5 knots. The pilot must maintain speed in a 5 knot range to avoid losing control.

This is because of a phenomenon called “coffin corner”. As altitude increases, stall speed increases, but the speed of sound also decreases. This means that fixed-wing, sub-sonic aircraft get more and more limited in the speed that they must fly as they get higher. Going to fast or too slow causes them to lose control and fall, and that can likely damage the aircraft with high g-forces as they try to recover.