Datum
What is datum?
The datum is an imaginary reference plane, chosen by the manufacturer, from which all horizontal distances (called arms) are measured for weight and balance calculations. It can be located anywhere—forward of the nose, at the wing's leading edge, or even aft of the main wheels—and all moments are figured relative to it. Think of it as the "zero point" on a ruler: every station and arm on the airplane is measured as a distance from the datum.
Also known as: reference datum, datum line
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