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?
In: 71
If the bolt broke because of metal fatigue, it will have distinct wear marks and you can see it slowly giving way over time. This tends to be the most common cause of a metal part failing in flight. Unless the plane was hit by something especially out of the ordinary (eg: severe winds or whatever) which should have caught the eyes of the investigators early as a cause or factor in the crash.
If it happened at the time of the crash, it would show a different tear pattern as it was ripped off through raw force in the crash.
There are also other factors to consider. Once a bolt has failed, it may cause other effects on what it was holding onto. For example, if an engine bolt broke, the engine might be sagging a bit loose now, wobbling around and bending metal as it does so. That might be visible as its own damage. So combined with a bolt that was ripped off, the conclusion is the bolt came off first, then the engine start flopping around causing this other damage.
All this is generic without seeing the episode and knowing more.
There is absolutely nothing wrong with the theories that others have suggested to you. I have another,
Part of the reason that it is obvious that the bolt must have come off during the flight could be that
a) the bolt is super-important and the craft cannot operate properly without it and the most likely chain of events if that bolt falls off IS a crash.
*as in, something quite important fails or falls off if the bolt fails.*
or b) the bolt was found somewhere where it wouldn’t have been after the crash, because other data speaks against it,
*example: the bolt was found INSIDE an engine, obviously making quite a lot of a mess in there that would only be possible if the engine was turned on, which it wasn’t when the craft went down; there is video that shows that it was doing a telling smoke plume instead of producing a jet stream when the craft was headed groundwards.*
in addition to the fine answers already given, the relative location of a given item of wreckage can help determine when a part failed.
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for example, if the item and its bolts were found some distance from “main” crash site in the direction the plane came form, then that would suggest it came off it during flight and fell short while the rest of the plane carried on at little futher.
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other elements can include data from the “black boxes” which show a failure in a given system , or the pilots talking about what they can see (“THE WHOLE BLOODY ENGINE IS GONE!”), or external witnesses that saw the part seperate.
Is it the flight that crashed into NY and the copilot rocked the rudder back and forth causing the vertical stabilizer to sheer off? We had a macabre joke at an old job about that.
There are a couple of ways, like if the bolt is found far away from the crash site, that would suggest that the bolt broke before the plane hit the ground. If the component is relatively intact after the crash, but the bolt is broken, it probably happened pre-crash.
An example of above is Alaska 261. They were able to retrieve the components from the tail section and were able to see that the components failed in a certain way that indicated consistent wear. Like, the threads on the nut of the bolt were shorn down almost evenly and the lubrication was missing, that is something that happened before the crash.
Without knowing the specific incident being mentioned, it’s probably a case where a single point of failure somehow caused/led to a loss of some flight control, or some important instrument.
That loss of information or additional workload could contribute to a loss of control of the plane.