Metallurgist Terry Heaslip of the Canadian company Accident Investigation and Research Inc. examined the aircraft skin from the tail and found signs of overheating, specifically that the skin had been repeatedly flexed, through a phenomenon known as flutter. This caused investigators to further scrutinize the tail of the aircraft. Furthermore, the investigation team found that the auxiliary power unit (APU), which was in the tail, generated heat which melted certain plastic parts, indicating that the APU was operating during the flight even though it normally would not be. The mechanic who had inspected the aircraft on the day of the accident flight told the investigators that one of the aircraft's two main generators had failed and that he was not able to repair the faulty generator. The investigators discovered that the pilots had noted in the flight log that they would operate the APU throughout the flight, because two power sources are required for flight. They also discovered that the APU's front mount was broken, which allowed it to vibrate excessively.