Message-Id: <3.0.3.32.20000418011555.008083c0@olsusa.com>
Date: Tue, 18 Apr 2000 01:15:55 -0400
To: lancair.list@olsusa.com
From: Marvin Kaye <marvkaye@olsusa.com>
Subject: seat belt attach points
Mime-Version: 1.0

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Posted for Srxtreme2@aol.com:

In response to your concerns of spinal injury from the shoulder harness in
a hard landing situation, your airforce comrade unfortunatly got the
physics backward.  During a hard landing the body is rapidly decelerating
in both a forward and downward direction.  This causes the weight of the
upper body multiplied by the G-force of your deceleration to compress the
spinal cord down into the pelvis resisted by the bottomed out seat.  If the
momentary maximum force generated exceeds the burst point of any of the
componants of the back, a spinal cord injury will occur.  The most common
failure points are the intervertebral disks, the vertebral bodies, and the
ligamentous attachments.  In addition there is a significant risk of
avulsion of the attachments of various internal organs from the same
mechanism of rapid deceleration.  While this is certainly some thing to
avoid if at all possible, the attachments of the upper ends of the shoulder
harness do not play a role in the genisis of these injuries.  Remember when
you feel the tug of the shoulder harness? Its with the negative Gs
generated during turbulence or acrobatics.  Hard landings generate
possitive Gs.  There is not much head room in the  smaller lancairs
especially if you are very tall,  and under negative G conditions it could
be possible to hit the canopy with enough force to injure the upper spinal
cord.  In this situation a lower placement of the upper shoulder harness
attachments would have a protective benifit.  

One could compromise the primary benifit of the shoulder harness,  that of 
forward deceleration during a frontal impact, however, by placing the
attachment points too low.  If the body is allowed to accelerate forward
any significant distance in relation to the decelerating aircraft, prior to
the shoulder harness coming taut, the "snap" that ensues can be
devastating.  The position of the attachment points in order to maximize
their overall benefit would be as close in a fore-aft direction to the
shoulders as possible and just below them vertically. One additional
protection that you can put in with little cost or weight, is a piece of
trocellin closed cell foam.  This foam unlike any foam utilized in the
upholstering industry is designed not to bottom out even under extreme
impacts.  It is the same foam used on the new top of the line gymnastics
floors and it greatly increases the curve during deceleration which allows
the force to be spread over a longer period of time therefore doing less
damage.  This comes in 1 3/8" and 2" thicknesses and is available from
Resilite and other gymnastic supply houses. I guess in the end the best
protection is the judgment we exibit when we finally mount the steeds we
have created.

Steve
IVP


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