buffet boundary

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 .
At flight level 310, the low speed buffet starts at 202 knots IAS, while the high speed buffet commences at 325 knots—the total spread of 123 knots. For the same weight and conditions at flight level 350, the speeds are 212 and 290 knots—a margin of 78 knots.
A typical nil turbulent buffet speed boundary graph for an airliner.

Aviation dictionary. 2014.

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