CHI04IA178

The scheduled domestic air carrier flight received minor damage during landing rollout when the left main landing gear rotated. Metallurgical examination revealed the landing gear fractured at the base of the lug where preexisting fatigue cracks were present. The material composition and hardness were within specifications. The only nonconformity noted was the lug radii were smaller than drawing specifications.

On July 6, 2004, at 1010 central daylight time, a Beech 1900D, N87SK, operated by Skyway Airlines Inc., as flight 1851, received minor damage during landing roll when the left main landing gear rotated at General Mitchell International Airport (MKE), Milwaukee, Wisconsin. Visual meteorological conditions prevailed at the time of the incident. The 14 CFR Part 121 passenger flight was operating on an instrument rules flight plan. The 2 flight crew members and 17 passengers deplaned on the runway, and no injuries were reported. The airplane was then towed to the hanger. The flight originated from Gerald R. Ford International Airport, Grand Rapids, Michigan, at 1025 eastern daylight time, and en route to MKE.

The main landing gear socket assembly and piston (part number 114-810021-607) and scissors with upper and lower torque link (part number 101-810032-5) were examined by the National Transportation Safety Board's Materials Laboratory. A report of the examination is available within the docket of this report.

The socket assembly was fractured in two pieces at the base of the lug that connects the socket assembly to the scissors. The fracture propagated all around the base of the lug into the bolt hole that goes through the piston, and continued down on the right side to within approximately 0.5 inches of the axle. Both lower corners of the lug appeared to have the original contour though localized impact damage was present from contact with the scissors. The fracture initiated from pre-existing fatigue regions at both the left and right radii at the base of the lug. Both sleeves that insert inside the pinhole into the lug were relatively undamaged with no evidence of looseness in the joint.

Examination of the fracture surfaces revealed that at the right side of the lug two separate fatigue regions with multiple origins were observed. One of these regions was approximately 0.45 inch wide by 0.025 inch deep, while the other region was approximately 1.1 inch wide by 0.020 inch deep. The fatigue regions covered approximately 5% of the cross-sectional area at this location. On the left radius, a single larger fatigue region with multiple origins approximately 2.2 inch wide by 0.025 inch deep was observed. The fatigue region covered approximately 10% of the cross-sectional area at this location. All fatigue regions were in the lug base radius and propagated in a flat plane normal to the surface. Beyond the fatigue region the fracture surface was typical of overstress fracture.

Energy dispersive spectroscopy of the base metal revealed the base was consistent with the drawing requirement of 7075 aluminum.

The average macroscopic hardness of the lug was 88 Rockwell Hardness B Scale (HRB), above the required hardness of 78 HRB minimum for 7075-T73 per Aerospace Material Specification (AMS) 2658.

The average conductivity was measured on an as-cut surface as being 39.4% International Annealed Copper Standard, within the acceptable range of 38.0 - 43.0% per AMS.

The radius on the right side lug was measured using a Smart-Cope optical measuring machine in multiple locations. The radius ranged between 0.025 and 0.029 inch; the requirement was 0.120 inch (nominal).

The main landing gear assembly accumulated a total time of 7,362.3 hours and 9,167 cycles.

The Federal Aviation Administration and Skyway Airlines Inc. were parties to the investigation.