Definition
The fixed reference point at an airport from which altimeter settings are determined, typically the airport's official elevation. When the local altimeter setting (QNH) is set in the Kollsman window, the altimeter reads height above mean sea level; when the airport's pressure setting (QFE) is set, the altimeter reads zero on the ground at the airport altimeter datum.
Plain English
The official height reference point at an airport that altimeter settings are based on. It's the spot the airport uses as its 'official elevation' for pressure and altitude purposes.
Context Anchor
Seen in instrument procedure discussions of QFE and altitude above ground, especially when comparing height above the airport with altitude above sea level.
Derivation
Datum' comes from the Latin datum, meaning 'something given' — a fixed, agreed-upon reference point. In surveying and aviation, a datum is any starting point from which other measurements are taken.
Why Pilots Care
When flying internationally or using QFE procedures, knowing the airport altimeter datum tells you exactly what your altimeter is referencing. With QFE set, the altimeter reads zero at this point — useful on approach, but easy to misread if you forget which setting is in the window.
Analogy
Think of putting the zero mark of a tape measure on the airport instead of at sea level. The same aircraft height can produce a different number depending on where the measuring starts.
Grounding Statement
On the ground at the airport datum with QFE set, the altimeter should read zero.
Intuition Check
Do not read “datum” as a piece of information or a set of data here. In this context, it means the fixed reference level the altimeter measurement starts from.
Example Sentence 1
With QFE set, the altimeter reads zero at the airport altimeter datum when the aircraft is on the ground.
Example Sentence 2
Charts list the airport altimeter datum so crews can apply the correct QFE reference during approach planning.