NTSB asks FAA to ground Zodiac CH-601XL

[bullwinkle]

That rate is not tolerated in certified aircraft by any means. Is it a trade off we are willing to make to the economies of LSA? Not my call, but let me know if it is acceptable so I can make my own decisions accordingly. As it is, just by looking at the LSAs at S-n-F and OSH, fully half of them I wouldn't fly because I spotted glaring design and construction errors that fall outside my personal standards, and that was without opening them up.

Care to cite examples? I'm very interested.

 

[Henning]

Care to cite examples? I'm very interested.

Bonanza, Malibu, C-441, Boeing 737 come to mind representing various levels of aviation from 91 to 121, plenty of others have had grounding Emergency ADs for various design or construction issues with lower failure rates than we're talking about here. Usually by the second failure, things are starting to be put in motion.

 

[EppyGA]

Bonanza, Malibu, C-441, Boeing 737 come to mind representing various levels of aviation from 91 to 121, plenty of others have had grounding Emergency ADs for various design or construction issues with lower failure rates than we're talking about here. Usually by the second failure, things are starting to be put in motion.

I think he was asking about examples of LSA you had seen that you wouldn't fly based on what you had seen at OSH and SnF

 

[Henning]

I think he was asking about examples of LSA you had seen that you wouldn't fly based on what you had seen at OSH and SnF

Honestly, at this point I don't remember names, my memory on those counts is horrible. I remember pictures. I'd recognize the plane if I saw it again. Everything I found I would find on a preflight. Now, some people here have witnessed my preflight and commented we weren't doing an annual or something like that...

I do remember a royal metallic blue and white monocoque/stressed skin wings with skins held on by hardware store pop rivets. Mmmmmm....no thanks, I'll pass. My personal experience says "yes, this will hold...for a while, then will fail catastrophically when it does." Now if the price had been more attractive I'd consider drilling and replacing the rivets with proper cored rivets, but they were asking $128,xxx for it. It had a glass panel though, that seems important to LSA, selling on the technical advantage available. I saw a FIAT is what I saw my opinion told long and short of it. Like I said, I don't pay close attention to LSA because it doesn't provide me an advantage, and I haven't had a client ask me to shop for one. I was generally curious enough to take a look.

 

[wanttaja]

That "1000 airplanes" number seems fairly high. I suspect that they are counting ALL 601 variations, including the 601 HD and the 601 HDS, neither of which has had these problems. There are only about 300 flying 601XLs registered in the U.S. Of those, perhaps 60-70 are factory made SLSAs from either AMD or Czech Aeroworks(this is from the FAA database).

I know that they are somewhat popular in europe and other countries, but 1000 601XLs actively flying seems a bit too high, but I could be wrong.

I took a look at my January 2009 copy of the FAA registration database. There were about 480 Zenair 601-type planes registered. Of those, about 360 had airworthiness certificates. Those are the only ones I personally count as "flying" (though exceptions are common).

29 were ELSAs, 54 were SLSAs, the rest (273) were Experimental Amateur-Built.

Ron Wanttaja

 

[mcjon77]

I took a look at my January 2009 copy of the FAA registration database. There were about 480 Zenair 601-type planes registered. Of those, about 360 had airworthiness certificates. Those are the only ones I personally count as "flying" (though exceptions are common).

29 were ELSAs, 54 were SLSAs, the rest (273) were Experimental Amateur-Built.

Ron Wanttaja

Those numbers are pretty close to what I saw. Just to make sure, when you say "601 type", you are refering to ALL 601 models (e.g. the 601XL, 601HD, 601HDS)? I am assuming so, since your total registered number is very close to the total registered number I found.

 

[Henning]

I took a look at my January 2009 copy of the FAA registration database. There were about 480 Zenair 601-type planes registered. Of those, about 360 had airworthiness certificates. Those are the only ones I personally count as "flying" (though exceptions are common).

29 were ELSAs, 54 were SLSAs, the rest (273) were Experimental Amateur-Built.

Ron Wanttaja

So we've lost approximately 10% of the SLSA fleet, or are the others in the same sampling?

 

[KSCessnaDriver]

So we've lost approximately 10% of the SLSA fleet, or are the others in the same sampling?

