Retreating blade stall is a hazardous flight condition in helicopters and other rotary wing aircraft, where the rotor blade with the smaller resultant relative wind exceeds the critical angle. Any stall is due to an excessive angle of attack. Retreating blade stall is the primary limiting factor of a helicopter's airspeed, and the reason even the fastest helicopters can only fly slightly faster than 200 knots (about 370 km/h).