The 737 is the world's most widely used airliner. As you read this, more than 700 are in the air. But when US Air Flight 427 plunged out of the sky over Pittsburgh in September 1994, investigators had no idea it would take years to uncover the deadly cause. What broke the case was a real cold clue. Using a thermal shock test, experts determined that an extreme temperature swing made the rudder valve jam and that a faulty hydraulic flow then made the valve reverse instead of going to neutral. After a four year investigation the NTSB report resulted in Boeing redesigning the rudder valves. The FAA approved a rule that required airlines to upgrade flight data recorders in all planes. New emergency procedures for 737 rudder malfunctions are now taught to all pilots.