What happened
During the takeoff roll of a turbine-powered aircraft, the crew felt a thump accompanied by a slight left nose pull. In response, the pilots aborted the takeoff. After taxiing clear of the runway and coming to a stop, several flight crews on the ground reported via tower frequency that the aircraft's left engine was on fire. The captain initiated an evacuation of the airplane. Airport Rescue and Fire Fighting (ARFF) personnel responded and determined that the engine fire was confined to the tailpipe area.
The investigation
An examination of the engine revealed that turbine blades had separated from the number 4 turbine disk. These blades penetrated the engine case, causing subsequent damage to the engine pylon and the empennage area. A teardown examination of the engine further showed that the Low Pressure Turbine (LPT) shaft was deformed and fractured into two pieces. The two pieces of the LPT shaft exhibited twisting and considerable permanent deformation, with a level of plastic deformation indicative of exposure to high temperatures. Heavy coke deposits were found on both the outer diameter of the LPT shaft and the inner diameter of the center tube. Additionally, investigators found that one of three 4 1/2 bearing stack retention pins was missing from the High Pressure Turbine shaft.