Definition
A theoretical reference level where atmospheric pressure equals 29.92 inches of mercury (1013.2 millibars) and the temperature is 15 °C. It is the fixed baseline from which pressure altitude is measured.
Plain English
An imaginary level in the atmosphere where the pressure and temperature have agreed-upon standard values. Altitudes can then be measured from that level instead of from constantly changing real-world conditions.
Context Anchor
Seen in altimeter, pressure altitude, density altitude, and aircraft performance discussions.
Derivation
‘Datum’ comes from the Latin for ‘something given’ — a fixed point everyone agrees to start from. ‘Plane’ here means a flat reference level, not an aircraft. Together: a given, agreed-upon reference level.
Why Pilots Care
It is the baseline for pressure altitude, which directly affects aircraft performance calculations, altimeter settings, and flight planning.
Analogy
Think of it like the zero mark on a ruler. The zero mark is not the object being measured, but it gives you a fixed place to measure from.
Grounding Statement
Picture a flat, invisible sheet sitting at whatever height in the real atmosphere has a pressure of 29.92 inches Hg today. That sheet is the standard datum plane, and pressure altitude is just how far you are above (or below) it.
Intuition Check
Do not read plane as an aircraft here. In this term, plane means an imaginary flat reference level used for measurement.
Example Sentence 1
When the pilot set the altimeter to 29.92, the indicated altitude became the height above the standard datum plane.
Example Sentence 2
Aircraft performance tables assume standard conditions at the standard datum plane so pilots can adjust for actual temperature and pressure at their airport.