Definition
The published height, referenced to airfield elevation using the QFE altimeter setting, at or below which an aircraft's vertical position is expressed as a height above the airfield rather than as an altitude above mean sea level. When climbing through the transition height, the pilot resets the altimeter from QFE to the appropriate setting (QNH or standard) so vertical position is then expressed as an altitude.
Plain English
It's the point during climb where you stop measuring how high you are above the airport and start measuring how high you are above sea level (or, higher up, above a standard reference). Below this point, your altimeter shows zero on the runway. Above it, you switch the altimeter to a different setting.
Context Anchor
Seen in international instrument procedure and altimeter-setting discussions; it is uncommon in everyday U.S. flying, where pilots normally use an altimeter setting that makes the altimeter read airport elevation on the ground.
Derivation
QFE comes from the international Q-code system used in early aeronautical and maritime radio. Each Q-code is a three-letter shorthand for a standard question or answer; QFE specifically refers to the pressure at field elevation. 'Transition' comes from Latin transire, meaning 'to go across' — the point where you cross from one altimeter reference to another.
Why Pilots Care
Ensures correct altitude separation from terrain and traffic once above the local airfield environment.
Grounding Statement
On the runway with QFE set, the altimeter should read about zero; in flight, it shows height above that runway or airport reference.
Intuition Check
Do not read “transition height” as any height where the airplane happens to be changing. Here it means a published height where the vertical reference changes from local QFE-based height to the standard reference used higher up.
Example Sentence 1
Climbing out of the airfield, the captain called 'transition height' and reset the altimeter from QFE to QNH.
Example Sentence 2
At the QFE transition height the crew changed from reading height above the field to flight level on standard pressure.