FTW97LA041

A witness stated that the airplane 'flared smoothly and touched down with what appeared to be no side load.' As the aircraft settled onto the runway surface, the right main gear collapsed, and the aircraft subsequently skidded off the right side of the runway. During examination of the right main landing gear, it was discovered that the gear pivot assembly was fractured adjacent to the spline. It has not been determined as to whether the fracture occurred during gear retraction, extension, or upon contact with the runway. According to maintenance records, Cessna Service Bulletin, SE390.1, April 20, 1990, was applicable to this aircraft and was complied with. The Service Bulletin's purpose was to inspect main landing gear pivot assemblies for cracks in the spline area. The pivot assembly that failed, P/N 2441100-9, had a total time of 1,611.8 hours. Visual inspection of the fractured surface showed evidence of overload, however a small area on the outer portion of the surface had several small cracks bordering the edge.

On November 1, 1996, approximately 1100 central standard time, a Cessna 172RG, N9520B, was substantially damaged following a collapse of the right main landing gear during landing roll out, at Riverside Airport, Tulsa, Oklahoma. The private pilot was not injured. No flight plan was filed, and visual meteorological conditions prevailed for the Title 14 CFR Part 141 solo instructional flight.

In an interview with the investigator-in-charge the pilot reported that he was practicing stall maneuvers in a designated practice area adjacent to the airport. Upon completion of the maneuvers, he returned to the airport and set up for landing on runway 1L. After executing the pre-landing checklist, he noticed that the green landing gear down light was not illuminated. After an unsuccessful attempt to change light bulbs, he tried to pump the manual gear extension handle, but "it would not move." He was able to visually check the left main gear but could not see the right gear. After contemplating a go around, the pilot decided to land.

One witness, who was observing the airplane as it was on final, reported that he thought that the nose gear and left main was down and locked, but the right main did not appear to be in the locked position. Another witness, who was observing, reported that he thought that all three landing gears were down and locked. He further reported that the airplane "flared smoothly and touched down with what appeared to be no side load." As the aircraft settled onto the runway surface, the right main gear collapsed. Subsequently, the aircraft skidded off the right side of the runway. The right wing was structurally damaged.

Inspection of the aircraft after the accident revealed that the landing gear down light bulb was burned out and was not related to the gear collapse. During examination of the right main landing gear, it was discovered that the gear pivot assembly was fractured adjacent to the spline (illustration is attached to this report). It has not been determined as to whether the fracture occurred during gear retraction, extension, or upon contact with the runway.

According to maintenance records, Cessna Service Bulletin, SE390.1, April 20, 1990, was applicable to this aircraft and was complied with. The Service Bulletin's purpose was to inspect main landing gear pivot assemblies for cracks in the spline area. The pivot assembly that failed, P/N 2441100-9, had a total time of 1,611.8 hours. Visual inspection of the fractured surface showed evidence of overload, however a small area on the outer portion of the surface had several small cracks bordering the edge. The part was sent to the NTSB metallurgy laboratory in Washington, D.C, for further analysis

The metallurgist who examined the pivot assembly reported that a large portion of the fracture face showed torsional mechanical damage. A roughly circular area of undamaged features surrounded the drilled hole near the center of the shaft. The undamaged area had features typical of an overstress separation. The damaged area had features typical of rotational smearing and mechanical deformation. Mechanically damaged ratchet mark features covered an arc of approximately 100 degrees around the outside edge of the shaft. Small longituidinally oriented cracks emanated from most of the ratchets. Hardness and electrical conductivity measurements were consistent with the engineering drawing specified material (7171-T74 aluminum alloy) for the part.