Air Tractor AT502 accident near Leipsic, Ohio, August 4, 2023

The NTSB's narrative final · quoted from the NTSB record

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.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.

Photographs from the investigation 81 pictures from the NTSB's docket, as the NTSB released them

Photograph 1 – View of airplane from leftPhotograph 2 – View of outboard left wingPhotograph 3 – View of airplane from rightAs received Friday, February 14, 2024 8:34 AM WO 23-1215 reviewedCasing split on c flange. Combustion can liner found missing from shipmentGroup picture of all parts receivedExhaust Friday, February 2, 2024 8:44 AM Dimple damages from departing PT bladesPT shaft forward lookingFOD in exhaust Combustion can Friday, February 2, 2024 8:45 AM Combustion liner missing in shipmentOil Filter Wednesday, February 14, 2024 4:48 PMFCU Friday, February 14, 2024 8:45 AMFuel pump Friday, February 14, 2024 8:46 AMPt disk and blades February 14, 2024 9:05 AMCasting number on PT bladesPT wheel part numberPT Wheel deformationPT wheel SNBlade part number AE 3013102T. All blades will need to be removed from disk, analyzed and PN verified5 blades were removed to confirm part numbers. Blades were identified.PT shroud February 14, 2024 9:15 AM PT shroudDeparting blades impact marks Ovalized shape of PT shroudInterstage baffle February 14, 2024 9:33 AM Stator vanes between CT and PT seen from trailing edge with damages on PT sideInterstage baffle inner part numberLeading edge’s view (center part show evidence of rotational signature with CT disk due to rubbing evidence)STI 72-50-4802 ROO part number therefore not PWC numberSTI part numberCT February 14, 2024 9:51 AM CT wheel Trailing edge looking aftCT blades were turned with starter gen outlet and free to spin no rubbing or noises. Back of CT blades show evidence of rubbing on inter baffle section vaneRGB chip plug February 14, 2024 12:29 PM RGB chip plug with metalT5 housing February 14, 2024 12:32 PM T5 casing with damagesRGB aft February 14, 2024 12:39 PM Second RGB stage gears First stage gears with debrisDebris at the bottom of RGB rear casing (hold first stage gears)First stage planet gear with damagesAll bolts on gear RGB rear case found sheared and #3 roller bearing at bottom (please do not mind the light…)Bearing 3 roller found loose at bottom of case. Bearing #3 race attachment bolts found sheared with part found at bottom of the case. Bearing #4 race bolts were found secured but damaged (bent)PT Shaft with damageBottom of RGB rear casing with debrisExhaust Casing liner found offsetRGB strainer with debrisRGB rear case all bolts found shearedPT Shaft between bearing 3 and 4 found with heat damage evidence and impact damage most likely from bearing cage spinning around after release of the rollersBearing #3 race found in 2 separate parts no evident signs of scraping or overheat on the race All 4 bolts holding the race found shearedBearing roller found with heat damage and rubbing2 bearing roller indicated signs of rubbing and overheat signaturesPT shaft overheat signature.First stage planetary gears balancing (metal removed from triangular plate)All sun/planetary gears turned without issues. Material missing is part of balancing process but need verification from PWC Two imprints inside tube FWD of bearing #4Imprint inside tube that enclose shaft forward to 3-4 showed an exact imprint of a roller bearing. Most likely one roller got stuck, rubbed and overheat. Shaft shows also sign of rubbingScoring/rubbing on PT shaftMost likely this roller was stuck between shaft and tubeDamages found on first stage planetary gearsTorque plate scoring. No visible damage found on ring gearBearing 3 race and found roller (One roller was not found). #3 bearing race did not exhibit sign of overheat or heavy scoringBearing #3 rollersPower turbine shaft cap between PT shaft housing and PT wheel exhibit evidence of crushing and torsional forceTip of power turbine shaft casing between bearing 3 and PT wheelRGB forward February 15, 2024 8:04 AM Forward RGB behind looking forward. The part number and serial number visible on the ring gear (5 o'clock) is indicative that the ring gear is installed the right way. Note that the 3 tabs are not engagOil nozzle with damagesInner core with damagesPlanetary gear second stageSecond stage planetary gear with damagesBearing 5 no visible issues. Note Ring gear not installed correctly (tab no in grooves)Bottom FWD RGB casing ring gear insert damaged Damage on RGB ring gear most likely due to ring gear migrating FWD.For demonstration ONLY. Ring gear correctly installed in groove to prevent axial playFor demonstration ONLY- Ring gear correctly installed in groove to prevent axial play. With ring installed incorrectly the ring would migrate forward until such point that the planetary gear would have spun the ring gear and damage the casiFor demonstration only- Ring gear correctly installed in groove to prevent axial playWhen the gear went forward it wear the bottom casing ring gear. Damage on the ring gear is indicative of thatRing gear part number. PN and SN were visible and therefore the groove on the ring gear was installed the right way (aft)Wear on top of brackets may indicate it was installed properly at some point during the life of the ring gear/tabs

The NTSB's photographs with the NTSB's captions, at screen size. Open a picture to see it full size or to report one that should not be shown.