Most crash worthy Sonex build?

[ryoder]

I am considering building a quick build Sonex to get a brand new airplane that is safe and economical.
However I am concerned about crash worthiness and was wondering how one could build the Sonex to be as survivable as a new Cessna 162.

One thing I noticed is that the seats are just cushions placed on a metal frame.
Is there any way to have more of a suspension? I assume not.
Also, I assume you guys install 6 point harnesses so that's good for crashing but are there are any guys who installed airbags, roll bars, or softened up the panel for impact? How about roll cages?

My biggest concern is probably the seating. I'd hate to have my spine compressed on a super hard landing.

[Bryan Cotton]

Hi,
One of the single most important things for vertical impacts is having a crush zone underneath you. So, if the belly hits, there should be volume underneath you to fold up, reducing the acceleration on your spine as compared to the belly skin. I would assume the lowered seat mod reduces this, so you may want to stick with the stock setup. It is a very conventional strategy in airplanes and helicopters. Also, generally you will fold the gear up before you crumple the floor. I can't comment on the 162. It would be hard to beat a 152, with its similar floor plus seat on too of that.

[EricS]

I'm not an engineer and I did not stay in a Holiday Inn Express last night, but from what I've seen I would consider the Sonex to be a very crash worthy aircraft. Here are some items that quickly come to mind:

- The titanium rod landing gear do a tremendous job of absorbing the shock on landing or during a crash. I've seen a few pictures now of crashed Sonex aircraft where the fuselage was only mildly damaged during a somewhat wings level landing because the gear rods absorbed the majority of the impact force and subsequently broke away. This fact may help ease your concern about the suspension of the seat section. I have completed the lowered seat mod and still have 2-3 inches between the seat and the belly.

- As far as needing roll bar protection, a properly built turtle deck provides excellent rollover protection. And usually in the event of a roll over, the canopy disintegrates making it easier to escape from the aircraft. If you speak with many builders they will say that the canopy cracks easily even during construction.

- Even though the fuel tank is located inside the cabin, it is one of the most important safety items on the aircraft. Again, going by prior crash photos and reports, the rotationally molded fuel tank will usually deform before rupturing.

- I would say that adding airbags or softening up the panel is a little outside of what the mission and design of the Sonex is about. The panel is a structural member of the aircraft as it keeps the forward fuselage sidewalls from bowing in during positive G maneuvers. Modifications to the panel's lower cross member needs to be carefully considered.

Just my personal opinion, but If cost is not a factor and you'd rather be flying more than building, the quick build kit may be the right choice. If you'd rather spend less, or invest it into the engine and instruments, remember that the new kits being shipped all have matched-hole parts. Several builders on this forum will attest to how quickly the parts go together with the new kits. By doing this, you know exactly how the aircraft has been built and have control over quality. Just to be clear, I'm not saying that the quick build kits are low quality at all.

Good luck with your research and feel free to throw more questions out there!

[vigilant104]

Well, the first thing to say is that the Sonex is already a very good design as far as crashworthiness. The plane has been tested in a few severe crashes and has held together very well.
Of course there are always ways to make improvements in any parameter, but there will be tradeoffs. For example, it's always better to avoid a crash than to survive one, and if adding 10 lbs of weight to enhance crashworthiness means taking off with 20 minutes less fuel then maybe it's safer to have the gas. Optimizing one characteristic can adversely affect others. With that said, here are some thoughts:
Minimizing vertical accelerations on occupants: Having empty space under the seat does no good if the seat pan merely collapses onto the floor. To be useful, the space needs to allow for controlled absorption of the acceleration forces. This might be done with a seat pan that "breaks away" at, say, 15Gs and then crushes metal honeycomb or foam for a few inches. These things are hard to design--much depends on occupant weight, and a seat pan that breaks away under normal use (e.g. when point-loading the seat underfoot when getting in and out of the plane) would be bad. I think a very practical way to reduce spinal loadings on occupants would be to build the airplane with the lowered seat and thereby make room for use of thicker impact-absorbent visco-elastic foam seat cushions. Unlike "regular" foam cushions these respond differentially to "fast" vs "slow" pressure (more resistance to "fast" pressure), they resist "bottoming out" abruptly, and they turn an appreciable amount of the impact energy into heat. They work very well at protecting objects, including people, from acceleration forces. A thick seat cushion of the right mix of viscoelastic foams would be an important first step in reducing the types of injuries you mention. References:
- EAR Confor Foam for seating: http://www.earsc.com/HOME/engineering/TechnicalWhitePapers/EngineersNotebookConfor/index.asp?SID=63
- Aircraft seating and Confor foam http://www.trelleborg.com/en/Applied-Technology/Products-and-Solutions/Energy-Control-Products/CONFOR-Cushioning-and-Impact-Absorbing-Safety-Foam/Glider-Sailplane-and-Light-Aircraft-Seat-Foam/

Of course, the space under the seat pan is still important, and it would be beneficial to put something between the floor and the seat pan. Suggestons might include metallic or Nomex honeycomb material or polystyrene. The choice would require more research, and it would definitely be important to pick something that is unlikely to burn/catch fire.

