SEA98LA158

Upon landing the pilot heard a bang and the right wing dropped. The airplane incurred substantial damage during a ground loop maneuver, following the separation of the right main wheel and lower strut end immediately after touchdown. Post-crash examination revealed that the lower end of the right main landing gear spring strut had broken off. Examination of the fracture surfaces revealed features characteristic of fatigue cracking in multiple locations. The fatigue cracks originated at the surface of the holes, at the edge between the wall of the hole and flat face of the leg. Separation of the spring strut was caused by fatigue cracks that originated from the surface of the two upper holes. No chamfer was found at the surfaces of the two upper holes. The surface of these holes contained for the most part a corner edge that is a stress raiser and, in turn, this stress raiser sensitized the hole to cracking. The two upper holes most likely were drilled oversize to remove anomalies such as corrosion or fretting damage. The diameter of the oversize holes was sufficient to have removed evidence of the chamfer. There was no airframe log information available which identified any maintenance on the spring strut.

On August 7, 1998, approximately 0915 mountain daylight time, a tailwheel equipped Cessna 182, N6437A, registered to and being flown by a private pilot, incurred substantial damage during a ground loop maneuver, following the separation of the right main wheel and lower strut end immediately after touchdown at the Johnson Creek airstrip, Yellowpine, Idaho. The pilot and two passengers were uninjured. Visual meteorological conditions prevailed and no flight plan had been filed. The flight, which was personal, was to have been operated under 14CFR91, and originated from McCall, Idaho, approximately 0830.

The pilot reported that after arriving in the Yellowpine area he executed "a normal approach for landing - - - touched down & rolled approx[imately] twenty or 50 feet when [he] heard a bang and the right wing dropped but did not come in contact with the runway."

Post-crash examination revealed that the lower end of the right main landing gear spring strut had broken off. The break was observed to pass through the two bolt holes at the top side of the strut end which attaches the wheel and brake assembly to the strut (refer to photograph 1). The lower end of the strut was found still attached by the two upper and two lower bolts to the wheel and brake assembly (refer to photograph 2). The lower end of the spring strut was removed from the wheel assembly and examined at the Safety Board's Office of Research and Engineering, Materials Lab.

Bench binocular microscope examination of the fracture revealed thumbnail features characteristic of fatigue cracking that stemmed from three separate locations. The fatigue cracks originated at the surface of the holes, at the edge between the wall of the hole and flat face of the leg. According to a representative from Cessna, a 45-degree chamfer is specified for the surface of the 4 holes. The edge of one of the holes that was intersected by a fatigue crack origin appeared to be slightly round. The edges of the holes that were intersected by the remaining two fatigue origins exhibited a right angle corner, and contained no chamfer. The fatigue cracks propagated away from the edge of the hole. The largest fatigue region propagated as much as 0.05 inch. Shear lips were noted on the edge of the fractures in areas outside of fatigue regions, and the fractures outside the fatigue regions contained features typical of overstress.

The corners between the upper holes and the face of the leg opposite from the fatigue initiation sites did not contain a chamfer. The corners of the two lower holes contained a chamfer on both faces. The same representative from Cessna indicated that the diameter of the upper holes should be 0.316 +/- 0.01 inch, whereas, the diameter of the lower two holes should be larger (0.377 +/- 0.01 inch). Although the fracture intersected the upper two holes, these two upper holes contained at least a half portion of a circumference that could be measured. The diameter of all four holes measured approximately 0.38 inch. These measurements indicated that the diameters of the upper holes were larger than specified and that the diameters of the lower holes were within specification limits (refer to attached Materials Laboratory Factual Report).

There was no airframe log information available which identified any maintenance on the spring strut.