When a plane crash occurs, usually there are hundreds of bits of plane shattered everywhere. I watch shows like “Air Disasters”, but they never explain how they know the “right wing flap came apart mid air” or know how the “fan blade dislodged on impact and not in the air”?
In: Engineering
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It has to do with the way an object is damaged.
An engine operating at high power settings at time of impact will have dirt and debris much deeper into the core than one that failed in flight.
A plane that breaks up in flight will have a larger debris field than one that impacted intact. Likewise a shallow impact makes a bigger field than a vertical impact.
When something falls off in flight it won’t be in the right spot in the debris field compared to parts that were still attached at impact.
Analog instruments will have an impact mark where the needle was at time of impact. Digital instruments are typically recorded so the last recorded value tells a similar story.
Metal that breaks from repeated fatigue has a distinct microscopic pattern that is distinct from metal which breaks due to sudden impact forces.
There are many more but you get the idea.
One thing they talk about on those shows is looking for the four corners of the aircraft— the nose, both wingtips and the tail. If they find something on the ground 4 miles away from where the four corners are then they would know that it fell off mid air.
Additional information comes from the black box which records all kinds of information, including the pilot control inputs and readings from the various instruments in the plane. If in the black box data, the instruments all show the plane suddenly rolling over despite the pilots inputting roll in the opposite direction, it means there must have been something with the control surface or wings. Since there’s a flap on the ground several miles away from the crash, that’s a good indication that the flap failed, changing the lift from that wing and causing a roll in that direction.
They can also pull up the recordings from the pilots who won’t know exactly what’s going on, but they will at least discuss the fact that they’re trying to roll in a direction but the plane isn’t doing what they want. Loud noises can often be heard on the recording, like the banging of an engine exploding, or maybe the screw jack that holds the flaps in place snaps.
So, they put it all together: there’s a loud bang in the voice recorder, then the gyroscope shows the plane rolling hard in that direction. The pilots yank on the controls in the other direction, but the plane won’t stabilize. The pilots call out to each other what they’re experiencing, verbalizing that they’re fighting the controls but the plane isn’t behaving. On the ground, people carefully label every single piece from the wreckage and figure out exactly what it is (or was). The flap is missing, and as they comb back along the flight path, they find it miles back.
Or, basically the same sequence but with engine noises and engine power feeds and the pilots talking about losing power in engine #whatever…
Airplanes have a lot of data that’s constantly recorded and is stored in a very hard to destroy device called a flight data recorder. A recording of the last 2 hours of pilot’s voices and other sounds heard in the cockpit are stored in another really hard to destroy device called a cockpit voice recorder. Those two devices are fire resistant, impact resistant, water resistant, painted bright orange, and may or may not be combined in a single device, and are normally referred to as the black boxes.
Using that data, along with detailed simulators, radar coverage recordings from air traffic control, the plans for the plane and all of its parts, expected flight performance details and even the maintenance records of the plane, investigators can recreate how the plane was flying before the incident. They can tell if some system was running slow or if something wasn’t getting enough power or even if there’s a large dent on a wing, because the plane wouldn’t fly like it’s supposed to. They know how a plane is supposed to fly given a certain set of conditions because there are hundreds of other planes with the exact same design flying constantly, maintenance is very controlled and everything is designed to be consistent between individual planes. If something in the data recorders doesn’t match expectations they can find it.
The investigators can also hear what the pilots said in reaction to whatever happens, they’re trained to handle a lot of emergencies with a plane, so their reactions can tell investigators what happened a lot of times. Or maybe hear a loud explosion or rattle on the voice recorder to give them a hint of what to investigate.
Finally they can look at the actual crash debris. They map exactly where each piece is found and then try to reassemble the plane with all of the pieces they find. If there’s some fan blades from the engine embedded in the cabin wall but most of the blades are completely missing, then it’s reasonable to suspect that the engine might have been destroyed mid flight. This could be matched with the data log showing that the engine lost power and started throwing errors. They might find that a bolt that holds the tail rudder on has some threads sheared off, or that it’s been stretched, and match that with unresponsive steering pedal inputs or erratic flying behavior to suspect that the rudder could’ve fallen off or lost control.
Investigators will piece all of the clues together to determine the likely cause of an accident. Basically they know how a plane should work given the conditions, and use the recorded data and the debris to work backwards to find out why it didn’t in that case.
I think the recovery teams collect everything they can in the area with multiple sweeps. Then they lay it out as best as possible and pretty much try and figure out what happened where.
I think they have logs and stats on damn near every part its pretty thorough from what I can remember.
another thing that I’ve heard is, they’ll check to see if the dead pilots wrist are snapped, if they are, they had the flight stick in hand when it hit the ground
I participated in 1 investigation (air to air collision). I am an expert in avionics. The investigators recovered the data from several units and gave them to me for analysis.
The primary question in front of me was “when did x unit fail?” and “could that failure have caused the collision?” From the hexidecimal data put in front of me, I was able to determine that the equipment was still functioning (and recording) correctly at the time of impact.
This (along with other data from other experts) led the investigation to conclude that it was a matter of pilot error.
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