FAA Aviation Mechanic General (AMG)Ground Operations and ServicingHard
During weighing of a small aircraft, the scale readings and arm distances from the datum are: nose wheel = 900 lb at arm -20 in, left main wheel = 4,550 lb at arm 100 in, right main wheel = 4,550 lb at arm 100 in. What is the aircraft's center of gravity location?
- A89.2 inches aft of datum
- B80.0 inches aft of datum
- C95.5 inches aft of datum
- D100.0 inches aft of datum
Show answer & explanationAnswer & explanation
Correct answer: A. 89.2 inches aft of datum
Total weight = 900 + 4,550 + 4,550 = 10,000 lb. Total moment = (900 × -20) + (4,550 × 100) + (4,550 × 100) = -18,000 + 455,000 + 455,000 = 892,000 lb-in. CG = 892,000 ÷ 10,000 = 89.2 inches aft of datum.
Why the other options are wrong
- B. Results from an arithmetic error omitting the nose wheel's negative moment correctly.
- C. Overstates CG location; does not match correct moment/weight division.
- D. Would only be correct if nose wheel moment were ignored entirely.
Aircraft Weighing/CG Calculation
CG location is found by dividing total moment (sum of each scale weight times its arm) by total aircraft weight.
- Moment = weight × arm (distance from datum)
- Total weight = sum of all scale readings
- CG = total moment ÷ total weight
Memory trick: Moment over weight tells you where the balance sits.