N6532MStinson 108 · 1948

On March 4, 2023, about 1100 central standard time, a Stinson 108-3 airplane was substantially damaged when it was involved in an accident near Lampasas, Texas. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot reported that he departed the Air Park-Dallas Airport (F69) Dallas, Texas, and was destined for the Kestrel Airpark (1T7) San Antonio, Texas. While enroute, he switched tanks from the right tank to the left tank. About two or three minutes later, the engine lost total power. The pilot switched back to the left tank but power was not restored. The pilot selected a bridge to land on. When the airplane neared the bridge, it contacted a cable that was placed across the threshold of the bridge and the airplane nosed over. The airplane came to rest on the bridge and sustained substantial damage to the rudder.

Postaccident examination of the airframe revealed no mechanical malfunctions or failures that would have precluded normal operation. Examination of the engine revealed no compression or vacuum in some of the cylinders and no rotation of the camshaft when the engine was rotated by hand. Further examination found the crankshaft timing gear exhibited damage to all teeth, with insufficient teeth material remaining to mesh with the camshaft gear. (See Figure 1.) The camshaft gear was not damaged.

Figure 1. Image of the engine showing the damaged crankshaft timing gear.

The oil was clean and not dark and the engine rotated freely. There was no visual evidence of engine overheating.

Three liberated gear teeth were recovered from the oil sump. The teeth and crankshaft timing gear were sent to the National Transportation Safety Board Materials Laboratory for examination. Figures 1 and 2 show the accident gear alongside an exemplar gear.

Figure 2. The accident gear (left) and an exemplar gear (right).

Besides the three teeth that had fractured and liberated, about 60 to 90 percent of the remaining gear teeth had been deformed and flattened. (See Figure 3.)

Figure 3. Side view of the accident gear (bottom) and the exemplar gear (top). The location of the three liberated teeth is identified with yellow numbers.

Examination of a fractured tooth using a scanning electron microscope revealed fatigue striations in the undamaged area, consistent with fatigue crack propagation. There were no indications of pits or inclusions at the fatigue initiation site. The opposite long edge of the fractured tooth exhibited ratchet marks, consistent with multiple crack initiation sites. The area just outside these multiple smaller cracks between the main fatigue crack displayed dimpled rupture. This area was consistent with overstress fracture of the final cross-section of the gear tooth at the end of the fatigue crack propagation.

The edge of the primary fatigue crack initiation site of the No. 1 liberated tooth revealed the lower flank and fillet radius of the driven side of the tooth, with the crack initiation site located along the fillet radius on that side. While the area outside the crack initiation site had been smeared, there was relatively little damage on the radius adjacent to the fracture surface edge. No indications of gouging, corrosion, or plastic deformation were located adjacent to the crack initiation site.

Cross-sections of the exemplar gear, worn accident gear teeth, and the No. 2 liberated accident gear tooth were mounted, polished, and etched with a 2% nital solution for metallographic examination. Examination revealed a difference in the case hardening values (HV).(See Figure 4.)

Figure 4. Chart of hardness (in HV500) changes based on depth from the surface of the accident gear, a liberated tooth from the accident gear, and the exemplar gear.

For all three, the surface exhibited a higher hardness than the core—however, this was most pronounced in the exemplar gear. The exemplar tooth exhibited a high hardness above 513 HV500 (50 HRC) for a depth of 0.04 inches—these hardness data were consistent with and typical of a case hardening surface treatment. In contrast, both the accident gear and liberated tooth exhibited hardnesses of 340 HV500 (35 HRC) near the surface. Neither of these hardness levels met the minimum value for case hardening. The core hardness of both accident teeth averaged around 195 HV500 (92.5 HRB), whereas the core hardness of the exemplar gear averaged 390 HV500 (40 HRC).

A review of the maintenance records revealed that the engine underwent an overhaul on September 1, 2020, at a tachometer time of 2316.89 hours. During the overhaul, all ferrous parts, which would have included the crankshaft timing gear, were non-destructive tested (NDT) with 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 identifies indications of surface and subsurface cracks. He further stated that crankshaft timing gears are included in the ferrous parts inspected. The last annual inspection of the engine occurred on November 10, 2022, at a tachometer time of 2366.53 hours. The tachometer time at the time of the accident was 2374.89, or 58 hours since overhaul. A review of the engine maintenance logbook going back to 1963 did not show a replacement of the crankshaft timing gear, consistent with the accident gear being the original gear for the engine.