Definition
Localized shock waves that form on parts of an aircraft when airflow over those areas reaches the speed of sound, even though the aircraft itself is still flying below the speed of sound. They occur in the transonic range, where some airflow is subsonic and some is supersonic at the same time, and they cause sudden changes in pressure, drag, and airflow behavior over the wings and control surfaces.
Plain English
When an aircraft flies fast enough, air speeds up as it flows over curved surfaces like the top of the wing. Even if the whole aircraft is below the speed of sound, the air over the wing can hit the speed of sound and form a small shock wave. These shock waves disturb the airflow and can cause buffeting, control problems, and a jump in drag.
Context Anchor
Seen in airspeed discussions for high-speed aircraft, especially when explaining why indicated airspeed, calibrated airspeed, equivalent airspeed, true airspeed, and Mach number are not all the same at high speeds.
Derivation
Transonic combines the Latin trans (across, through) with sonic (relating to sound). It describes the speed range where the aircraft is crossing through the sound barrier — partly subsonic, partly supersonic — which is exactly when these mixed-flow shock waves appear.
Why Pilots Care
These waves produce buffeting, increased drag, and possible loss of control effectiveness, directly affecting aircraft handling and airspeed indicator accuracy near the critical Mach number.
Analogy
It is a little like water piling up and changing shape around a fast-moving boat, except here the change is in air pressure at very high speed. The airplane is still moving through air, but the air no longer behaves as if it can move smoothly out of the way.
Grounding Statement
Air speeds up as it flows over a curved wing, so the wing can grow its own little supersonic patch — and a shock wave — long before the aircraft itself goes supersonic.
Intuition Check
Do not read shock as an electrical shock or a blast. Here it means a sudden jump in air pressure caused by very fast airflow. Do not assume transonic means the whole aircraft is exactly at the speed of sound. It means sound-speed effects are starting to appear in parts of the airflow.
Example Sentence 1
As the jet approached its critical Mach number, transonic shock waves formed on the upper wing surface and the crew felt the onset of Mach buffet.
Example Sentence 2
Transonic shock waves can appear on the upper wing surface even though the airplane has not yet reached the speed of sound.