Seat Belt Replacement After an Accident

[CraigCantwell]

What I was stating, is for a free falling person with no fall protection....just like falling off a roof or ladder. Force of impact for a free falling body is estimated as W*(1+(D/d)), where W= weight of the object, D is the free fall distance and d is the deceleration distance. For what I quoted, W=200, D=9 feet and d is approximately 1.1 for a human body falling flat. Land on your feet and collapse, and you reduce the number as d increases. Plug the numbers and you get around 3600 pounds. Our decelerators on our fall protection gear, limit the maximum fall to under 6 feet, with a decel distance of 2.5 to 3 feet. The impact numbers drop greatly. For a seat belt, the fall distance is going to be very small, with the decel distance being quite large in comparison. Typical estimates are that a seat belt will stretch about 6" in a 30-40 mph accident, and that's assuming that there is no displacement of any of the anchors. Higher speeds are going to result in higher stretch, until you reach enough speed, that the belts can no longer stretch and end up failing.


As to the parachute Wes, it adds a bit of complexity to the calculations, putting you into the realm of differential equations to come up with the impact force. I do know that one our pilots at work, had a near zero/zero ejection recently and was slightly injured, as his chute was not fully inflated at the time he touched down. Makes a big difference. On a chute, you are in a controlled decent and not a free fall.

[WLIU]

This is indeed a complex subject. An inelastic object falling the distance specified and stopping in zero distance does indeed experience very large forces. But that is pretty much never the case. Even the human body is elastic and not long ago a close relative put that to the test by accidentally slipping and falling off the roof of the fly bridge of his boat. I believe that "D" in the calculation was actually > 9'. Unfortunately, the boat was not in the water, but was on a trailer parked on asphalt. A broken arm, 3 broken ribs, and I think a concussion resulted. The arm broke when he landed with it between his head and the pavement. Better the arm broke than his skull. He might be an advertisement for one of the fall arresting harnesses mentioned previously.

As mentioned in post #11 we most often replace web products not because we know that they have been compromised in use but rather out of an abundance of caution since we do not have good tools in the field to determine their actual condition. In the parachute world there is a non-destructive test protocol for older canopies to determine that the fabric has not weakened with age (PIA TS 108.1). I am not aware of a similar test for seat and shoulder belts that can be done in the field.

Question about "indicate a 10 year lifetime" - I just stepped out to my hangar and none of my older belts have any date on it other than the date of manufacture. Is putting some sort of "replace by" date on the TSO tag a new thing?

Reading the comment above about builders modifying shoulder harness installations from plans, I will suggest that builders thinking of changing their seat belt and shoulder harness anchors be familiar with FAA AC 43-13.2B Chapter 9 Shoulder Harness Installation before they go to work. Of course, this comment is speaking to the choir.

Fly safe,

Wes