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Sign of a negative arm

Aviation Mechanic General (AMG) practice question 12 of 25 on Weight and Balance. ACS element AM.I.C.K4 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONA Type Certificate Data Sheet lists a baggage compartment with an arm of (-22.5). Where is the compartment located?

  1. A22.5 inches forward of the datum
  2. B22.5 inches aft of the datum, measured to the compartment's own CG
  3. C22.5 inches below the datum plane
Show the answer and explanation

Correct answer: A. 22.5 inches forward of the datum

  • A — correct Arms are measured in inches, and a negative sign indicates the item is located forward of the datum.
  • B — incorrect A positive sign indicates a location aft of the datum; the measurement is indeed taken to the item's own CG, but the direction is wrong.
  • C — incorrect Arms are horizontal distances from the datum, not vertical ones.

BACKGROUND

The arm is the horizontal distance from the datum to any point within the aircraft, always measured in inches and preceded by a positive or negative sign. A positive sign means the item is aft of the datum and a negative sign means it is forward of the datum. If the manufacturer places the datum at the most forward point of the aircraft, all arms are positive. When an arm is determined for a piece of equipment, the measurement is taken from the datum to the equipment's own CG.

STUDY TIP

Positive is aft, negative is forward; an item exactly on the datum has an arm of zero.

KEY POINTS

  • Arms are horizontal distances in inches from the datum.
  • Negative arm means forward of the datum.
  • Measure to the item's own CG.

SOURCES

  • FAA-H-8083-30B ch06 "Weight & Balance Terminology - Arm"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.