Aircraft systems and instrumentsQuestion 553 of 934

What may happen when the vacuum pressure is low?

  1. The altimeter may read higher than the true altitude
  2. The airspeed indicator may read lower than the true speed
  3. The attitude and heading indicators may be unreliable
  4. The magnetic compass may become unreliable
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Correct answer: The attitude and heading indicators may be unreliable

The gyros slow down and become more susceptible to deflection. The pilot should cross-check the instruments.

Source: Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 8: Flight Instruments — Gyroscopic Flight Instruments — Sources of Power, page 218

Practice aircraft systems and instruments

Questions last reviewed: 18 October 2026