The Boeing 737 has a history of rudder system-related anomalies, including numerous instances of jamming. A number of accidents and incidents were the result of the airplanes' unexpected movement of their rudders. During the course of the four and a half year investigation of the crash of USAir Flight 427 near Aliquippa, Pennsylvania, killing 132 people, the NTSB discovered that the PCU's dual servo valve could jam as well as 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. This finally solved the mystery of sudden jamming of the rudders of this aircraft.