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Periodic realignment of a directional gyro

Aviation Mechanic Airframe (AMA) practice question 2 of 20 on Aircraft Instrument Systems. ACS element AM.II.H.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 light aircraft is equipped with a vacuum-powered gyroscopic direction indicator. What must the pilot do with this instrument periodically during flight?

  1. AAdjust its vacuum relief valve setting.
  2. BRealign it with the magnetic compass.
  3. CReset it against the airspeed indicator.
Show the answer and explanation

Correct answer: B. Realign it with the magnetic compass.

  • A — incorrect The vacuum relief valve is a system component set during maintenance to hold the correct suction; it is not a flight deck adjustment for heading.
  • B — correct Vacuum-driven direction indicators drift from precession, so the handbook states that periodic manual realignment with the magnetic compass is required during flight.
  • C — incorrect The airspeed indicator carries no directional information, so it cannot serve as a heading reference for the gyro.

BACKGROUND

The gyroscopic direction indicator, or directional gyro, is often the primary direction instrument because a magnetic compass fluctuates too much for steady reference. Its operating principle is rigidity in space: the rotor spins in the vertical plane and stays aligned with the direction to which it was set while the aircraft and instrument case move around it. A vertical compass card geared to the rotor gimbal is calibrated in degrees, usually labeled every 30°, and the nose of a small fixed airplane on the glass shows the heading. Vacuum-driven direction indicators share the shortcomings of other vacuum gyros, including precession, so built-in compensation varies and a caging device is usually fitted. Because of that drift, the pilot must periodically realign the indicator with the magnetic compass during flight.

STUDY TIP

Remember that the gyro supplies steadiness while the magnetic compass supplies the reference; that pairing explains the realignment requirement.

KEY POINTS

  • A directional gyro works on rigidity in space with the rotor spinning in the vertical plane.
  • Precession causes drift, so the indicator must be realigned in flight.
  • The magnetic compass provides the heading reference for that realignment.

SOURCES

  • sources/ama-airframe/ch10.txt "Gyroscopic Direction Indicator or Directional Gyro (DG)"
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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.