What happened
During the ground run for takeoff, a critical mechanical failure occurred within the engine assembly. The incident began with a fatigue-precipitated failure of the third-stage turbine blade. This initial breakage resulted in secondary damage to several guide vanes as well as the fourth-stage turbine wheel.
The imbalance created by the damaged turbine wheel led to the subsequent failure of the number six bearing. This bearing failure allowed oil to leak into the exhaust area, which ultimately caused a low-intensity fire.
The investigation
The examination of the event focused on the sequence of mechanical failures within the turbine section. Investigators identified that the root cause was a fatigue failure in the third-stage turbine blade. This failure propagated through the system, damaging guide vanes and the fourth-stage turbine wheel.
Findings
The primary causal factor was fatigue failure of the third-stage turbine blade. This initial defect caused an imbalance in the turbine wheel, leading to the failure of the number six bearing and subsequent oil leakage into the exhaust area. The combination of leaking oil and hot exhaust components resulted in a low-intensity fire during the takeoff roll.