Definition
The range of flight speeds, roughly Mach 0.75 to 1.20, in which airflow over different parts of an aircraft is partly subsonic and partly supersonic at the same time. Within this region, shock waves form locally on the airframe even though the aircraft itself has not yet reached the speed of sound, producing mixed-flow conditions that affect lift, drag, and control response.
Plain English
The speed band where some airflow over the aircraft is below the speed of sound and some is already above it. The aircraft is neither fully subsonic nor fully supersonic — it is in between.
Context Anchor
Seen in discussions of high-speed flight, especially when comparing subsonic, transonic, and supersonic airflow over an aircraft.
Derivation
Transition comes from the Latin transire, meaning to go across or pass over. The word captures the idea of an in-between zone — the airflow is in the middle of crossing from one regime to another.
Why Pilots Care
Recognizing this region explains the sudden rise in drag and changes in aircraft handling that occur as an airplane nears the speed of sound.
Grounding Statement
Picture a jet approaching the speed of sound: air over the thicker parts of the wing is already supersonic and forming small shock waves, while air over the rest of the airplane is still subsonic. That mixed condition is the transition region.
Intuition Check
Do not read “transition region” as a marked physical area in the sky. Here it means a range of speeds and airflow conditions where the air changes from subsonic behavior toward supersonic behavior.
Example Sentence 1
As the jet accelerated past Mach 0.8, it entered the transition region and the pilot noticed small trim changes from shock formation on the wings.
Example Sentence 2
Charts in the handbook show how the size of the transition region grows with increasing Mach number.