Seat belt mounting........

[1600vw]

I need some help........ My wife says I need a lot of help but thats another story.

The mounting point for my shoulder belt is located all the way back by my tail wheel, she is a taildragger. My bird goes into the shop in a few days and I believe this should be changed. I worry, again I worry alot, about an incident where I might break my fuse in half and then I am hooked to the tail of the bird by my shoulder straps.

The way we do it on the dragster is the shoulder belt should not go down but is mounted even with your shoulders, so no down pressure in the event of a crash, you will not compress the spin.

I would think it does not matter what the belt system is in but it should be mounted this way.


Am I correct for thinking this.

H.A.S.

[1600vw]

The shoulder mount is connected to a cable and this cable runs to the tail of my bird. I know this can not be good, I could be wrong........

[WLIU]

You worry too much.

There are a number of airplanes that anchor the shoulder harness that way. The shoulder harness' job is to keep your face from slamming into the instrument panel. That said, it will not guarantee that your face will not contact the instrument panel. If you impact so hard that your tail section deforms, when the tail is the last part of the airplane to the scene of the accident, your cockpit has already deformed in an manner that is likely not survivable. At lease in the most common accident situation which is a straight on impact. If you manage to have a landing accident that cartwheels the airplane, typically the wings and engine area take the most abuse. The tail section is pretty light and the mechanics of that sort of crash appear to involve more repeated impact of the heavier parts of the airplane (I hope I formulated that right).

So the shoulder harness mounting is intended to do its job in a survivable accident. Your job is to have an accident that stays within its design parameters.

Best of luck,

Wes
N78PS

[martymayes]

If the shoulder harness is anchored aft like yours, up to a 5 degree downward slope to the anchor point is acceptable. It won't compress your spine. All in all, it sounds like the shoulder harness is installed IAW AC 43.13-2B. Not sure I would try to "improve" it.

[1600vw]

I will leave it as is. Thanks for the info.

[steveinindy]

f you impact so hard that your tail section deforms, when the tail is the last part of the airplane to the scene of the accident, your cockpit has already deformed in an manner that is likely not survivable. At lease in the most common accident situation which is a straight on impact.

Except that the tail frequently bends or the attachments are prone to failure at forces well below the human


All in all, it sounds like the shoulder harness is installed IAW AC 43.13-2B. Not sure I would try to "improve" it.

The problem with AC 43.13-2B is that it leaves much to be desired with regard to crash survivability. It's in dire need of a revision based on science from the last 30-50 years.


up to a 5 degree downward slope to the anchor point is acceptable.

Just for the sake of thoroughness, a 5-10 degree upward slope is better.



I worry, again I worry alot, about an incident where I might break my fuse in half and then I am hooked to the tail of the bird by my shoulder straps.

You would be more likely to be left with no shoulder protection (the attachments would fail if it is designed anything like your "standard"/AC 43.13-2B arrangement) and would allow you to be thrown forward striking your face or chest on the instrument panel or control stick. Having the shoulder harness attach directly to an intertial reel which is directly bolted to a substantial part of the airframe is more ideal.

Feel free to PM me if you'd like to discuss this further. I am always happy to help with occupant protection issues.

[martymayes]

The problem with AC 43.13-2B is that it leaves much to be desired with regard to crash survivability. It's in dire need of a revision based on science from the last 30-50 years.


AC 43.13-2B was revised 3/3/08.

Signing off on a non-STC'd seatbelt installation, probably best to use data in the AC vs. SGOTI.

[steveinindy]

AC 43.13-2B was revised 3/3/08.

But it still uses largely the same assumptions about human tolerances that the previous version did. The FAA is very hesitant to increase the standards most likely because of pressure from the certificated industry.


Signing off on a non-STC'd seatbelt installation, probably best to use data in the AC vs. SGOTI.

This is another case where an STC simply means someone did some paperwork that allows them to charge more for something than it is worth.

Personally, I'd go with the US Army's crash survival design guide standards. It is probably one of the single best scientifically based sources of information available and has a large role to play in why I believe what I do. The AC is a better than nothing but it is far from being ideal or anything I would want to rely upon if it were my butt on the line. We have the ability as homebuilders to exceed the basic standards created by some bureaucrats in DC so why should we settle for the minimum especially when it is something that is directly connected to our safety (literally and figuratively)?

[WLIU]

Everything in aviation is a trade-off. The Army can apply $$ and horsepower to the problem. An installation appropriate to an AH-64 is not practical in an ultralight. And the military sends aircraft places where the expectation is for a higher crash rate. Most civilian aviators are much less likely to have an uncontrolled contact with the ground than any military aviator. So the trade-offs are different.

The safest airplane is one that never leaves the ground. But they aren't any fun either.