Reducing horizontal acceleration on occupants: A well-adjusted lap/shoulder harness system helps tremendously (in helping to avoid contact with the aircraft structure). Also, I'm sure the long nylon shoulder harnesses provide a bit of "give" that is useful in reducing peak loads. The sloped Sonex firewall helps the plane avoid "digging in" if it impacts soil, which can be a very large factor in reducing acceleration. I suppose adding a small section with a slightly more gentle angle at the bottom of the firewall might go a bit further in this direction, but it would reduce access to important engine/exhaust components.

Other: I've thought about a lightweight foam head restraint attached to the rear canopy crossmember. This would reduce cervical loads and cranial exposure to the crossmember when the heads of occupants move violently rearward (they do this even in "regular" forward crashes).

Air bags: I know that shoulder-harness airbags are available for GA aircraft. I think we are in the "very low bang for the buck/pound" region with these in a Sonex if you've got your harness on correctly. Rollover protection is very good in the Sonex, but, like any canopy-equipped aircraft you'll probably be stuck in the plane until help arrives if it is flipped over.

Anyway, there are some ideas. Again, the plane is very strong and its slow stall speed can be a tremendous aid in reducing the energy of a crash. It makes sense to think of prudent steps to enhance crashworthiness, but even more sense to avoid crashes in the first place.

Mark W.

[Rynoth]

Oddly enough, in seeing what the molded fuel tank does during a heavy collision, my untrained eye sees it as sort of a pseudo-airbag itself. Or at least a crush zone between you and the firewall.

[fastj22]

The new Viking Sport, based on the Sonex will have a great deal of space between the seat pan and the bottom of the fuselage.

[daleandee]

Another part of this discussion has to center on the pilot. Vertical impacts in crashes appear to be what causes the the most injury. The point is that the pilot must absolutely avoid letting the airplane "stall & fall" in an emergency situation. I know, this sounds easy from behind a computer keyboard. Still, you must have your brain wired to understand that a controlled crash into almost anything is better than a vertical fall or spin in from even a few dozen feet. Fly the airplane ... always!

As far as the fuel tank ... I'm only aware of one Sonex crash with a post crash fire. The photos were disturbing to say the least. NTSB report here:

http://www.ntsb.gov/aviationquery/brief ... 183&akey=1

Dale
N319WF

[sonex293]

As a Sonex Pilot who has survived the unplanned engine out, I can only sing the praises of the Sonex design. (http://www.ntsb.gov/aviationquery/brief.aspx?ev_id=20130909X40737)

The Sonex is one of the only aircraft designs with such a wide margin of high top speed and low stall speed. All aspects of the Sonex design held up well as I descended through 70+ feet of NC pines! The crumple zones did their work and I was able to walk away with only slight injuries. Being able to slow the aircraft down to 40 mph or so before impact only adds to the safety factor. I did use multi-layered memory foam on my design. (http://www.mykitlog.com/users/category.php?user=mcrowder&project=36&category=484)

The rotationally molded fuel tank held up well, although the fuel cap did get squeezed out and some AvFuel did splash out.

In the end, the airplane was totally destroyed. I don't think there was a skin on the airplane that didn't have a crinkle or tear. I was also amazed how the spars held up, even with a direct impact to a tree top. We (the recovery team) removed the spar bolts and the wings came off with very little effort.

As I hope you can tell, the Sonex is one tough design.

The cause of engine out? I had wired something too close to a hot point in the engine that was critical. Once the wire melted and created a short, the engine quit. I encourage everyone to get a second set of eyes on your project to see things you might overlook.

==
Michael Crowder
ex Sonex N293SX

[Bryan Cotton]

I lost a friend, fellow tow pilot and glider pilot from one of my old clubs. He was set up for an outlanding in his glider, and he stalled/pancaked in. Gliders are a lot worse. Not much structure to crush. Never should have happened. If you fly the aircraft all the way down vertical impact should not be a big factor. It can be a bigger factor in helicopters.

I don't know that the seat, as designed, would collapse to the floor. There are a lot of rivets to shear or edges to tear out. I also think you would see the bottom floor collapse up toward the pan if you had gotten far enough to flatten the gear.

I do think that keeping the aircraft light will only help survivability.

[sonex892]

The new Viking Sport, based on the Sonex will have a great deal of space between the seat pan and the bottom of the fuselage.

Compared to a Sonex the viking sport has a substantial amount of pilot and passenger well above the turtledeck which is a worry.

The turtle deck on the Sonex is a great roll bar. It saved a friend when his flipped on its back during a forced landing. The turtledeck and the vertical fin crumpled slightly which no doubt helped soften the impact. The canopy as you would expect disintegrated.

Whilst upside down the fuel cap stayed put and not a drop of fuel leaked out, the aircraft was upside down for a few hours. His planes fuel vent is as per the plans, plumbed to the bottom of the firewall, I believe this method to be the safest. An upside down Sonex with fuel remaining in the tank fitted with a top mounted gunsight style vent would leak.

Steve 892