Regarding mass and balance calculation, the datum is...
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A
the point on the lateral axis of an aeroplane or its extension from which the centers of gravity of all masses are referenced.
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B
the point on the vertical axis of an aeroplane or its extension from which the centers of gravity of all masses are referenced.
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C
the point on the longitudinal axis of an aeroplane or its extension from which the centers of gravity of all masses are referenced.
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D
the distance from the reference plane to the center of gravity of an aircraft.
Refer to figure.
CENTER OF GRAVITY AND DATUM
The longitudinal axis is the axis that runs from the nose to the tail of the aircraft. It's the primary axis used for measuring the fore-and-aft position of the center of gravity (C.G.). In order to calculate the C.G., the weights and the moments of the loads should be measured. In order to calculate the moments, the distances from a datum are needed. (Moment = Weight x Arm)
The datum is a fixed point on this longitudinal axis, chosen by the aircraft manufacturer. It serves as the reference point from which all horizontal distances are measured. When calculating the mass and balance, the distance of each component's weight (e.g., fuel, passengers, cargo) is measured from this datum point. These distances are used to determine the moments and ultimately the overall C.G. of the aircraft.
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