Aircraft Weight and Balance
Weight and balance asks one question: with this load, is the centre of gravity inside the approved envelope for every phase of flight? The exam tests the arithmetic, envelope interpretation, and the operational limits that make the arithmetic matter.
Core relationships
- Weight x arm = moment. Arm is the horizontal distance from the datum, usually in inches.
- CG = total moment / total weight.
- Basic empty weight includes unusable fuel, full oil and fixed equipment. Useful load = maximum ramp weight minus basic empty weight.
- Limits include maximum takeoff weight, maximum landing weight, maximum zero fuel weight, and forward and aft CG limits. The forward limit is usually set by elevator authority; the aft limit by stability.
- Ballast may be permanent or temporary and must be recorded.
Worked example
Basic empty weight 2,300 lb at 82.0 in; add 200 lb of baggage at 120.0 in. Moments: 2,300 x 82.0 = 188,600 lb-in; 200 x 120.0 = 24,000 lb-in. Total = 212,600 lb-in; total weight 2,500 lb. CG = 212,600 / 2,500 = 85.04 in. If the aft limit is 84.5 in, the aircraft is out of limits even though it is under maximum weight. Move load forward or add forward ballast. The shift formula is shift = (weight moved x distance moved) / total weight.
Traps that cost marks
- Mixing kilograms and pounds, or inch-pounds and metre-kilograms.
- Using station numbers as arms when the datum is not at station zero.
- Ignoring fuel density changes with temperature; use the density actually given.
- Checking takeoff weight but not zero-fuel weight or landing weight.
- Forgetting that adding weight at an aft station also changes total weight, not just moment.