FAA Aviation Mechanic General (AMG)Aircraft Drawings, Weight and BalanceMedium
A technician is computing the CG of a small aircraft for a weighing report. The basic empty weight is 1,500 lb at an arm of 78.0 in, and the two occupants together weigh 340 lb at an arm of 85.0 in. What is the resulting center of gravity?
- A81.50 in
- B79.29 in
- C78.00 in
- D85.00 in
Show answer & explanationAnswer & explanation
Correct answer: B. 79.29 in
Moment = weight × arm. Empty weight moment = 1,500 × 78.0 = 117,000 in-lb; occupant moment = 340 × 85.0 = 28,900 in-lb. Total weight = 1,840 lb; total moment = 145,900 in-lb. CG = total moment ÷ total weight = 145,900 ÷ 1,840 ≈ 79.29 in.
Why the other options are wrong
- A. This is an average of the two arms, not weighted by mass.
- C. This is just the empty weight arm, ignoring the occupant moment.
- D. This is simply the occupant arm, not the combined CG.
Center of Gravity (CG) Calculation
CG is found by dividing the total moment (sum of weight × arm for all items) by the total weight of the aircraft.
- Moment = Weight × Arm
- CG = Total Moment ÷ Total Weight
- Arm is measured from a fixed reference datum
Memory trick: CG = 'Center of Gravity' — total moment split by total weight