The European Aviation Safety Agency (EASA) recently published EASA SIB 2015-07 as a quick review of the basic physics related to aircraft loss of control above FL300. The bulletin focused not on the aerodynamic stall itself, but on the recognition of conditions preceding a stall to help avoid the loss of control entirely.

The document begins with a reminder that at high altitudes, maximum available thrust is significantly reduced. EASA said, “When Mach [number] decreases from cruise, aerodynamic drag initially decreases, consistently with that Mach [number] reduction. With a constant thrust setting—assuming level flight—the aircraft has a natural tendency to recover initial Mach. In this range and as Mach [number] decreases, less thrust is required to maintain level flight. However, if Mach [number] continues to decrease, the aircraft will reach the point of minimum drag where, due to the aerodynamic characteristics of the wing profile, drag will start to increase. If the deceleration is not halted, drag will increase rapidly to a point where maximum available thrust cannot compensate.”

Trying to maintain level flight in this situation will inevitably end in an aerodynamic stall. The only safe recourse is to reduce the angle of attack and allow the aircraft to descend to regain flying speed.