Mass and Balance
Mass and Balance tests the loading calculations that keep the aircraft within structural and CG limits, and the performance and handling consequences when it is not. You must be fluent with moments, index units, %MAC and the effect of fuel burn.
Core rules
- Moment = mass × arm from the datum. Index = moment divided by a constant, plus a fixed offset, which keeps load-sheet numbers small.
- %MAC = (CG arm − leading-edge arm) / MAC × 100. Forward and aft CG limits are usually quoted in %MAC.
- Basic empty mass, dry operating mass (including crew and catering) and zero fuel mass build up to take-off mass with fuel.
Worked example
Take-off mass is 50,000 kg with the CG at 11.0 m from the datum. MAC is 5.0 m and its leading edge is 10.0 m from the datum, so %MAC = (11.0 − 10.0)/5.0 × 100 = 20%. Move 2,000 kg from arm 8.0 m to 14.0 m: moment change = 2,000 × 6.0 = 12,000 kg·m, so CG shift = 12,000/50,000 = 0.24 m. The new arm is 11.24 m and the new %MAC is 24.8%. The CG has moved aft but is still inside a typical aft limit of about 30% MAC - always check the actual limit.
Effects and traps
- An aft CG reduces static stability and stick force and can reduce trim drag; a forward CG increases elevator effort and can restrict rotation.
- Traps: mixing mass and weight; forgetting that fuel burn moves the CG; adding last-minute changes without recomputing the index; using the MAC leading edge as the datum.