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Old 8th Sep 2010, 08:03
  #2138 (permalink)  
PickyPerkins
 
Join Date: Jul 2002
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Silly question

There seem to be many questions about the fact that bodies found floating on the surface of the sea were not belted to seats, but that in contrast their spinal injuries are consistent with them having been in seats and belted at the time of impact, and subject to high vertical acceleration.

Ruminating on this I am tempted to ask a “silly question”, and simultaneously to take cover behind the notion that there ought to be no “silly questions”, particularly when the known facts and ideas do not add up neatly.

My “silly question” is rooted in the fact that all objects dropped on a surface bounce unless their properties are such as to be able to EXACTLY absorb all the kinetic energy of the falling object (which isn’t very often).

Pretty well everything bounces, not just tennis balls, etc. When you click a computer mouse button, the computer sees a long series of make and break events as the contact bounces, and special software exists in the computer to interpret this series as a single “make“ event.

Now for my “silly question”.

What happens when a passenger lap belt buckle is slammed down on a passenger’s lap with decelerations of say 20g-100g?

There is going to be some bounce.

The buckle as a unit is going to bounce upward, and the release lever may also impact the buckle body and bounce upward as well.

In either case, could the upward momentum of the release lever be enough to turn the release lever through 90+ degrees and thereby release the belt? The spring which normally holds the buckle closed is not very strong.

If one knew the weight of the release lever and the strength of the spring, it might be possible to calculate the accelerations needed to release the belt under “bounce” conditions.

Of course, if you have a seat belt and buckle available, like the one carried on every aircraft and used on every flight by flight attendants to demonstrate how to release the buckle, you could take it to the nearest “bouncy” surface and see how hard you have to slam it down in order to release the buckle.

I presume that no tests were required at the time that the seat design was approved since no 20g-100g forces were considered relevant.

The fact that most of the bodies were found with spinal injuries consistent with being subjected to high accelerations while being seated and belted, but that the bodies were recovered from the sea unbelted, strongly suggests to me there must be some common mechanism by which they became unbelted.

Taking cover ……….

Last edited by PickyPerkins; 9th Sep 2010 at 01:54. Reason: typo
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