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Definition of the datum

Aviation Mechanic General (AMG) practice question 11 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

QUESTIONFor weight and balance purposes, the datum is best described as

  1. Athe point at which the aircraft balances when in the empty weight condition
  2. Ba horizontal line through the center of the main landing gear axles
  3. Can imaginary vertical plane from which all horizontal arms are measured
Show the answer and explanation

Correct answer: C. an imaginary vertical plane from which all horizontal arms are measured

  • A — incorrect That describes the empty weight center of gravity, not the datum.
  • B — incorrect The datum is a vertical plane chosen by the manufacturer, not a horizontal line through the gear.
  • C — correct The handbook defines the datum as an imaginary vertical plane from which all horizontal measurements are taken for balance purposes, with the aircraft in level flight attitude.

BACKGROUND

The datum is an imaginary vertical plane from which all horizontal measurements are taken for balance purposes, with the aircraft in level flight attitude. On a drawing it appears as a vertical line perpendicular to the longitudinal axis. The manufacturer chooses its location, often the nose, the wing leading edge, a helicopter's mast, or a specified distance from a known point. The datum location is identified in the Aircraft Specifications or Type Certificate Data Sheet (TCDS).

STUDY TIP

Keep datum (reference plane), arm (distance from it), and CG (balance point) as three separate ideas.

KEY POINTS

  • The datum is a vertical reference plane for horizontal measurements.
  • The manufacturer picks its location.
  • It is listed in the Aircraft Specifications or TCDS.

SOURCES

  • FAA-H-8083-30B ch06 "Weight & Balance Terminology - Datum"
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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.