FAA Aviation Mechanic General (AMG)Aircraft Drawings, Weight and BalanceHard
An aircraft weighs 3,000 lb with the CG currently at 90.0 in aft of datum, which exceeds the forward CG limit. A technician plans to add ballast at fuselage station 40.0 in to bring the CG to the forward limit of 88.0 in. How much weight must be added?
- A150 lb
- B125 lb
- C187.5 lb
- D100 lb
Show answer & explanationAnswer & explanation
Correct answer: B. 125 lb
Using the added-weight formula: Weight to add = [Total weight × (Actual CG − Desired CG)] ÷ (Desired CG − Arm of added weight) = [3,000 × (90.0 − 88.0)] ÷ (88.0 − 40.0) = (3,000 × 2) ÷ 48 = 6,000 ÷ 48 = 125 lb.
Why the other options are wrong
- A. This results from using the wrong distance in the denominator.
- C. This results from doubling the numerator incorrectly.
- D. This underestimates by not properly dividing by the arm distance.
Adding Weight to Shift CG
To calculate ballast needed to move CG to a target location, use: Weight to add = [Total weight × (Actual CG − Desired CG)] ÷ (Desired CG − Arm of added weight).
- Used when CG is out of limits and ballast must be added
- Arm of added weight must be forward of desired CG to move CG forward
- Formula rearranges the weighted-average moment concept
Memory trick: Add weight far from CG for maximum leverage effect