Just watched an Air Crash Investigation episode in which the investigators studying the wreckage say that a certain bolt shearing off during flight caused the crash. How can they tell that the broken bolt was during flight and not because of the crash?
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In maintenance engineering there’s a huge amount of people dedicated to looking at why things break and how it occurred- any company that uses heavy equipment will have a section of the business to do this, or a supplier to provide the service. They get enough practice to spot how things fail and work out why.
When a part fails, it fails from the weakest point – a whole wing doesn’t just fall off, it falls because the bolts holding it break. The way it breaks tells you a lot about the event – bolts that stretch and fail will look different from bolts that are instantly overloaded and shear at the head, because one will have a fracture in the shaft and one will be a clean shear at the head. There’s so many examples in mechanical engineering that it’s often easy to understand the mechanism of failure. Other surrounding parts will have wear or damage that will support the investigation.
If you look at the crash scenario, you can tell a difference between parts that were destroyed by the impact because they will have impact damage – upper surfaces may be less damaged than lower ones, or there might be dirt and debris in the areas that hit the ground first, for example. Parts that failed earlier- like bolts – will likely survive the crash damage because their weakest part has already failed, and will either be housed within a part that will take the impact, or the bolt will be so solid it won’t have much damage anyway. If you drop a solid steel bolt from height, you’ll get damage on the small section that had the impact, but the rest is solid steel and will often survive just fine. The head is often a failure point but the shaft will survive a hefty impact because it harder than the surface it impacts.
Aviation parts are made specifically and metalurgically to withstand high stresses, so they don’t take damage they way a lower grade of steel will. So you can then expect the bolts to perform a certain way, and determine when they likely failed because you will usually know the speed of the crash and the mass of the plane. All this is used to determine, through failure tracing, where the issue started.
I believe in the case of AA191 they found parts of the broken thrust bushing bolt on or near the runway associated with debris from the detached #1 engine some distance from the crash site. This indicated that the bolt was broken and separated from the aircraft before the aircraft impacted the ground.
Because evidence.
You are classifying all types of broken bolts as the same when in fact they are not the same.
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