Use of Metallurgical Analysis to Pinpoint a Brake Problem and the Cause of a Bus Fire

The incident of concern involves a tour bus that caught fire. The fire took place while the bus was traveling at highway speeds. Bus fires can be catastrophic, consuming the entire vehicle and threatening the lives of passengers. In this case, the fire was detected soon enough to allow the bus to be evacuated. Further, the fire was extinguished in sufficient time to allow the origin of the fire to be traced by burn patterns to the brake assembly at the passenger’s side rear axle. With the origin established, it remained consequential to determine the cause. It was here that metallurgical analysis came into play. Two competing theories as to the cause were presented. One theory was that heat generated by a dragging brake ignited the adjacent tire. The dragging brake, in turn, would have been the result of a leak in the air line servicing the brake. The second theory was that an underinflated tire had overheated and ignited. To prove if either theory was correct, the brake drum was subjected to metallurgical examination. It so happened that the changes in microstructure of the drum material as a function of location and of temperature exposure verified that the dragging brake theory was correct. It is also important to realize that a correct and well-documented analysis provided knowledge as to the cause of the fire and is one step in preventing another such fire.