Weight, arm, moment, and center of gravity — the formula, a worked example, and why the CG limits matter as much as the weight limit.
Weight and balance comes down to two checks. First, is the airplane under its maximum weight? Overweight hurts climb and landing performance and stresses the structure. Second, is the weight in the right place? The center of gravity has to fall between the forward and aft limits, or the airplane can become unstable or uncontrollable — even at a perfectly legal weight.
The airplane itself plus unusable fuel and full operating fluids — from the POH weighing record.
Max gross weight minus empty weight — everything you can add: people, bags, and fuel.
The horizontal distance (in inches) from the datum to a station where weight is placed.
Weight × arm. A measure of the turning effect a load has about the datum.
The point the airplane balances on — total moment ÷ total weight.
The forward and aft CG limits (and weight limit) you must stay inside, plotted in the POH.
Moment = Weight × Arm
CG = Total Moment ÷ Total Weight
List every item with its weight and arm, compute each moment, sum the columns, then divide. Confirm the total weight and the CG both land inside the POH envelope.
| Item | Weight (lb) | Arm (in) | Moment |
|---|---|---|---|
| Empty weight | 1,500 | 87.0 | 130,500 |
| Pilot & front passenger | 340 | 85.5 | 29,070 |
| Rear passengers | 170 | 118.0 | 20,060 |
| Fuel (48 gal × 6 lb) | 288 | 95.0 | 27,360 |
| Baggage | 50 | 142.8 | 7,140 |
| Total | 2,348 | — | 214,130 |
CG = 214,130 ÷ 2,348
CG ≈ 91.2 in
At 2,348 lb (under a 2,440 lb max) with a CG of 91.2 in inside an 83.0–93.0 in envelope, this loading is legal. Burn-off keeps it inside the envelope here — always confirm for your aircraft.
More stable, but heavier pitch forces, a higher stall speed, and longer takeoff and landing distances. Can make the flare harder.
Lighter controls and slightly more efficient, but less stable — and stall and spin recovery get harder. The aft limit is the safety-critical one.
Burning fuel shifts the CG in flight. A legal takeoff loading can drift toward a limit, so check that the CG stays in the envelope for the whole flight — not just at engine start.
FlightKit’s weight & balance tool stores your aircraft’s stations and limits, so you just enter people, bags, and fuel — it computes the CG and checks it against the envelope.
Two reasons. Too much weight degrades takeoff, climb, and landing performance and can overstress the airframe. And where that weight sits — the center of gravity — directly affects stability and controllability. An aircraft loaded outside its limits may be unsafe or impossible to fly, even if every individual item seems reasonable.
Multiply each item’s weight by its arm to get its moment, add up all the weights and all the moments, then divide: CG = total moment ÷ total weight. Compare the result against the POH CG envelope to confirm it falls between the forward and aft limits at your loaded weight.
The arm is a fixed distance from the datum to a loading station. The moment is weight × arm — the leverage that load exerts. The CG is the single balance point of the whole airplane, found by dividing total moment by total weight. Arms are given to you; moment and CG you calculate.
A forward CG makes the airplane more stable but heavier in pitch, with a higher stall speed and longer takeoff and landing distances. An aft CG lightens the controls and can be slightly more efficient, but it reduces stability and makes stall and spin recovery harder — which is why the aft limit is the more safety-critical one.
Yes. Burning fuel changes both weight and CG, depending on where the tanks sit relative to the datum. A legal loading at takeoff can drift toward a limit as fuel burns off, so you check that the CG stays within the envelope for the whole flight, not just at the start.
The datum is an arbitrary reference plane the manufacturer chooses — often the firewall or a point ahead of the nose. Every arm is measured from it. The datum’s exact location does not matter as long as every arm and limit is referenced to the same one, which the POH guarantees.