Refer to figure.
In the same way that separated airflow prior to a low speed stall can cause airframe buffet, shock induced separation (shock stall) at high speed can also cause buffeting.

Aerodynamic buffet is a valuable stall warning, but it can damage the aircraft structure. Because of the higher dynamic pressure when an aircraft is operating in the transonic speed region, any shock induced buffet will have a greater potential for severe airframe damage.

High speed buffet must be completely avoided. The aircraft must therefore be operated in such a manner that a (safety) margin exists before aerodynamic buffet will occur.

If the variables which affect both high speed and low speed stall are considered it will be possible to identify the conditions under which buffeting will occur and a chart can be drawn to show all the factors involved.

This is called a ‘Buffet Onset’ chart which is used by flight crews to ensure their aircraft is always operated with a specified minimum buffet margin.

In this exercise:

1. Start by drawing a line from de 50° bank that intersects the 100-ton solid black line.
2. From here, draw a straight line from right to left that intersects the FL360 curved solid black line.
3. The goal is to have to intersection points, which correspond to the lowest and highest limits of the buffet boundaries. In this case, the low speed boundary and because there is no intersection for the high-speed boundary, as we can read on note 2 we’ll consider it to be M > 0.84.