Similar Chute; Different Day

[EricJ]

4 hours ago, Blue on Top said:

@PT20J  I have heard 3-point is better than 4, but I have no hard data  … and 5-point (4-point with crotch strap) is better than 3.  The reason I was told is with 4-point, the shoulder harnesses have a tendency to pull up the waist belt and allow the occupant to slide underneath (submarine).  From high-speed, slow-motion cameras, I am amazed at how much these belts stretch in a crash.  Wichita State University (a local university) does seat testing … for a $$$$$$ price .

Yes, in a frontal impact it is easy to submarine and wind up essentially unrestrained with a poorly designed or poorly installed 4-pt.    The issue is that the center of body mass is near the lap belt, so if it winds up at all below the lap belt it just pulls you under it, and this is not an unusual occurrence.   For this reason only certain sled-tested 4-pts are allowed for track days with the organization that I run with.   The allowed 4-pt belts are basically designed to work like 3-pt belts, which prevent submarining by allowing enough of your upper body mass to tilt forward and be in front of the lap belt (i.e., in the direction of impact).   This balances the forward forces in front of the lap belt so there is insufficient net force to pull you under the lap belt. 

As you mention, this is aggravated in a 4-pt by the shoulder belts tending to pull the lap belt further up.   I wear a 4-pt in my track car, but I don't pull the shoulder belts very tight, and one of the shoulder belts is designed to yield so that the same forward rotation of the upper body happens as it does with a 3-pt.   Otherwise the shoulder straps do the job of holding you firmly in the seat during driving, which a 3-pt doesn't do as well.

This is why a 3-pt is safer in a crash with a forward deceleration component than a 4-pt.

All of that is avoided with a 5-pt, and all the 5-th strap does is hold the lap belt low over the iliac crest of the hip-bone, which is the most mechanically strong part of your body.   6-pt harnesses, that are used in some open-wheel cars where the driver sits very reclined, are designed to rotate the hip-bone up during a forward impact to provide better mechanical restraint of the body by the lap belt.   The shoulder harnesses will always stretch, and that's a good thing to give more deceleration time for the upper body, and allowing the hip to rotate up a bit goes along with that.

I've had two significant race accidents with high forward impact forces.   I had a 5-pt harness in each.   In the first everything worked properly and I was completely uninjured, up and walking around immediately, no soreness later, etc., etc.   Harnesses are one-time use, though.   Once they've stretched they're done and need to be replaced, and standard procedure with our safety/tech guys is to chop up your harness after it's been in an accident in order to prevent re-use.   So that harness got chopped up.   I switched brands and used a different harness after that and about a year later got in another high-forward-impact crash (both were due to mechanical failures).   Unfortunately, that brand of harness had an issue with the lap-belt tightening hardware that affected being able to fully tighten the lap belt.   It had just enough slack that the delay in tensioning the lap belt meant that my upper body mass was delayed in stretching the shoulder belts, which results in the shoulder harness pushing down on your shoulders instead of restraining your full upper torso in deceleration...this is a known thing in how harnesses are supposed to work.   The result was a spinal compression that was very, very un-fun.

I think in airplanes the whole story is a bit different, though, in that the harness is primarily to keep you secure in the seat during flight.   Sliding around in the seat too much affects the ability to accurately manipulate the controls (just like in a car on the track), so that's especially important for things like aerobatics.   I'm sure it all helps for crashworthiness, too, and we all know that having a shoulder strap is a LOT better than not having any.   Having a 3-pt is probably better than having a 4-pt for the same reasons as in a car, and a 5-pt will restrain you best, but is also a pain to deal with and you can't reach anything once you're strapped in (a common thing in a race car is to get yourself strapped in, hans on, get the nets up, and then remember you forgot to close your door or turn your camera on or do something just out of reach...too late).

Anyway, this is a long post, but related to something I've spent a lot of time learning about and using, etc., etc.

Edited January 30, 2020 by EricJ

[Ibra]

30 minutes ago, Hank said:

Over here, it's just 8000 msl; FLs start at FL180 = 18,000 msl.

There may be a pile of dirt to ski on somewhere in OK, but real mountains are 1000 miles west of there . . .

Yes, I forgot proper skiing is done at Mt Whitney somewhere at 14.6kft and 1000nm to the west, so flight levels should start FL170 at least

AFAIK, there is not much ski going at "Mt" Ben Nevis (4kft), so FL60 does a great job across Scotland while in England, region ATC offer FL30 or FL40 depending how high they sit on the chair that day and if it is a spring/neap tide  but they have plans this year to sort this mess to FL60 and then higher on next years, so we can fly at 8000msl   

[Blue on Top]

2 hours ago, Ibra said:

... any snow mountains around Oklahoma ?

@Ibra  Ummmmm … no ,  Oklahoma is flat … and typically little if any snow.  Here is the image that came up when I Googled it.   

Skiing in Salt Lake City, Utah is much, much better.  Greatest Snow on Earth (per their license plates). 

[Blue on Top]

1 hour ago, EricJ said:

Anyway, this is a long post, but related to something I've spent a lot of time learning about and using, etc., etc.

@EricJ Thanks for the great post (information).  We can ALL learn from each other (and our experiences).  I don't have enough time (or lives) to make all the mistakes myself.

[PT20J]

3 hours ago, EricJ said:

Having a 3-pt is probably better than having a 4-pt for the same reasons as in a car, and a 5-pt will restrain you best, but is also a pain to deal with and you can't reach anything once you're strapped in

The commercial 5 point harness I had in the Decathlon I used to rent was like that. The military style I’ve used in the BT-13, AT-6 and C-45 had a lock on the shoulder harness straps that you could release in flight and lock for takeoff and landing and aerobatics. They had big, wide green webbing and padded shoulders and were very comfortable but they looked, well, military. In a Mooney, they’d look more at home if you replaced the interior plastic with quilted olive drab padding 

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