Seat Belt Replacement After an Accident

[Marc Zeitlin]

Now, how do you answer the original question....which is obtuse since we don't know whether the question is about experimental or standard aircraft?

Whether Exp. or TC'd, I agree with Ron W.'s and your subsequent posting that there is no "requirement" for replacing the seat belts, and that they should be replaced on condition for all the reasons you guys state.

Now, most of the seatbelt labels I've seen indicate a 10 year lifetime (by the MFG), so on my CI reports I indicate to my customers when their seatbelts are over 10 years old, but if they look to be in good shape (and the airplane is hangared without much UV exposure) and the stitching looks good, I don't refuse to sign off a CI because of older seat belts. But when they get 15, 20, 25 years old, I start pushing harder for a replacement, even if they don't look too ratty, and give my customers a few years to do so before I get belligerent about it :-).

[dougbush]

I've heard that child car seats must be replaced after an accident. Somehow they are only able to absorb the maximum energy once and are thereafter compromised. It was recommended never to buy or accept a used child car seat if you can get a new one.

[FlyingRon]

I've heard of HELMETS needing to be replaced after they receive a shock (usually from an accident) and also climbing ropes once you've got a fall. The idea is that you've deteriorated the energy reducing material.
Seatbelts on the other hand aren't designed to stretch or give.

[CraigCantwell]

For a data point, any fall arresting harness or lanyard at work is removed from use and destroyed if it was involved in a fall. Otherwise, they are replaced on a 5 year cycle, and we have about 750 full body harnesses and several hundred lanyards and reels.

For force numbers...a 200 pound person falling 9 feet will have an impact force of about 3600 pounds....impact velocity of only 16 mph....

[Mike M]

"Seatbelts help reduce the risk of injury in many types of crashes by helping to properly position occupants, while the seat belt is designed to stretch at a controlled rate to absorb impact energy and reduce the severity of the occupant’s deceleration."

This is why one shouldn't simply alter the seat belt fabric lengths and mounting methods of certificated systems. Nor systems designed by qualified engineers with expertise in the area. For example, well-known EAB kits with quality engineering that have cables attaching the shoulder belts to the mount points. Builders sometimes eliminate the cables and lengthen the belts to the attach points. May or may not be a good idea, I'm not qualified to say. Your smilage may vary.

https://gwrco.com/seatbelts/seat-belts/

[melann]

"Seatbelts help reduce the risk of injury in many types of crashes by helping to properly position occupants, while the seat belt is designed to stretch at a controlled rate to absorb impact energy and reduce the severity of the occupant’s deceleration."

This is why one shouldn't simply alter the seat belt fabric lengths and mounting methods of certificated systems. Nor systems designed by qualified engineers with expertise in the area. For example, well-known EAB kits with quality engineering that have cables attaching the shoulder belts to the mount points. Builders sometimes eliminate the cables and lengthen the belts to the attach points. May or may not be a good idea, I'm not qualified to say. Your smilage may vary.

https://gwrco.com/seatbelts/seat-belts/

A good example of this is; Van's Aircraft originally used the full length of shoulder harness into the tail cone. Testing showed that the "stretch" of the material in a forward impact could allow the occupant to contact the instrument panel. The change was to shorten the shoulder harness material and, instead, extend the harness with a steel cable through the baggage bulkhead to the aft anchor bolt in the tail cone.

[Dana]

For force numbers...a 200 pound person falling 9 feet will have an impact force of about 3600 pounds....impact velocity of only 16 mph....

That depends on the deceleration distance.

[WLIU]

That calculation is definitely not correct. Or I should have died every time that I landed a round parachute. The load depends on deceleration distance and how the impact is absorbed by whatever restraint system used. Please look up the material characteristics of the seat belt webbing. It stretches and fails at specified loads. This affects the calculation for instance.

Best of luck,

Wes

[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