The reason is that the art of flying a plane can quickly change from complicated to complex, and sometimes to chaotic, situations in which even pilots and their human judgment and experience are often not able to respond adequately.
When both engines of the Airbus A320 being piloted by Captain Chesley B. “Sully” Sullenberger III were knocked out shortly after takeoff from LaGuardia Airport, with the plane soaring three thousand feet above the Bronx, Sully drew on his extensive flight experience to courageously land the plane on New York’s Hudson River. Having been a superb air force pilot who had spent years in the cockpit of the heavy, supersonic F-4 Phantom fighter jet, he knew the theory of how to slow a powerless descent. In just seconds, he was able to triage the situation and take decisive action. Ironically, the extraordinarily difficult feat was made even harder by the plane’s automated flight control system. As Sullenberger later explained, one of the features of the system, which is meant to avert disaster, is that at low flight speeds, “regardless of how hard the pilot pulls back on the sidestick, the flight control computers will not allow him to stall the wings and lose lift,” which is exactly what he had to do to land on the Hudson. As he attempted to pitch the nose of the plane to slow his airspeed over the water, computer overrides nearly made that impossible. There is good reason that when a flight hits turbulence over the North Atlantic, those on board hope and pray that there is an experienced pilot up front who can draw upon expertise potentially broader and more comprehensive than what system designers can imagine.