ERA23LA327

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.

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.

On August 4, 2023, about 1930 eastern daylight time, an Air Tractor AT-502B, N5048S, was substantially damaged when it was involved in an accident near Leipsic, Ohio. The pilot was not injured. The flight was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. The pilot had been applying fungicide to several fields, during which he noted no engine performance anomalies. Shortly after starting the second spray run in a field, he 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 ground-looped in the high crops. Postaccident examination of the wreckage revealed that the airplane came to rest upright with substantial damage to the left wing. Borescope examination and partial engine disassembly revealed broken power turbine blades, guide vanes, and stator housing; additional examination of the engine revealed no visual anomalies on the exterior surfaces of the RGB front housing. The RGB second-stage carrier and planet gears were consistent in condition with their time in service. The examination revealed that the RGB second-stage ring gear was not assembled in accordance with the PWC technical publications. The ring gear retaining slot was located forward of the three retaining plates and could move freely in the axial plane within the RGB housing splines. None of the three second-stage ring gear retaining plates, nor their respective RGB housing mounting holes, showed signs of distress. Removal of the second-stage ring gear revealed distress over the full circumference of the front RGB housing mating splines that were consistent with the material loss of the spline teeth forward ends. Matching contact/wear marks were observed on the second-stage ring gear spline teeth ends. The second-stage ring gear evidence of contact with the RGB front housing surfaces adjacent to the housing splines, with those regions appearing to have been machined flat. The RGB first-stage sun gear and planet gear mating teeth showed evidence of distress and material loss. The propeller shaft assembly exhibited no evidence of distress. However, the propeller shaft retaining nut and key washer exhibited tool marks and localized damage. Tool marks were observed on the key washer swaged section that were not consistent with dedicated PWC tooling being used. The PT disc and the PT blades exhibited markings consistent with rotational contact on the upstream face. A portion of the balancing rim was deformed towards the disc center axis in a manner consistent with rotational contact. All of the PT blades exhibited damage and fractures at different lengths. The PT shroud ring was fractured into multiple pieces and all shroud blade contact surfaces exhibited signs of circumferential contact wear. The PT containment and reinforcement rings exhibited an oval shape indicative of two high-energy release areas. The PT vane and baffle exhibited rotational contact marks from contact with the upstream PT blades and PT disc. The PT vane was identified as a non-PWC approved repair. The bolts retaining the PT shaft housing to the RGB rear housing were all fractured. The PT shaft housing and the No. 3 bearing cover exhibited rotational contact damage. The PT shaft exhibited signs of heat discoloration and rotational scoring on the midsection surface. The No. 3 bearing flanged outer ring was fractured into two halves. The bearing inner ring raceway exhibited signs of overheating similar to the PT shaft in color. The No. 3 bearing cage was deformed in an oval shape. The bearing rollers, which were all released from the cage, were found in the No. 4 bearing housing region. The No. 4 bearing did not show any signs of distress. The engine was overhauled on June 20, 2013, by a repair station. The work order records detailing the overhaul were no longer available. The engine was installed on the airplane on March 27, 2014, at 7,784 hours engine total time. After the overhaul, the engine was inspected nine times during the airplane’s subsequent annual inspections. The most recent annual inspection was completed on May 1, 2023, at 8,274.2 hours engine total time and 490.2 hours since overhaul, (a Hobbs reading of 6,680.2 hours). The maintenance logbook entries did not note any specific maintenance related to the RGB second-stage ring gear or the propeller shaft assembly. There were no unresolved maintenance discrepancies noted. At the time of the accident, the Hobbs meter indicated 6,753.3 hours, 73.1 hours since last inspection, bringing the engine total time to 563.3 hours since major overhaul.