Best of luck,

Wes
N78PS

[1600vw]

2712

My little bird......Fisher Avenger....Not an UL...Should be...but not.....If you ask me...if you wiegh half what your plane weighs....Its an UL....But we all know the story....so she is a EAB...

[martymayes]

My little bird......Fisher Avenger.....

Looks good!

[rosiejerryrosie]

Looks good!

It would look better if he stopped calling it a bird - its an AIRPLANE!

[1600vw]

It would look better if he stopped calling it a bird - its an AIRPLANE!


LOL.......Agreed......My airplane was built by Charles Lovett. He was a small man of 5'5" 150 lbs. I am 6' 180 lbs. Its a bit of a tight fit but I do fit. I need to move my peddles and or reshape my conopy. The peddles I can do with some help, the canopy that is outside what I can do. These mods won't happen this year but maybe next.

This year its a panel upgrade. Transponder with Encoder, Traffic alert system with GPS along with a EIS.

H.A.S.

[steveinindy]

Everything in aviation is a trade-off. The Army can apply $$ and horsepower to the problem. An installation appropriate to an AH-64 is not practical in an ultralight. And the military sends aircraft places where the expectation is for a higher crash rate. Most civilian aviators are much less likely to have an uncontrolled contact with the ground than any military aviator. So the trade-offs are different.

The safest airplane is one that never leaves the ground. But they aren't any fun either.

It doesn't stop us from learning from our money that they spent. The issue of weight in safety is minimal and is a cop-out that gets touted out when people don't want to put forth the effort or want to adhere to their old beliefs. The difference between a crashworthy restraint and a top of the line system in terms of weight can be under five lbs even if you have to re-engineer the attachments. Most people waste more weight on excessive upholstery or unnecessary instruments.

[Bob Dingley]

Amazing how wise wives are. I once wanted to put shoulder harness in a Beechcraft, but the STC was real steep. Most of the cost was beefing up the semi-monocoque roof structure where it had to anchor. Why couldn't it have been a Piper or Maule? Why the roof? As you say, the anchor must be upward. never down.

I once took a course in A/C crash survivability at Arizona State and they did a lot of work in this area. (Steve, you have heard of DRs Turnbow & Robertson) Their guidance is that shoulder harness should come off the shoulders and angle upward at least 20 deg. They cite that if angled down, enough "eyeballs out" Gs will cause bilateral collar bone fractures. This is based on crash tests with dummies and cadavers.

I flew some commercial & military A/C that the straps went over a small SS bar on the seat back a little above the shoulders. Some passed over a simple roller, some had a pulley in the headrest. Some anchored in the heavy sub floor structure, some on the base of the seat. Those ( Huey variants) depended on the seat-to-floor tracks. They prove to be reliable and effective. I've crawled through some wrecks.That was my area to check.

There is a T-28B restoration in the area that did away with the little roller structure on the seat back leaving the pilot's shoulders the high point. A popular kit plane is apparently designed this way from the get-go. Both are easy to fix.
A pal of mine has a Fisher bi-plane and they have a wood structure. I bet that you can come up with a plan to put a small hunk of oak in the right place a get a good anchor. Good luck.

Bob

[steveinindy]

A pal of mine has a Fisher bi-plane and they have a wood structure. I bet that you can come up with a plan to put a small hunk of oak in the right place a get a good anchor. Good luck.

If I had to bet my life on the integrity of a piece of wood as a restraint anchorage I would be looking for some pieces of live oak. It's expensive, hard to locate and very dense but the stuff is tough as hell if well maintained. After all, they used it for the walls of the USS Constitution ("Old Ironsides")

[WLIU]

We should get you up to speed on the names and characteristics on the different species of wood. Prof Hoadley's book "Understanding Wood" is a great place to start. The USDA Forest Products Lab's book is another great reference. Oak is not well suited for aircraft sontruction.

To construct a hard point in a wood aircraft, I suggest a fine grained and strong wood - hard maple. Or even better, West System epoxy with structure filler mixed in to the consistency of peanut butter...

Best of luck,

Wes
N78PS
woodworker...

[martymayes]

A pal of mine has a Fisher bi-plane and they have a wood structure. I bet that you can come up with a plan to put a small hunk of oak in the right place a get a good anchor. Good luck.

...and luck is something you'll need a lot of to find an attach point 30 degrees above the pilots shoulders in a fisher biplane. I guess you could anchor the shoulder harness to the top of the vertical stab. That would be cool, it would automatically tighten the faster you go so you won't fall out.
:D
I would think having the "acceptable" 5 degee angle below the horizontal shoulder harness is much preferable to nothing at all and while other geometry might be better, it's not an option here because of the contraints of the Fisher Avenger fuselage. So it's not a matter of choosing between good and better. The choice here is good or bad.



