Flight Performance and Planning - Mass and Balance
Mass and Balance
This domain tests whether you can compute aircraft weight and centre of gravity (CG) and keep both inside the certified envelope. You must know empty weight, basic empty weight, maximum take-off and landing weights, datum, arm, moment and CG limits.
Core rules
- Weight: total the empty aircraft, occupants, fuel, baggage and any usable stores. Compare against maximum take-off weight and maximum landing weight.
- Moment: moment = weight × arm. Arms are measured from the datum; forward of the datum may be negative depending on the chart.
- CG: total moment divided by total weight. The result must fall between the forward and aft CG limits for that weight.
- Fuel: aviation gasoline is about 6 lb per US gallon; do not mix gallons with pounds.
- Loading: aft CG reduces stability and can shorten take-off distance but worsens stall/spin recovery; forward CG increases stability but can reduce elevator authority.
Worked example
Empty aircraft: 700 lb at arm 40 in → 28,000 lb·in. Pilot: 170 lb at 37 in → 6,290. Fuel: 240 lb at 48 in → 11,520. Baggage: 30 lb at 95 in → 2,850. Total weight = 700 + 170 + 240 + 30 = 1,140 lb. Total moment = 28,000 + 6,290 + 11,520 + 2,850 = 48,660 lb·in. CG = 48,660 ÷ 1,140 = 42.7 in. If the envelope allows 40.0–46.0 in at 1,140 lb, the loading is legal.
Traps that cost marks
Using empty weight instead of basic empty weight, forgetting that fuel arm changes as tanks burn, and ignoring maximum landing weight after a long trip are common. Always recheck the CG after moving baggage or passengers.