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Half-deviation adjustment on a south heading

Aviation Mechanic Airframe (AMA) practice question 12 of 20 on Aircraft Instrument Systems. ACS element AM.II.H.K2 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONWhile swinging a compass, the N-S screw has been set facing north and the E-W screw set facing east. The aircraft is then aligned facing south and the compass indicates 184°. To what indication should the N-S compensating screw now be adjusted?

  1. A186°, doubling the indicated deviation
  2. B180°, removing all of the deviation
  3. C182°, removing half of the deviation
Show the answer and explanation

Correct answer: C. 182°, removing half of the deviation

  • A — incorrect Adjusting away from 180° increases the error rather than splitting it, which is the opposite of the intent of the procedure.
  • B — incorrect Removing the entire south-facing deviation would destroy the adjustment already made facing north; the procedure deliberately splits the error in half.
  • C — correct The procedure calls for eliminating half of the south-facing deviation from 180°; the deviation is 4°, so half of it is 2° and the compass is set to read 182°.

BACKGROUND

Swinging a compass means noting the deviation on the north-south radial and the east-west radial of a compass rose, then adjusting the compensating magnets to reduce that deviation as far as possible. The technician first zeroes the compensating magnets with a nonferrous screwdriver, aligns the aircraft north and sets the N-S screw for 0°, then aligns east and sets the E-W screw for 90°. Facing south, the compass is unlikely to read exactly 180°, so the N-S screw is used to eliminate half of the deviation from 180°. The same half-correction is then made facing west using the E-W screw against 270°. The aircraft is returned to north, indications are recorded every 30° around the rose, and the results are entered on a compass correction card that is dated, signed, and placed in full view of the pilot.

STUDY TIP

Practice the half-deviation arithmetic on both the south and west headings; the exam favors the step where only half the error is removed.

KEY POINTS

  • North and east headings are set to read exactly 0° and 90°.
  • South and west headings are corrected by only half of the observed deviation.
  • Results go on a dated, signed compass correction card near the compass.

SOURCES

  • sources/ama-airframe/ch10.txt "Magnetic Compass Maintenance & Compensation"
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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.