Incident Description
A Boeing 727, dispatched with its auxiliary power unit (APU) inoperative, underwent a pushback operation conducted by contract personnel. The captain had elected to start all engines prior to pushback due to a short taxi. After pushback, during a forward tow, a shear pin on the tow bar failed, and the aircraft rolled into the tug.
Examination Findings
Subsequent examination of the tow bar revealed looseness in its head and worn holes for the shear pins. Metallurgical examination of the failed shear pins disclosed pre-existing cracking and that the pins were softer than specified. The failed tow bar had been inspected daily by the contract personnel, with no problems noted, and it continued in use.
Maintenance and Training Issues
A fleet-wide campaign found that 63 percent of the airline's tow bars required maintenance. The investigation revealed that the airline did not have a preventative maintenance program for its tow bars. Additionally, there was no program to train contract personnel in the inspections of tow bars or to monitor their inspections.