Definition
A theoretical level in the atmosphere at which the air pressure is exactly 29.92 inches of mercury (1013.2 millibars) and the temperature is 15 degrees Celsius. It is the fixed reference used to measure pressure altitude.
Plain English
An imaginary flat surface in the sky where the air pressure is set at the standard value of 29.92. Altitudes measured from this imaginary surface are called pressure altitudes.
Context Anchor
Seen in instrument flying, altimeter setting, and pressure altitude discussions.
Derivation
‘Datum’ comes from Latin meaning ‘something given’ — in engineering and surveying it means a fixed reference point that everything else is measured from. ‘Plane’ here means a flat, level surface. So a standard datum plane is simply a standard reference surface that altitudes can be measured against.
Why Pilots Care
Pressure altitude, which affects aircraft performance charts and true altitude calculations, is always measured upward from this fixed reference.
Analogy
It is like choosing the zero mark on a ruler before measuring. Once everyone uses the same zero mark, the measurements can be compared consistently.
Grounding Statement
Picture a flat sheet floating somewhere in the atmosphere where the pressure is exactly 29.92. On a high-pressure day it sits below sea level; on a low-pressure day it sits above sea level. Your pressure altitude is just your height above that sheet.
Intuition Check
Do not read plane as “airplane” here. It means an imaginary flat reference surface used for measuring altitude.
Example Sentence 1
When the pilot set the altimeter to 29.92, the indicated altitude was the height of the aircraft above the standard datum plane.
Example Sentence 2
Density altitude calculations begin by finding how far the aircraft is above the standard datum plane.