FAA Flight Instructor Airplane (FIA)Performance, Weight and BalanceHard
A pilot must determine the baggage compartment loading needed to bring the airplane's CG to exactly 85.0 inches aft of datum. Loaded with fuel and passengers but no baggage, the airplane weighs 2,300 lb with a CG of 84.5 inches. The pilot plans to add 100 lb of baggage. What baggage station arm is required to achieve the desired CG?
- A96.5 inches
- B84.5 inches
- C102.0 inches
- D90.0 inches
Show answer & explanationAnswer & explanation
Correct answer: A. 96.5 inches
Total weight after adding baggage is 2,400 lb, and the desired total moment is 2,400 × 85.0 = 204,000 lb-in. The existing moment is 2,300 × 84.5 = 194,350 lb-in. The baggage moment needed is 204,000 − 194,350 = 9,650 lb-in, so the required arm is 9,650 ÷ 100 = 96.5 inches.
Why the other options are wrong
- B. This is the existing CG, not the required baggage arm.
- C. Overestimates the arm; would move the CG beyond 85.0 inches.
- D. Underestimates the arm needed to shift the CG aft to 85.0 inches.
Solving for Required Arm
When a target CG is known, the moment equation can be rearranged to solve for the arm of an added item: Arm = (Target Moment − Existing Moment) ÷ Added Weight.
- Target moment = desired total weight × desired CG
- Existing moment = current weight × current CG
- Required arm = moment needed ÷ weight of item being added
Memory trick: Work backward from the target moment to find where it must sit