Weight and Balance
The arithmetic
Weight and balance is moment arithmetic: moment equals weight times arm, and center of gravity equals total moment divided by total weight. The datum is a reference plane with no physical meaning, and arms forward of it are negative. Loading is a two-step process: take empty weight and empty CG from the current equipment list and weight and balance, add fuel, occupants, and baggage as new rows, then compare total weight and CG against the envelope.
Interpreting the envelope
Exceeding maximum gross weight overloads the structure and lengthens takeoff and landing distances. Forward of the forward limit, the airplane is hard to rotate and flare, stalls at a higher speed, and may be elevator-limited. Aft of the aft limit, stability and stall recovery degrade and cruise speed rises. Fuel burn moves the CG toward the CG of the remaining load, which can carry it out of limits on a long flight.
Traps
- Maximum ramp, takeoff, and landing weights differ; fuel burn-off may permit a legal overweight landing.
- Empty weight changes when equipment is installed; use the current document, not the POH sample problem.
- Do not average limits; check each point of the envelope.
Worked example
A 3,000-lb airplane with an item of 100 lb moved 40 in aft shifts its CG by (100 times 40) divided by 3,000, or 1.33 in aft. In reverse, to shift the CG 2 in, move (2 times 3,000) divided by 40, or 150 lb, across a 40-in arm.