The flight data recorder (FDR) was an antiquated analog model which used metal foil strips scratched by moving pins. Here, it did not record vertical acceleration readings and misrecorded heading indications. One needle recorded some lines twice, initially confusing the investigators, leading the team to send the FDR to the American company which manufactured it. The manufacturer asked an ex-employee, the highest expert regarding the company's flight data recorders, to temporarily leave retirement to examine the recorder. The expert concluded that the needle supposed to have been recording the altitude had been shaking so much that it left other stray marks on the foil. This particular FDR recorded for hundreds of hours; further investigations found that the needle had been shaking abnormally for months. This told investigators that another component, not just the APU with the broken mount, had also been vibrating. The investigators charted the vibrations and found that two months before the crash, the vibrations stopped for two weeks, from immediately after the aircraft received a major overhaul in Canada by the airline's previous owner. During the Canadian company's test flights of the aircraft and its first several passenger flights for Partnair, the FDR recorded almost no abnormal vibrations. A review of the maintenance records of the aircraft revealed that during the overhaul, the mechanic discovered wear on one of the four bolts that held the vertical fin and fuselage together and replaced it. The vibrations stopped after the one bolt, with its associated sleeves, was replaced. When the vibrations later returned, they steadily increased until the accident flight.