Definition
A fixed reference point, line, or surface from which measurements are taken. In aircraft weight and balance, the datum is an imaginary vertical plane established by the manufacturer, from which all longitudinal distances (called arms) are measured to determine the location of the center of gravity.
Plain English
A starting line you measure everything from. For weight and balance, it's the zero point the manufacturer picks on the aircraft, and every item's position is described as a distance forward of or behind that line.
Context Anchor
Seen in weight-and-balance calculations, aircraft records, maintenance manuals, and aircraft drawings.
Derivation
From Latin datum, meaning 'something given.' In measurement, it's the value or point that is 'given' as the starting reference, so every other measurement is taken from it.
Why Pilots Care
Accurate use of the datum ensures correct center of gravity calculations, which are essential for safe aircraft handling and stability.
Analogy
A datum is like the zero mark on a ruler. Once everyone agrees where zero is, every other measurement can be made from the same place.
Intuition Check
Do not read datum as “data” or general information. In this aviation use, it means the fixed starting reference used for measurement.
Example Sentence 1
The pilot looked up the aircraft's datum in the Pilot's Operating Handbook before calculating the loaded center of gravity.
Example Sentence 2
Pilots must use the correct datum location specified in the aircraft's weight and balance records to avoid errors in center of gravity determination.