
  3. CThe vacuum is within normal limits.
Show the answer and explanation

Correct answer: A. The gyros are spinning too slowly.

  • A — correct A reading below the stated minimum means the airflow is not spinning the gyros fast enough for reliable operation.
  • B — incorrect Excess suction would show as a reading above the desired value, not below the minimum.
  • C — incorrect The value is below the stated minimum of 4.6 inches, so the system is outside its acceptable operating range.

BACKGROUND

In a vacuum system a stream of air is directed against vanes on the gyro rotor to spin it, much like a water wheel, after passing through a filter. Either a venturi or a vacuum pump supplies the suction, and the amount required for instrument operation is usually between 3½ and 4½ inches of mercury, set by a vacuum relief valve in the supply line. The suction gauge, also called the gyro pressure gauge or vacuum gauge, is calibrated in inches of mercury and shows how much less pressure exists in the system than in the atmosphere. Desired values and minimum and maximum limits vary with system design. If the desired vacuum is 5 inches and the minimum is 4.6 inches, any reading below that minimum tells the crew that the gyros are not turning fast enough to be trusted.

STUDY TIP

Treat a suction gauge question as a limits comparison: read the number, compare it to the stated minimum, and describe the effect on gyro speed.

KEY POINTS

  • Vacuum systems spin gyro rotors with filtered air and are set by a vacuum relief valve.
  • Required suction is typically 3½ to 4½ inches of mercury.
  • A reading below the stated minimum means the gyros are turning too slowly to be reliable.

SOURCES

  • sources/ama-airframe/ch10.txt "Vacuum Systems"
  • sources/ama-airframe/ch10.txt "Typical Pump-Driven System"
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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.