Air Tractor AT502 N5048S accident near Leipsic, Ohio

Probable cause

The incorrect assembly of the turbine engine’s reduction gear box second-stage ring gear, which resulted in an internal failure of multiple engine components and a total loss of power.

NTSB analysis

During the first aerial application flight of the day, the pilot noted no engine performance anomalies. After starting the second flight over the intended field, the pilot heard a “pop” and observed flames emitting from the engine exhaust ducts; then the engine sustained a total loss of power. He performed a forced landing to a field and the airplane sustained substantial damage to the left wing.

About 8½ years before the accident, the engine was overhauled and reinstalled on the airplane. The records for the engine overhaul work order were no longer available. Since the overhaul, the airplane and engine received nine annual inspections and had accumulated 563.3 hours of service.

Postaccident examination of the engine revealed that the reduction gear box (RGB) second-stage ring gear was assembled incorrectly in front of the retaining plates instead of the retaining plates being fixed within the dedicated slot on the ring gear. The incorrect assembly allowed the second-stage ring gear to migrate forward into the RGB front housing. This allowed reduction in the second-stage ring gear axial meshing spline engagement, to the point where the remaining contact surface could no longer sustain the ring gear load. The small overlapping region sheared off circumferentially, allowing the ring gear to spin freely and resulting in the decoupling of the power turbine (PT) disc from the propeller shaft. The reduced support by the second-stage ring gear resulted in an increase in movement of the second-stage carrier assembly. That movement then transmitted to the RGB first-stage carrier sun gear, where damage was observed on the mating teeth; the movement also resulted in excessive loads on the No. 3 bearing outer ring that resulted in its fracture. The loss of support from the No. 3 roller bearing resulted in an imbalance of the PT disc assembly that resulted in the blades fracturing. The PT vane previously had a repair performed that was not approved by the manufacturer; however, it likely did not influence the initial engine malfunction.

Text from the NTSB. Open the NTSB docket for ERA23LA327 for the full record.