Accident Overview
On August 4, 2023, at approximately 1930 eastern daylight time, an Air Tractor AT-502B, registration N5048S, was substantially damaged during an accident near Leipsic, Ohio. The pilot, who was the sole occupant, was not injured. The flight was conducted under Title 14 Code of Federal Regulations Part 137 as an aerial application operation.
The pilot had been applying fungicide to multiple fields and reported no engine performance anomalies before the incident. Shortly after starting the second spray run in a field, he heard a "pop" and observed flames emitting from the engine exhaust ducts, followed by a total loss of engine power. He executed a forced landing to a field, during which the airplane ground-looped in the high crops.
Postaccident Examination
Postaccident examination of the wreckage revealed that the airplane came to rest upright with substantial damage to the left wing. A borescope inspection and partial engine disassembly disclosed broken power turbine blades, guide vanes, and stator housing. Further examination of the engine showed no visual anomalies on the exterior surfaces of the reduction gearbox (RGB) front housing. The RGB second-stage carrier and planet gears appeared consistent with their time in service.
The examination identified that the RGB second-stage ring gear was not assembled in accordance with Pratt & Whitney Canada (PWC) technical publications. The ring gear retaining slot was located forward of the three retaining plates, allowing the ring gear to move freely in the axial plane within the RGB housing splines. None of the three second-stage ring gear retaining plates or 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, consistent with material loss at the forward ends of the spline teeth. Matching contact and wear marks were observed on the second-stage ring gear spline teeth ends. Evidence of contact between the second-stage ring gear and the RGB front housing surfaces adjacent to the housing splines was noted, with those regions appearing 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 had tool marks and localized damage. Tool marks on the key washer swaged section were not consistent with the use of dedicated PWC tooling.
Engine Findings
The power turbine (PT) disc and blades exhibited markings consistent with rotational contact on the upstream face. A portion of the balancing rim was deformed toward the disc center axis in a manner consistent with rotational contact. All PT blades were damaged and fractured at different lengths. The PT shroud ring was fractured into multiple pieces, and all shroud blade contact surfaces showed circumferential contact wear. The PT containment and reinforcement rings had 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 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 showed signs of heat discoloration and rotational scoring on the midsection surface. The No. 3 bearing flanged outer ring was fractured into two halves, and its inner ring raceway exhibited signs of overheating similar to the PT shaft in color. The No. 3 bearing cage was deformed into an oval shape, and all bearing rollers were released from the cage and found in the No. 4 bearing housing region. The No. 4 bearing showed no signs of distress.
Maintenance History
The engine was overhauled on June 20, 2013, by a repair station; however, 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 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 (based on 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. No unresolved maintenance discrepancies were recorded.
At the time of the accident, the Hobbs meter indicated 6,753.3 hours, 73.1 hours since the last inspection, bringing the engine total time to 563.3 hours since major overhaul.