was broken, which allowed it to vibrate excessively. The two shroud doors on the aircraft tail were not present in the recovery. They were constructed with an aluminium honeycomb liner, and aluminium's reflective properties allowed the doors to appear on radar when floating free. This led the AIBN to conclude that the unidentified objects tracked at high altitude by Swedish radar for 38 minutes were likely the shroud doors, which had separated from the aircraft tail. From this, the AIBN found that the tail failed at . If the rudder moved in a violent manner, the weights behind the doors would also move violently and hit the shroud doors. Therefore, the rudder had made a violent movement as the accident unfolded. Partnair suggested that the F-16 fighter jet had been flying at a faster velocity and closer to the Convair than reported in the media, reaching supersonic speed and creating a pressure wave that could have caused the Convair to disintegrate in midair. The National Aeronautical Research Institute, a Swedish aviation technology research facility, said that a 60% chance existed of this being the cause. The F-16 pilot testified that his aircraft was more than above the Convair. The investigators concluded that the jet would have had to have been within a few metres of the Convair to have affected the passenger aircraft in such a manner, and had found no evidence that the two aircraft had been so close. The AIBN investigation found no connection between the F-16 and the accident. The Convair’s flight data recorder (FDR) was an antiquated analog model that 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 that manufactured it. The manufacturer asked an ex-employee, the highest expert regarding the company's FDRs, 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