Definition
The upper and lower airspeed limits, at a given weight and altitude, between which an aircraft can fly without encountering buffet. The lower limit is the low-speed (stall) buffet, where the wing is approaching its critical angle of attack. The upper limit is the high-speed (Mach) buffet, where airflow over the wing reaches a Mach number high enough to form shock waves and disturb the airflow. As altitude increases, these two limits converge.
Plain English
The narrow speed window at high altitude where the aircraft is flying fast enough not to stall but slow enough not to hit shock-wave shaking. Fly too slow, the wing buffets from low speed; fly too fast, it buffets from going too close to the speed of sound.
Context Anchor
Seen in high-speed and high-altitude flight discussions, especially when comparing stall buffet on the slow side with Mach buffet on the fast side.
Derivation
Mach' is named after physicist Ernst Mach and refers to a fraction of the speed of sound. 'Buffet' comes from the Old French 'buffet,' meaning a blow or strike — here describing the rapid shaking of the airframe when airflow over the wing breaks down. 'Boundaries' marks the two edges of the safe-speed envelope.
Why Pilots Care
Staying inside these boundaries prevents loss of control, structural stress, and the onset of uncommanded pitch or roll oscillations.
Analogy
Picture a tightrope that gets narrower the higher you climb. Near the ceiling, the rope is so thin that one small step left or right — slower or faster — and you fall off.
Grounding Statement
At high altitude, an airplane may have less room between the speed where it begins to stall-shake and the speed where it begins to Mach-shake.
Intuition Check
Do not read buffet here as food service, and do not picture boundaries as painted lines in the sky. Here, buffet means aircraft shaking from disturbed airflow, and boundaries means the speeds where that shaking starts.
Example Sentence 1
At cruise altitude and heavy weight, the crew checked the Mach buffet boundaries chart and found only fifteen knots between low-speed and high-speed buffet.
Example Sentence 2
As the aircraft accelerated, the crew watched for the first signs of buffet that would mark the upper Mach buffet boundary.