What happened
During the final approach phase of flight, the aircraft experienced an uncontained failure of the number two (right) engine. The pilot reported feeling a shudder and hearing a loud bang, followed by the observation of a fire ball emanating from the right engine as the landing gear was being extended. Despite the engine failure, the aircraft completed a single-engine landing without further incident.
The investigation
A postflight inspection of the aircraft revealed an exit hole on the inboard side of the right engine cowling. A similar sized penetration was found in the right wing, located above and inboard of the cowling hole. Investigators identified a rupture on the left side of the engine plenum case, which was aligned with the damage to the airframe.
The physical evidence suggested that a single, high-energy rotor fragment exited the engine case on an upward and slightly forward trajectory. While the 2nd stage turbine wheel was not recovered, the axial location of the rupture and the intact condition of the 1st stage turbine wheel indicated that a piece of rim material, approximately 4 to 5 inches in size, had separated from the 2nd stage turbine wheel. Damage to the engine structure was consistent with a released fragment striking the 3rd stage turbine stator assembly forward rib support casting and deflecting toward the rupture location.
Further examination showed heavy curvic teeth smearing damage, indicating gapping at the rotor curvic connections during operation. This condition is typically caused by gross rotor mass unbalance. The observed shaft separation and extensive non-uniform rotational damage were also consistent with severe rotor unbalance and sudden shaft separation.
Findings
Materials analysis performed by Honeywell suggested that a section of wheel rim material separated due to low cycle fatigue cracks originating at the 2nd stage turbine wheel stress-reduction rivet holes. These fatigue cracks likely progressed to failure due to long-term exposure to excessive temperatures. Metallurgical evidence supported findings that 2nd stage turbine vane airfoils were exposed to temperatures exceeding 2000 degrees F for short periods.
Although no specific material defect or anomaly was found, investigators could not rule out other failure mechanisms because critical components, including the 2nd stage turbine disk, were not recovered during searches.