Definition
That portion of controlled airspace wherein aircraft change from one phase of flight or flight condition to another.
Plain English
The chunk of controlled airspace where aircraft are shifting between phases of flight — such as climbing out from a departure into the en route structure, or descending from cruise toward an approach.
Context Anchor
Seen in airspace and chart discussions, especially around airports and routes where aircraft are entering or leaving another segment of flight.
Derivation
From the Latin transire, meaning 'to go across.' Transitional airspace is the airspace pilots are 'going across' as they move from one phase of flight to the next — it's the in-between zone, not a destination.
Why Pilots Care
Maintains safe clearance from terrain and obstacles during the critical low-altitude transition between en route flight and airport operations, directly supporting obstacle-avoidance and approach safety.
Intuition Check
Do not read transitional as meaning temporary or unimportant. Here it means airspace used during a change from one phase or condition of flight to another.
Example Sentence 1
After takeoff, the flight passed through transitional airspace as it climbed out of the terminal area and joined the en route structure.
Example Sentence 2
Airport expansion required recalculating the transitional airspace boundaries to keep the 7:1 slope clear of new buildings.