Definition
The altitude in the vicinity of an airport at or below which the vertical position of an aircraft is controlled by reference to true altitudes, using the local altimeter setting. When climbing through the transition altitude, pilots reset their altimeters from the local pressure setting to the standard setting of 29.92 inches of mercury (1013.2 hPa).
Plain English
The altitude during a climb where you switch your altimeter from the local pressure setting to the standard setting. Below it, you fly using local pressure; above it, everyone uses the same standard setting so all aircraft read altitude the same way.
Context Anchor
Seen in instrument procedures, international operations, altimeter-setting procedures, and air traffic control altitude assignments.
Derivation
From Latin transitio, meaning a passing across or going over. The name reflects the moment of passing from one altimeter reference system to another.
Why Pilots Care
Maintains safe vertical separation between aircraft by ensuring everyone above this level uses the same pressure reference regardless of local weather.
Intuition Check
Do not read “transition” here as a general phase of flight or a training maneuver. It means a specific published altitude where the altimeter reference changes.
Example Sentence 1
Climbing through the transition altitude of 18,000 feet, the pilot set the altimeter to 29.92 and began reporting flight levels.
Example Sentence 2
Below the transition altitude the aircraft reported its height as an altitude; above it the same aircraft reported a flight level.