Page:CAB Accident Report, Pan Am Flight 102.pdf/4

 helical fracture originated at this peen mark and ran completely around the truck beam to the left, terminating at the longitudinal crack. This peripheral fracture ended 1-1/2 inches forward of its point of origin.

At the forward end of the longitudinal crack, another instantaneous fracture ran helically to the right completely around the beam and beyond to a point on the bottom of the beam three inches to the rear of the front axle centerline. A third instantaneous fracture occurred at the rear of the longitudinal crack and ran peripherally to the left. The terminus of this fracture was ground off by the sliding action on the runway.

The left main gear lower torsion link assembly antirotation bolt was still in place but its nut was missing. Some threads were damaged and the end was slightly bent. The bolt end surface had two peened areas, each the same size and shape as the peen mark on the truck beam. The lower spacer was also peened on the edge of its lower surface.

In a metallographic examination conducted by the National Bureau of Standards numerous cracks were found, some of which appeared to be intergranular. Indications of intergranular cracking were also found on the main fracture, suggesting the possibility of hydrogen embrittlement contributed to the cause of the failure.

The microstructure was typical of quenched and tempered steel but in some areas the nonmetallic inclusions were more numerous than is considered normal for aircraft quality steel.

Hardness testing of the steel near the fracture produced a Rockwell number of 53C, showing that the beam had been heat-treated to the specified tensile strength range of 260,000 to 280,000 pounds per square inch.

The truck beam is considerably harder than the antirotation bolt and any contact between the two which would deform the truck beam would also damage the softer bolt.

During takeoff and landing the landing gear may be subjected to extreme and violent vertical oscillation over runway pavement at high speeds. In addition, it has been determined that unequal braking during the retraction cycle may also cause similar violent oscillations. Therefore, the landing gear truck is fitted with a "hydraulic shock absorber" snubber assembly. Its purpose is to damp these oscillations of the truck beam around its single pivot point at the lower end of the oleo strut. It also serves to limit the displacement at the truck beam to a maximum of 10 degrees above the 15 degrees below the horizontal. Without this limiting action by the snubber the truck beam is free to pivot to an angle at which it will contact the antirotation bolt in the lower torsion link assembly. Investigation disclosed that the position of the peen marks on the truck beam of N 707PA corresponded precisely with the point at which the beam would contact the antirotation bolt during over travel of the truck beam.

A review of the history of the landing gear on this aircraft revealed that a snubber failure had occurred on takeoff from Boston on July 7, 1959. After landing in New York, the snubber was replaced. The antirotation bolt was damaged but the nut which was missing was replaced. On July 9, 1959, at Paris, it was noted that the snubber terminal attach bolts were sheared. The were