If I had to bet my life on the integrity of a piece of wood as a restraint anchorage I would be looking for some pieces of live oak. It's expensive, hard to locate and very dense but the stuff is tough as hell if well maintained. After all, they used it for the walls of the USS Constitution ("Old Ironsides")

Excluding the wood IKEA makes furniture out of, I can't think of a more unsuitable wood to use in an aviation application. Even a statisfactory hardpoint is only as good as the remainder of the structure. Is the next of kin supposed to feel better because the hardpoint ripped away from the structure vs. the harness attachment pulling free from the hardpoint?

[1600vw]

:D
I would think having the "acceptable" 5 degee angle below the horizontal shoulder harness is much preferable to nothing at all and while other geometry might be better, it's not an option here because of the contraints of the Fisher Avenger fuselage. So it's not a matter of choosing between good and better. The choice here is good or bad.




My A&P was looking at this last winter and said just what you did. He did say it might be better to have the seat belt mounted to the seat down low so the seat belt and seat stays intact incase of plane breakup.
He then said its a trade off. Crush your spin..or be hooked to the tail? He thought go for the crushed spin. Speaking from someone whom has broke his back 5 times, I did not think that was a good idea.
I thought I would ask this question here now because my bird is going back into the shop and I know the A&P will ask about this again.

H.A.S.

[WLIU]

The most important part of the anchoring of the shoulder harness is to do it in a way that the hard point will not pull loose from the remaining structure. Just wrapping a cable around a spruce bulkhead part creates a point load that is likely to fail sooner than say a bolt through enough material that the stress is spread out to be below the yield strength for the wood and the glue joint(s). In plain english this usually translates into plywood doublers on either side of the material that the bolt goes through. If you are attaching to a horizontal member of the fuselage, you might also look at whether the gussets that attach the horizontal member to longerons need to be beefed up. Don't want the cable attachment to hold and the whole horizontal member to rip out...

FAA AC43.13-2b actually has a good discussion of mounting belts, but the structures described are metal, not wood. You have to go back to CAM-18 to get guidance as the older publication was written closer to a time when wood structure was state of the art.

Best of luck,

Wes
N78PS

[Bob Dingley]

Marty, please don't even joke about attaching the shoulder harness to the vertical stabilizer. This is the least valuable suggestion. Its the flimsiest structure possible and someone may try it and call it an antenna.

One builder of a small open cockpit I saw has a 6 inch dowel (white pine) solidly mounted an inch or two aft of his shoulders and a touch above. The shoulder straps pass over this then run down behind the seat and attach to something way down there out of sight. Landing gear for all I know. Does not put a down force on your tender pink bod. If I could not avoid a high speed taxi into a fuel truck, this would be the one. A good, simple, light weight option. Yes, could be improved. I will go with the research over the burro crats every time.

I learn best from experience. My very first emergency was when I caught fire. I haven't flown in polyester underwear since.

[martymayes]

One builder of a small open cockpit I saw has a 6 inch dowel (white pine) solidly mounted an inch or two aft of his shoulders and a touch above. The shoulder straps pass over this then run down behind the seat and attach to something way down there out of sight. Landing gear for all I know. Does not put a down force on your tender pink bod. If I could not avoid a high speed taxi into a fuel truck, this would be the one. A good, simple, light weight option.

If he had the structure engineered and tested to ensure appropriate strength margins, great. Otherwise, it's only a "feel good" installation.

[steveinindy]

Bob, as the other crash survivability trained person on this discussion, you might have a take or more to add to an idea that struck me. Another option might be to extend the back up the seat up a couple of inches and then bring the restraints either straight back or slightly down to a more closely located and secure point than the tail. This should give the needed twenty degree clearance across the shoulders without having to resort to anything too dramatic. What is really important here is the strength of the anchorages and the angle of the shoulder straps (and, yes, there should be two of them) as they cross the collarbones and shoulder musculature.

Just out of curiosity 1600vw, have you given any thought to inclusion of inertia reels on your shoulder restraints if you don't already have them?

[steveinindy]

Otherwise, it's only a "feel good" installation.

The same can be said for a lot of certificated restraints and seat attachments honestly. When the top end of the federal standard is below even what is required for a passenger car let alone the threshold of reasonable survivability, something is- with due apologies to Shakespeare- rotten in Denmark.

[steveinindy]

He did say it might be better to have the seat belt mounted to the seat down low so the seat belt and seat stays intact incase of plane breakup.

