What happened
The aircraft was in the climbout phase of flight, passing through 4000 feet MSL, when it experienced a sudden failure of the #2 engine. The flight crew reported hearing an explosive sound, which was immediately followed by a drop in fuel flow and a decrease in N1 speed to approximately 30% RPM on the affected engine. Following this event, the aircraft returned to Seattle for an uneventful landing.
The investigation
Post-accident examination located the engine exhaust tail cone and numerous turbine blades in a rural area approximately 11 miles south of the airport. The investigation revealed that the turbine stator support structure had weakened due to sulfidation. This degradation was caused by exhaust gases escaping through an inspection port. The failure of this port allowed the hot gases to contact the turbine blades, resulting in blade detachment and subsequent engine imbalance.
Findings
The primary mechanical issue was the failure of the inspection port, which permitted exhaust gas leakage. This leak led to sulfidation of the turbine stator support structure, ultimately causing the turbine blades to detach. The resulting imbalance necessitated an immediate return to the departure airport.