Definition
An airframe vibration caused by airflow separation that occurs at relatively low indicated airspeeds in high-altitude flight, when the local airflow over portions of the wing accelerates to supersonic speed even though the aircraft itself is well below its critical Mach number indication. As altitude increases, the margin between the stall speed and the speed at which Mach buffet begins narrows, eventually meeting at a point sometimes called 'coffin corner.'
Plain English
A shaking of the aircraft that happens at high altitude, even when the airplane isn't flying particularly fast, because the air over the wing is moving fast enough to cause shock waves and disturbed airflow.
Context Anchor
Seen in high-altitude performance and Mach-speed discussions, especially when comparing the slow-speed stall boundary with the high-speed Mach buffet boundary.
Derivation
Mach' comes from Austrian physicist Ernst Mach, whose name is used as the unit for the ratio of an object's speed to the speed of sound. 'Buffet' comes from Old French 'buffet,' meaning a blow or strike — here it describes the repeated striking sensation of disturbed airflow hitting the airframe. Knowing this helps the term feel less abstract: it's literally a Mach-related buffeting of the airplane.
Why Pilots Care
Signals the aircraft is nearing the edge of controlled flight; continued flight can lead to loss of lift or pitch-up.
Grounding Statement
At high altitude, the air is so thin that the airplane must fly at a high angle of attack to stay aloft, and air over the wing accelerates to shock-wave speeds even though the airplane itself feels slow — and that produces a shaking through the airframe.
Intuition Check
Do not read low-speed as simply slow over the ground; here it means the slow side of the airplane’s safe Mach operating range at that altitude. Do not read buffet as weather turbulence; here it is shaking caused by airflow separating from the wing.
Example Sentence 1
As the crew climbed into the upper thirties, they monitored airspeed carefully to stay clear of both the stall and low-speed Mach buffet.
Example Sentence 2
The instructor used low-speed Mach buffet as the cue to reduce angle of attack before the onset of pitch-up.