FAA Flight Instructor Airplane (FIA)Performance, Weight and BalanceHard
During an annual inspection, an airplane is weighed on three scales: the nose wheel reads 400 lb at an arm of 25 in, the left main wheel reads 900 lb at an arm of 90 in, and the right main wheel reads 900 lb at an arm of 90 in. What is the airplane's empty weight CG?
- A71.7 in aft of datum
- B90.0 in aft of datum
- C75.0 in aft of datum
- D78.2 in aft of datum
Show answer & explanationAnswer & explanation
Correct answer: D. 78.2 in aft of datum
Total weight = 400 + 900 + 900 = 2,200 lb. Total moment = (400×25) + (900×90) + (900×90) = 10,000 + 81,000 + 81,000 = 172,000 lb-in. CG = 172,000 ÷ 2,200 ≈ 78.2 in aft of datum.
Why the other options are wrong
- A. Results from an incorrect total weight or arithmetic error in the division.
- B. Uses only the main wheel arm, ignoring the nose wheel's contribution.
- C. Averages the arms incorrectly instead of using weighted moments.
Empty Weight CG from Weighing
An airplane's empty weight CG is determined by weighing it on scales at each wheel and computing the total moment divided by total weight, a required step in establishing the weight and balance data.
- Each scale reading is a weight at a known arm (wheel location)
- Total moment = sum of (weight × arm) for each wheel
- Empty weight CG = total moment ÷ total weight
Memory trick: Three scales, three arms, one CG — sum the moments to see.