Aircraft CG Envelope Check Tool

Loading an aircraft safely means checking two things together: that it is under its maximum weight, and that its centre of gravity falls within the certified envelope. This tool computes the moment for each item you load, sums to a gross weight and CG, and flags whether the result is inside a generic light-aircraft envelope.

How it works

Each item’s moment is its weight times its arm (distance from the datum). Summing all moments and dividing by total weight gives the CG, which is then checked against the weight and CG limits:

moment_i   = weight_i × arm_i
total_wt   = Σ weight_i
total_mom  = Σ moment_i
cg         = total_mom / total_wt
in_limits  = total_wt ≤ max_weight  AND  fwd_limit ≤ cg ≤ aft_limit

A loading is legal only when it passes all three checks: under maximum weight, not forward of the forward CG limit, and not aft of the aft CG limit.

Worked example

Suppose an empty aircraft weighs 1,500 lb at arm 39.0, a pilot of 170 lb sits at arm 37.0, 120 lb of fuel sits at arm 48.0, and 50 lb of baggage at arm 95.0:

CG = 75,300 ÷ 1,840 ≈ 40.9 inches. If the envelope runs from 35.0 to 47.3 inches forward-to-aft and max weight is 2,300 lb, this loading is legal on all three checks.

Understanding what each check catches

Maximum weight: An aircraft structure and its certification are designed around a specific maximum gross weight. Exceeding it overstresses airframe components, degrades climb performance, and extends the landing roll. In some loading configurations a light aircraft can feel fine handling-wise at modestly above MTOW while being structurally outside certification.

Forward CG limit: When the CG is too far forward, the elevator must work harder to rotate on takeoff and to flare for landing. At an extreme forward CG the elevator may not have enough authority to raise the nose — rotation speed climbs and landing flare may be impossible. This is the more common failure mode in training aircraft loaded with heavy baggage in a nose-forward position.

Aft CG limit: An aft CG reduces the restoring moment that makes an aircraft inherently stable. The aircraft becomes progressively harder to control and may depart controlled flight at an aft CG beyond the certified limit. Aft CG exceedances are particularly dangerous because there is often no warning before stability breaks down.

How fuel burn affects the loading

Fuel stations typically sit near the wing spar, which is close to (or slightly aft of) the centre of the envelope on many training aircraft. As fuel burns off during the flight, the total weight drops but the CG shift depends on the fuel arm relative to the total CG. Always check both the takeoff and estimated landing loading — a flight that is inside limits at departure can go outside limits after fuel burn if the fuel arm is significantly different from the total CG. Recompute CG at landing weight with remaining fuel.

Always pull the real arms and envelope from your aircraft’s specific pilot operating handbook (POH). This tool uses whatever limits you enter and is for teaching the method, not for dispatching real flights.