A typical nil turbulent buffet speed boundary graph for an airliner.
The speed boundaries within which airflow separates from the wing and the buffet is experienced. The low-speed buffet is caused by flow separation as the aircraft approaches the stalling angle of attack. At higher Mach numbers, the high-speed buffet is caused by flow separation from the wings as occurs behind the shock wave. Normally, high-speed aircraft flight manuals have at least three graphs called initial buffet boundary charts
. One is for level flight and nil turbulence, the second is for 40° bank angle/moderate turbulence, and the last one is for 50° bank angle/high turbulence. These correspond to load factor values of 1, 1.3, and 1.6, respectively. These graphs are used in selecting a suitable flight level, given the atmospheric conditions forecast or actually experienced. The turbulence penetration speeds are also displayed, along with maximum normal operation speeds, as indicated air speed (VMO
) up to a specified flight level and as a Mach number (MMO
) thereafter. The illustration here is for the first case and does not show penetration speed and VMO