What happened
During the climb following takeoff, the aircraft experienced an uncontained failure of the No. 1 engine. Following the failure, the flight crew returned to the departure airport, where the aircraft landed without further incident.
The engine failure resulted in a significant distribution of debris, which was located across a city block area measuring eight blocks long by three blocks wide. The separation of the No. 1 engine's left thrust reverser also occurred during the event. Examination of the aircraft revealed damage to the fuselage, the tail cone, the rudder, the vertical and horizontal stabilizers, and the high lift devices on the left wing.
The investigation
Technical examination of the No. 1 engine showed a 360-degree separation in plane with the high pressure turbine (HPT) rotor. This failure caused the HPT shaft and the HPT forward rotating air seal to exit the engine. While the HPT rotating air seal was recovered within the debris field, metallurgical analysis of the component revealed two radial fractures passing through the bore and the HPT front shaft-to-HPT disk attachment bolt holes.
Investigators identified fatigue cracking in one attachment bolt hole. This crack originated from a heat affected layer, which was the result of abusive machining during the manufacturing process caused by a loss of coolant during drilling. Analysis of the striation count indicated that the fatigue crack had reached a detectable length during two previous in-service inspections, but it was not detected at those times.