What happened
While descending from FL 145 above an airport, the pilot experienced an engine failure caused by the liberation of a fragment from the 3rd-stage turbine wheel. This fragment, which broke away due to a low-cycle fatigue crack, became lodged in the combustion case. The loss of this material created a significant rotational imbalance, which subsequently caused the main and torsion shafts to fracture. As components from the turbine section detached, they exited the engine and caused damage to the exhaust assembly. The failure was contained, as no fragments exited the combustion case with enough energy to pose an external hazard to the aircraft. The pilot had enough time to perform necessary engine failure procedures and successfully executed a forced landing on runway 18, a decision that minimized risk to parachutists in the area.
The investigation
Investigators examined engine logbooks and maintenance records to determine why the component failed. The inquiry revealed that the initial maintainer had made several traceable errors when calculating and recording engine cycles and equivalent cycles. Because these calculations were performed by a single individual without independent verification, the errors went undetected. These inaccuracies meant the 3rd-stage turbine wheel had accumulated 477.6 more equivalent cycles than the logs indicated, meaning the part had actually exceeded its life-limit by 357.6 equivalent cycles at the time of the failure. The operator had intended to replace the part once the recorded limit was reached.
Furthermore, the investigation looked into the operator's method for estimating daily engine shutdowns. The practice of estimating shutdowns at the end of each day, rather than following the specific monitoring methods outlined in the Honeywell service bulletin, introduced potential inaccuracies. The maintainers relied on the number of landings and shutdowns recorded in maintenance releases, a method that could deviate from actual usage if the records were not perfectly accurate.