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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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.
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.*
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.
in addition to the fine answers already given, the relative location of a given item of wreckage can help determine when a part failed.
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.
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.
If jump off your roof and break your leg, the break point on the bone will look different than if you smash it with a hammer and then jump off the roof. Same for this. Investigators know what a bolt looks like when it breaks due to the forces of impacting terrain and what a bolt looks like if it breaks due to the force that caused it to shear mid-flight.
Beyond that, they can tell by the crack if it was a sudden break or the result of repeated stress over hundreds/thousands of flights as those two breaks will leave very different effects on the bolt.
The key thing will be inspection of the fracture surface of the bolt. This is literally the face left behind where the bolt snapped.
A bolt that snapped in a single overload event will have a very “clean” surface because the break is very recent (there hasn’t been time for it to corrode). Even if inspected much later, the corrosion and colour will be the same across the broken face.
A bolt that has fatigued will have a colour gradient across it – darkest where the crack originated, as that’s been exposed to the elements the longest, and then very clean at the bit which snapped off once the bolt was weakened.
A bolt that’s had some kind of stress-corrosion cracking or other chemical attack will look slightly different again.
Most failures like the one you’re describing will be from fatigue. A failure caused during a crash will be a single overload event.
They don’t find a broken bolt and say, “Oh, that’s what caused the crash.”
They’ll find something like a wing section that came off during flight immediately before the crash that would have been found farther from the rest of the debris (indicating that it came off and started falling out of the sky before the plane started falling). Then they would consider that part and how it was affixed to the aircraft, which would lead them to examine the fasteners or welds in the area where the parts would have been held together, and that’s how they find the broken bolt.
From their knowledge of the aircraft and mechanics and physics that would be able to piece together likely scenarios that might describe how that piece broke off. If the bolt was sheared off (impact) you might start looking for evidence of a collision with something in the air that could cause a bolt to break like that. If it’s fatigue, that will show up in testing.
So if you’ve got a critical bolt that is necessary for holding a part of the plane on and it fails due to fatigue, the investigators can piece that together from looking at the big picture, not just that one broken bolt.
One thing to remember is that all of the engineering and “forensic” evidence that’s been well explained by folks here doesn’t occur in isolation – there is a huge multidisciplinary team involved in investigating an airliner crash, smaller for a private aviation crash, but still more than one discipline. Police, forensic investigators, materials scientists and all sorts of folks make observations about the evidence they find and then work together to come up with theories. Smoking guns do exist, but it’s also likely someone didn’t pick up that bolt and say, “hey, this caused the crash.” Maybe a forensic anthropologist noticed an injury pattern on the passengers in one part of the plane, and some materials engineers also found metal shrapnel in the area that came from a part with a an alloy composition matching the wotzenhoffer thingamabob, and then they found the bolt that was supposed to keep the thingamabob secured to the hull and discovered that it had metal fatigue failures. All of those discoveries together support a theory that the bolt was the cause of the initial failure.
The reason planes are really safe is that the NTSB has zero sense of humor about crash investigations, and they’re really thorough.
Edit: a word correction
Well if you watch more of them you’ll realize it’s not a broken bolt that cause the crash… It’s a series of incidents that is often incredibly long and statistically almost impossible but from minor default to minor default it generated a catastrophic failure.
This shit was my jam a 3am for a long time, you might have gotten me back hooked on it.
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