High-Altitude Stall Buffet

A Mach number-altitude operating envelope is a useful concept for jet airplanes. With Mach number plotted as the abscissa, the right-hand boundary gives the maximum operating Mach number at each altitude. This is ordinarily established by buffet for com­mercial airplanes. Military airplanes have higher design load factors, and structural strength or controllability ordinarily sets the right-hand maximum operating Mach number. When buffet, structural, or controllability limits do not apply, maximum Mach number is estab­lished by the highest speed attainable in dives from maximum altitude.

The left-hand boundary gives the minimum operating Mach number at each altitude. Ordinary stall generally defines the low-altitude minimum operating Mach number. Air­frame buffeting due to flow separation can define the minimum operating Mach number at high altitude, since that Mach number can be high enough for compressibility-induced flow separation. There is a significant stability and control involvement when stall buffet is responsible for the minimum operating Mach number at high altitude. This is because when a high-altitude stall buffet boundary is breached when an airplane is upset by turbulence, the pilot may use the controls too vigorously to recover.

This seems to have happened in 1992 with a McDonnell Douglas MD-11 airliner cruising at a Mach number of 0.70 at an altitude of33,000 feet. The airplane was upset by turbulence. In recovering from the upset, it slowed to a Mach number of 0.50 while making four successive penetrations of the stall buffet boundary.