Definition
An imaginary vertical plane or line from which all horizontal distances are measured for weight and balance purposes. The location of the datum is established by the aircraft manufacturer and is shown in the aircraft's Type Certificate Data Sheet and weight and balance documents.
Plain English
A fixed starting line on the aircraft that the manufacturer chooses. Every weight on the airplane (seats, fuel, baggage, passengers) is measured as a distance forward of or behind this line. That distance is what's used to calculate balance.
Context Anchor
Seen in weight-and-balance charts, loading examples, and aircraft records that list distances for seats, fuel, baggage, and equipment.
Derivation
From the Latin 'datum,' meaning 'something given.' In aviation, it is literally a 'given' reference point — chosen by the manufacturer and fixed for that aircraft type. Knowing this helps explain why the datum isn't standard across all aircraft: each manufacturer 'gives' their own.
Why Pilots Care
Correct use of the datum ensures the center of gravity stays within limits, directly affecting aircraft stability and safety.
Analogy
Think of the datum like the zero mark on a ruler. The ruler only makes sense because every measurement starts from the same zero point.
Grounding Statement
All arm measurements for weight and balance begin at this imaginary line and extend aft or forward from it.
Intuition Check
Datum does not mean a piece of data here. In this context, it means the fixed starting line used for measuring distances on the airplane.
Example Sentence 1
The pilot measured each station's distance from the datum to calculate the moment for the loaded baggage.
Example Sentence 2
Using the wrong datum location will throw off the entire center-of-gravity calculation.