After investigators recovered all four bolts, sleeves, and pins, they found that the bolt and parts installed by the Canadian firm were properly approved equipment, but the other three bolts and their parts were counterfeit and were incorrectly heat-treated during manufacture. Those bolts each could bear only about 60% of their intended breaking strength, making them less than practical to use on the aircraft. The fake bolts and sleeves wore down excessively, causing the tail to vibrate for 16 completed flights and the accident flight.