Pratt & Whitney J75 turbojet. The U-2S and TU-2S variants incorporated the more powerful General Electric F118 turbofan engine. High aspect ratio wings give the U-2 glider-like characteristics, with an engine out glide ratio of about 23:1, comparable to gliders of the time. To maintain their operational ceiling of , 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 . This narrow window is called the "coffin corner", because breaching either limit was likely to cause airflow separation at the wings or tail. For most of the time on a typical mission the U-2 was flying less than above stall speed. A stall would cause a loss of altitude, possibly leading to detection and overstress of the airframe. The U-2's flight controls are designed for high-altitude flight; the controls require light control inputs at operational altitude. However, at lower altitudes the higher air density and lack of a power-assisted control system make the aircraft very difficult to fly: control inputs must be extreme to achieve the desired response, and a great deal of physical strength is needed to operate the controls. The U-2 is very sensitive to crosswinds, which, together with its tendency to float over the runway, makes the aircraft notoriously difficult to land. As it approaches the runway, the cushion of air provided by the high-lift wings in ground effect is so pronounced that the U-2 will not land unless the wings are fully stalled. A landing U-2 is accompanied on the ground by a chase car, which is driven by a second U-2 pilot who assists the landing U-2 by reporting the aircraft's altitude and attitude.Smith, Sam. "Chasing the U-2 spy plane— in a Pontiac GTO." Popular Mechanics, 28 August 2012. Retrieved: 12 September 2014. In practice, once the aircraft has descended to an altitude of above the runway the pilot initiates a stall and the aircraft falls from this height. Chase cars and live calling of aircraft altitude are necessary because the landing gear is not designed to absorb the weight of the aircraft when falling from altitudes much above . Instead of the typical tricycle landing gear, the U-2 uses a bicycle configuration with a forward set of main wheels located just behind the cockpit and a rear set of main wheels located behind