Event
On August 26, 2008, about 1315 central daylight time, a Schweizer G-164B, registration N3633S, operated by Howard Flying Service, Inc., was taxiing after landing at Fletcher Field Airport (CKM) in Clarksdale, Mississippi. The flight, an aerial application flight under 14 CFR Part 137, had originated from CKM at about 1243. Visual meteorological conditions prevailed.
The pilot reported that while taxiing, he applied the left brake and heard a loud pop from the left landing gear. Further brake applications were required to maintain directional control, and upon the third application, the left axle and wheel assembly separated. The airplane then veered left and momentarily nosed up before the tail wheel contacted the ground. Inspection revealed damage to the lower left wing spar.
Examination
A Federal Aviation Administration airworthiness inspector examined the aircraft. Two bolts (MS20008H22) and one screw (AN509-616-35) that secure the axle to the left spring leg gear were found fractured. These components were sent to the National Transportation Safety Board's Materials Laboratory for further analysis.
The laboratory examination showed that the fracture surface of the screw was damaged, preventing determination of failure mode, but the threads were deformed. Both bolts exhibited ratchet marks consistent with multiple fatigue crack initiation sites.
Maintenance History
The airplane was manufactured in 1987 and had accumulated 10,282.0 total flight hours at the time of the accident. The last annual inspection was performed on March 4, 2008, at 9,945.0 hours. The mechanic who conducted that inspection reported using his own checklist and did not perform a torque check on the axle attachment bolts as specified in the manufacturer's Annual/100 Hour Airframe Inspection Check List.
Damage
The aircraft sustained substantial damage. The pilot, a certificated commercial pilot, was not injured.