The rudder fails on a Boeing 737 operating as United Airlines Flight 585 and the aircraft spins out of control during approach to Colorado Springs, killing 25. Investigators are unable to determine the cause although they suspect a problem in the power control unit (PCU) which controls the rudder. Then three years later, another Boeing 737 flying as USAir Flight 427 crashes after a similar event during final approach. This time, the PCU survives intact but functions normally during testing. Two years on, Eastwind Airlines Flight 517 experiences two massive rolls to the right but the pilot is able to control the rolls and lands the 737 safely. This time an intact PCU is available for inspection. Investigations determine that thermal shock caused the rudders of the three aircraft to "hardover" and jam.