19980525036029I

(-5) As take off roll was initiated, a shimmy developed and the captain aborted the takeoff and exited the runway. Aircraft sustained major damage to upper and lower torque links, nose wheel tires, and lower TCAS antenna. Skidmarks from nose tires as well as aircraft debris were on the runway. Investigation revealed that service bulletin SB737 32-1129 rev 3 2/9/95 was not complied with. Summary of investigation of the vanguard airlines flight 424 incident at mdw 5/25/98. On 5/25/98, a B737 operated by vanguard airlines, aborted take-off roll on runway 31C at mdw airport in Chicago, Illinois due to an increasing nose wheel shimmy. The pilots slowed the aircraft but had difficulty turning off the runway. There were no injuries reported. The aircraft had to be towed back to the gate. Although the aircraft never exceeded 35 knots, it left a trail of damaged parts and skidmarks from the beginning of the take-off roll. There was no trail found from the gate to where the aircraft was positioned on the runway for initial take-off roll. Subsequent investigation of the nose wheel shimmy during take-off was the result of the lower torque link separating from the aircraft causing an immediate loss of steering control and directional stability. As the nose wheel gyrated, pieces of the fractured lower torque link punctured the LH nose tire increasing the vibration and skidding. Damage to the belly of the aircraft was limited to the lower TCAS antenna which appeared to have been damaged by one of the parts departing from off the lower torque link. Photos taken during the initial on site investigation in addition to runway sketches documented, in detail, the locations of scattered aircraft parts. This information will be kept on file at this office. Final analysis by Boeing, of the parts that were recovered will be made available to the pmi in late August. Initial findings by this office suggest that the separation of the torque link may have been caused by a suspected migration of the torsion link pin and shaft due to corrosion and cracking originating at the antirotation bolts. Boeing has been aware of this type of problem which prompted the first issue of a service bulletin in 1983. The most recent service bulletin sb 737 32-1129, rev 3 dated 2/9/95, called for the installation of a modified torsion link pin. No compliance time was given in the service bulletinat the time of the incident, vanguard airlines had not complied with the service bulletin. However, during the course of repairs, the maintenance log indicated that the service bulletin was performed and had now been complied with. The NTSB noted that since there were no injuries and no emergency evacuation was ordered, their participation in the investigation would be limited and allowed the FAA to continue with the investigation. Since no violation occured, this office considers this matter closed. This summary was generated by inspectors sam latorre and clem corpus. Additional information can be found in ptrs records GL319809604 and GL319810279.