MIA08LA174

While applying the left brake during taxi after landing, the pilot first heard a "loud pop from the left gear" and during further brake application, the left axle and wheel assembly separated. Two MS20008H22 bolts and one AN509-616-35 screw that secure the axle to the spring leg were fractured. The failure mode of the bolts was due to fatigue, and was typical of bolts that are insufficiently torqued. The mechanic who last inspected the airplane used his own checklist and did not torque check the bolts and screw, which is required by the manufacturer's Annual/100-Hour Airframe Inspection Checklist.

On August 26, 2008, about 1315 central daylight time, a Schweizer Aircraft Corporation G-164B, N3633S, registered to Howard Flying Service, operated by Howard Flying Service, Inc., experienced separation of the left main landing gear axle and wheel assembly while taxiing after landing at Fletcher Field Airport (CKM), Clarksdale, Mississippi. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 Code of Federal Regulations (CFR) Part 137 aerial application flight from CKM. The airplane was substantially damaged and the certificated commercial pilot was not injured. The flight originated about 1243, from CKM. The pilot stated that while taxiing after landing, he applied the left brake and heard a "loud pop from the left gear." Further application of the left brake was required to keep the aircraft going straight and when he applied the left brake for the third time, the left axle and wheel assembly separated. The airplane then veered to the left and nosed up momentarily before the tail wheel contacted the ground. Damage to the lower left wing spar was noted. Examination of the airplane by a Federal Aviation Administration (FAA) airworthiness inspector revealed that two MS20008H22 bolts and one AN509-616-35 screw that secures the axle to the left spring leg gear were fractured. The bolts and screw were retained for further examination by the National Transportation Safety Board (Safety Board's) Materials Laboratory. Examination of the fractured bolts and screw by the Safety Board's Materials Laboratory revealed the fracture surface of the screw was damaged which precluded determination of failure mode; however, the threads were deformed. Examination of both bolts revealed ratchet marks consistent with multiple fatigue crack initiation sites. The airplane was manufactured in 1987, and at the time of the accident had accrued 10,282.0 hours. The last annual inspection was performed on March 4, 2008, at airplane total time 9,945.0 hours. The mechanic who performed the last annual inspection stated that he used his own checklist and did not torque check the axle attachment bolts as specified in the manufacturer's Annual/100 Hour Airframe Inspection Check List.