Aircraft Upset
As an engineer, non-aviation, I am interested in aircraft upsets.
As I understand it, and please correct me if I am wrong, for an aircraft in cruise at high altitude and weight, the allowable range (margin) of air speed is small, to avoid stall at the lower limit or Mach overspeed at the higher limit. If the aircraft flies at a lower altitude, the margin increases; at a higher altitude the margin decreases.
It seems then, that the risk of aircraft upset would be inversely proportional to the size of the speed margin. (A margin of 5knots would be riskier than a margin of 20knots).
Who decides at what cruise altitude to fly, and hence decides what risk of upset is acceptable?
Is it the aircraft manufacturer, the certifying authority, the airline, or the captain?