"...To maintain their operational ceiling of 70,000 feet (21,000 m), the early U-2A and U-2C models had to fly very near their never-exceed speed (VNE). The margin between that maximum speed and the stall speed at that altitude was only 10 knots (12 mph; 19 km/h). This narrow window is called the "coffin corner", because breaching either limit would likely cause airflow separation at the wings or tail. For most of the time on a typical mission the U-2 was flying less than five knots above stall speed. A stall would cause a loss of altitude, possibly leading to detection and overstress of the airframe."