Mounting them on the seat might actually make the strain on the seat attachments (which are often underdesigned to be brutally honest) sufficient to wrench the seat loose. I would suggest mounting the belt attachments as close to the seat as possible so if the tail snaps off, you don't lose your shoulder harnesses or have nasty injuries inflicted as the attachment wire or whatever is pulled away.

[WLIU]

For better or worse, the seat back and structure immediately behind the seat on that type of wood aircraft will not support the types of alterations that you are talking about. Seat backs in light aircraft are only expected to support the load of the pilot pushing on the rudder pedals and a little weight from the torso of an upright pilot. You can see this in TSO C-39 for aircraft seats. The seats in military aircraft use more steel and supporting structure.

The currently installed shoulder harness anchor in the Fisher is likely the only practical installation. Upgrading as discussed above likely requires throwing away the fuselage and designing a new one. Then designing new wings to support a heavier aircraft, and a bigger engine......

Of course, the best approach is to don't crash. 99.9% of recreational pilots don't.

Best of luck,

Wes
N78PS

[steveinindy]

Seat backs in light aircraft are only expected to support the load of the pilot pushing on the rudder pedals and a little weight from the torso of an upright pilot. You can see this in TSO C-39 for aircraft seats.

The mindset of that approach is quite frankly another example of what is wrong with the "official" approach to aircraft design. I am really hoping that the ASTM involvement in trying to improve the design standards will help to move us away from largely holdovers from the 1930s and other times before the broader requirements for structures were considered.


The currently installed shoulder harness anchor in the Fisher is likely the only practical installation. Upgrading as discussed above likely requires throwing away the fuselage and designing a new one. Then designing new wings to support a heavier aircraft, and a bigger engine......

It would likely be possible to redesign the seat without having to throw out everything. It could be an engineering challenge but then again why should we shy away from something because it is hard. Taking the "easy way out" is what moved us from being innovators to a group of kit assemblers many of whom had to be told by the FAA to stop using hired guns to do it for them. It is common on this forum to grouse about the "decline of the EAA" or the "new EAA" but I think we miss what has brought that about. It is not the leadership but the change in the mindset of the membership at large. The few of us who choose to speak up on here are outliers and are probably somewhat holdovers from the original era of the EAA when advancement was first, foremost and penultimate.

I don't think you would need a steel seat for this to work. The vertical loads on a seat would likely be within the range that common aluminum alloys would be sufficient. Granted, I haven't fully run the numbers for this particular approach.


Of course, the best approach is to don't crash. 99.9% of recreational pilots don't.


While I agree on that concept on it's face, I have a problem with such approaches because it is rather fatalistic and counter to the point of experimental aviation (to advance things). Based upon that approach, we should stop trying to develop new engines because they meet the 99.9% of the needs out there. We should stop crash testing cars because most people survive in the current models.

Sorry....I couldn't resist playing devil's advocate.

[WLIU]

The original poster HAS a flying airplane. Most folks would rather fly than tear their machine apart and spend months trying to make it so that it is 100% set up for handling a very unlikely event. Aviation is an exercise in compromise. So we size up the risks and decide to fly or not. It will never ever be accident free. There are too many imponderables. And even if you crash, you have more control of the outcome than you think. I have a number of friends who have had airplanes give up on them and in fact one day an acquaintance stopped me and recounted that when his engine quit, he remembered some advice that I had given him. I was surprised by that and I will offer that advice here - Never stop flying until the crunching noises stop. You are welcome to repeat that. An old aviator once said, in the days before cell phones, to crash near a telephone so you can call your base and let them know where you wound up. But we all flew old ratty ships for low budget operators in those days..... Obviously my experience is different than that of a lot of folks.

Best of luck,

Wes
N78PS

[braywood]

Interesting and important topic. I read through the document steveinindy referred to - excellent! it is full of way more info than I need (or care to understand) but has very good recommendations for basic seat, harness and cockpit design considerations. I hope to incorporate some of these in my Pietenpol!

[steveinindy]

Interesting and important topic. I read through the document steveinindy referred to - excellent! it is full of way more info than I need (or care to understand) but has very good recommendations for basic seat, harness and cockpit design considerations. I hope to incorporate some of these in my Pietenpol!

Glad to know it was helpful. If I can offer any further assistance (or references should you desire to want to read more), let me know. Is your Piet nearly complete? If so, let's see some pictures. I am not known for being a "low and slow" kind of guy but that has always been a design that I have found aesthetically charming and it would probably be really fun to fly.

[braywood]

Hi Steve - my Piet is still a long way off!! Tail is built, working on the centre section, metal parts and wire wheels. I am enjoying the journey!! If I get my rear seat design following this US Army Design Guide drawn out, I will send you a copy to have a look and critique. Can't have too many eyes looking at this sort of thing.
Thanks again,
Mark