Weight and balance

The center of gravity (CG) of an aircraft is the point over which the aircraft would balance. Its position is calculated after supporting the aircraft on at least two sets of weighing scales or load cells and noting the weight shown on each set of scales or load cells. The center of gravity affects the stability of the aircraft. To ensure the aircraft is safe to fly, the center of gravity must fall within specified limits established by the aircraft manufacturer.

Center of gravity (CG) is calculated as follows:

The arm that results from this calculation must be within the center of gravity limits dictated by the aircraft manufacturer. If it is not, weight in the aircraft must be removed, added (rarely), or redistributed until the center of gravity falls within the prescribed limits.

Aircraft center of gravity calculations are only performed along a single axis from the zero point of the reference datum that represents the longitudinal axis of the aircraft (to calculate fore-to-aft balance). Some helicopter types utilize lateral CG limits as well as longitudinal limits. Operation of such helicopters requires calculating CG along two axes: one calculation for longitudinal CG (fore-to-aft balance) and another calculation for lateral CG (left-to-right balance).

The weight, moment and arm values of fixed items on the aircraft (i.e. engines, wings, electronic components) do not change and are provided by the manufacturer on the Aircraft Equipment List. The manufacturer also provides information facilitating the calculation of moments for fuel loads. Removable weight items (i.e. crew members, passengers, baggage) must be properly accounted for in the weight and CG calculation by the aircraft operator.

To find the center of gravity, we divide the total moment by the total mass: 193,193 / 2,055 = 94.01 inches behind the datum plane.

In larger aircraft, weight and balance is often expressed as a percentage of mean aerodynamic chord, or MAC. For example, assume the leading edge of the MAC is 62 inches aft of the datum. Therefore, the CG calculated above lies 32 inches aft of the leading edge of the MAC. If the MAC is 80 inches in length, the percentage of MAC is 32 / 80 = 40%. If the allowable limits were 15% to 35%, the aircraft would not be properly loaded.

When the center of gravity or weight of an aircraft is outside the acceptable range, the aircraft may not be able to sustain flight, or it may be impossible to maintain the aircraft in level flight in some or all circumstances. Placing the CG or weight of an aircraft outside the allowed range can lead to an unavoidable crash of the aircraft.

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Wikipedia