Introduction
On March 4, 2023, at approximately 1100 central standard time, a Stinson 108-3 airplane sustained substantial damage during a forced landing on a bridge near Lampasas, Texas. The pilot, who was the sole occupant, was not injured. The flight was conducted as a personal flight under 14 CFR Part 91.
Accident Sequence
According to the pilot, the flight departed Air Park-Dallas Airport (F69) in Dallas, Texas, destined for Kestrel Airpark (1T7) in San Antonio, Texas. While en route, the pilot switched the fuel selector from the right tank to the left tank. Approximately two to three minutes later, the engine experienced a total loss of power. The pilot immediately switched back to the left tank, but power was not restored. The pilot then selected a bridge for an emergency landing. As the airplane neared the bridge, it struck a cable positioned across the bridge threshold, causing the airplane to nose over. The airplane came to rest on the bridge, and the rudder sustained substantial damage.
Postaccident Examination
A postaccident airframe examination revealed no mechanical malfunctions or failures that would have precluded normal operation. The engine was examined and found to lack compression or vacuum in some cylinders, and the camshaft did not rotate when the engine was turned by hand. Further inspection revealed that the crankshaft timing gear had damage to all teeth, with insufficient tooth material remaining to mesh with the camshaft gear. The camshaft gear itself was undamaged. The engine oil was clean and not dark, and the engine rotated freely. There was no visual evidence of overheating.
Three liberated gear teeth were recovered from the oil sump. The teeth and the crankshaft timing gear were sent to the National Transportation Safety Board Materials Laboratory for examination. The accident gear was compared to an exemplar gear. Besides the three teeth that had fractured and separated, approximately 60 to 90 percent of the remaining gear teeth had been deformed and flattened.
Metallurgical Analysis
Scanning electron microscope examination of a fractured tooth revealed fatigue striations in the undamaged area, consistent with fatigue crack propagation. No pits or inclusions were found at the fatigue initiation site. The opposite long edge of the fractured tooth exhibited ratchet marks, indicative of multiple crack initiation sites. An area outside these smaller cracks showed dimpled rupture, consistent with an overstress fracture of the final cross-section at the end of fatigue crack propagation.
The primary fatigue crack initiation site on the No. 1 liberated tooth was located along the fillet radius on the driven side of the tooth. The area outside the initiation site was smeared, but there was relatively little damage on the adjacent radius. No gouging, corrosion, or plastic deformation was found near the initiation site.
Cross-sections of the exemplar gear, worn accident gear teeth, and the No. 2 liberated tooth were analyzed metallographically. Hardness testing revealed that the surface of all three samples was harder than the core, but this difference was most pronounced in the exemplar gear. The exemplar tooth exhibited a high hardness above 513 HV500 (50 HRC) to a depth of 0.04 inches, consistent with a case hardening surface treatment. In contrast, both the accident gear and the liberated tooth had surface hardnesses of approximately 340 HV500 (35 HRC), which did not meet the minimum for case hardening. The core hardness of the accident teeth averaged around 195 HV500 (92.5 HRB), while the exemplar gear core averaged 390 HV500 (40 HRC).
Maintenance Records Review
A review of the maintenance records showed that the engine had been overhauled on September 1, 2020, at a tachometer time of 2316.89 hours. During the overhaul, all ferrous parts, including the crankshaft timing gear, were non-destructive tested (NDT) using a wet-method magnetic particle testing machine by a level II NDT technician. The crankshaft timing gear was not replaced at that time. According to the accountable manager for the overhaul facility, the wet-method magnetic particle test can identify surface and subsurface cracks, and crankshaft timing gears are included in the ferrous parts inspected. The last annual inspection occurred on November 10, 2022, at a tachometer time of 2366.53 hours. At the time of the accident, the tachometer read 2374.89 hours, meaning the engine had accumulated only 58 hours since overhaul. A review of the engine maintenance logbook dating back to 1963 did not show a replacement of the crankshaft timing gear, consistent with the accident gear being the original.