S4·E5 Hidden Danger (United Airlines 585, USAir 427 and Eastwind Airlines 517)

On 3 March 1991, a Boeing 737 operating as United Airlines Flight 585 is on approach to land, when it suddenly rolls into a dive and crashes within eight seconds, killing all 25 people on board. On 8 September 1994, USAir Flight 427 is another Boeing 737 on approach to land, when it rolls and crashes within thirty seconds, killing all 132 people on board. On 9 June 1996, Eastwind Airlines Flight 517 is another Boeing 737 that unexpectedly rolls in similar circumstances, but the crew successfully regains control of the aircraft and lands safely. The cause of all three incidents was the rudder experiencing a "hardover" (suddenly going to full deflection) and jamming due to thermal shock of the hydraulic control valve.