Accidents
On March 3, 1991, United Airlines Flight 585, a 737-200, crashed in Colorado Springs, CO, killing 25 people.
On September 8, 1994, USAir Flight 427, a 737-300, crashed near Pittsburgh, PA, killing 132 people.
During the course of the investigation of Flight 427, the National Transportation Safety Board (NTSB) discovered that the PCU's dual servo valve could jam as well and deflect the rudder in the opposite direction of the pilots' input, due to thermal shock, caused when cold PCUs are injected with hot hydraulic fluid. As a result of this find, the Federal Aviation Administration (FAA) ordered that the servo valves be replaced and that new training protocol for pilots to handle unexpected movement of flight controls be developed.
Read more about this topic: Boeing 737 Rudder Issues