FAA Aviation Mechanic General (AMG)Aircraft Drawings, Weight and BalanceMedium
A student mechanic asks why the datum for weight and balance computations on one aircraft is at the nose, while on another it is 100 inches forward of the firewall. What is the correct explanation?
- AThe datum location is arbitrary and chosen by the manufacturer as a fixed reference point
- BThe datum must always be located at the aircraft's CG
- CThe datum location changes each time the aircraft is loaded differently
- DThe datum must be located at the leading edge of the wing on all aircraft
Show answer & explanationAnswer & explanation
Correct answer: A. The datum location is arbitrary and chosen by the manufacturer as a fixed reference point
The datum is simply an arbitrary reference point or line established by the manufacturer from which all arm (horizontal distance) measurements are made for weight and balance purposes. Its location varies between aircraft models but remains fixed for a given aircraft.
Why the other options are wrong
- B. The datum is a fixed reference, not the CG, which moves with loading.
- C. The datum is fixed for the aircraft and does not change with loading.
- D. There is no universal rule placing the datum at the wing leading edge.
Datum (Weight & Balance)
An imaginary vertical plane or line from which all horizontal distances (arms) are measured for weight and balance computations; its location is arbitrary and set by the manufacturer.
- Location varies by aircraft model, often nose, firewall, or a point ahead of the aircraft
- Fixed reference—does not move with loading
- All arms are measured forward (negative) or aft (positive) of the datum per convention
Memory trick: Datum = the 'starting line' manufacturer picks for measuring arms