It was my understanding that only 3 of the crashes were American registered, with the remainder being European aircraft.

 

[SkyHog]

I took a look at my January 2009 copy of the FAA registration database. There were about 480 Zenair 601-type planes registered. Of those, about 360 had airworthiness certificates. Those are the only ones I personally count as "flying" (though exceptions are common).

29 were ELSAs, 54 were SLSAs, the rest (273) were Experimental Amateur-Built.

Ron Wanttaja

That would indicate that the 1000 number may be pretty close to accurate, since those are only American numbers.

I really like the Zodiacs, and I'd hate to see irresponsibility on the part of the NTSB (yet again) cause the fall of a great alternative to building yet another RV.

 

[Henning]

It was my understanding that only 3 of the crashes were American registered, with the remainder being European aircraft.

Ok, so 3 out of 54 is what, about 7%? And this isn't the only sampling showing failure. That's a problem, and at this point in any certified application the fleet would be grounded until there was an answer which would require finding the problem. Again I ask simply, is this a level we are willing to trade off on, and what level of failure should trigger a grounding in LSA? What is the applicable standard? I find it interesting that the Germans retracted their grounding. Does anyone know if or to what level TUV is applicable to LSA?

 

[Henning]

That would indicate that the 1000 number may be pretty close to accurate, since those are only American numbers.

I really like the Zodiacs, and I'd hate to see irresponsibility on the part of the NTSB (yet again) cause the fall of a great alternative to building yet another RV.

What about the Zodiacs is it that you really like? What is it that makes you think that there isn't a design or construction issue with the factory S/LSA units as these seem to be the ones effected from what I'm gathering?

 

[wanttaja]

Those numbers are pretty close to what I saw. Just to make sure, when you say "601 type", you are refering to ALL 601 models (e.g. the 601XL, 601HD, 601HDS)? I am assuming so, since your total registered number is very close to the total registered number I found.

Yes, that's right...any variation on 601. I exclude cases where the model name include "Aerostar", or the aircraft manufacturer includes Canadair, PZL, Piper, Airbus, and Aerospatiale."

I took a look at my homebuilt aircraft accident database (1998 through 2006, inclusive), and found that the Zenair 601's fleet accident rate (number of aircraft accidents a year vs. the number on the registry) is pretty typical. No one cause seemed to stick out among the 19 CH601 accidents in that nine-year period. The pilot error rate was almost half that of the overall homebuilt fleet, but the sample size (19 vs. 1900) was so small I wouldn't put much stock in it.

Only one Experimental Amateur Built CH601 wing-failure in my analysis period, though it was in final year.

Which makes one wonder: What has changed on the Zenair that would induce such a spate of structural failures?

I know the stock wing of the CH601 uses the skin itself as the aileron hinge, and that Zenair eventually offered a "conventional" hinge arrangement. How were the wing-failure airplanes equipped? I'd heard complaint for years that the "skin-hinge" ailerons were stiff...but maybe that stiffness was helping the flutter situation. Do the E/SLSAs have conventional hinges?

The CH601 situation reminds me of two similar past cases involving homebuilts. The FAA suspended RV-3 airworthiness certificates in the early '80s due to wing failures during aerobatics (new certificates were issued banning acro). Six more failures over the following ten years or so led to another FAA notice. In the meantime, Van had developed the "B" wing to correct the problem.

The other case was the Bowers Fly Baby. About a quarter of Fly Baby accidents (14/52) have been caused by wing failures. The FAA issued a warning notice in the 80s.

Scary? Well, it's not quite so bad. Of the 14 failures, half involved aerobatics. Two involved airplanes that sat outside for long periods and suffered dry rot in the spar carry-throughs. Another was due to corroded turnbuckles. One crash was pilot error (didn't replace the spar pins after unfolding the wings), two involved improper maintenance, and another was due to a modification to the design.

So... despite a fairly bad record in the wing-failure department, I feel pretty safe flying my own Fly Baby. I don't do aerobatics and my plane sits in a nice dry hangar.

The spate of Zenith accidents bothers me, because we AREN'T hearing anything reassuring out of the post-crash investigations. The two US accidents were the investigation has finished both concluded that the accident was due to "structural failure of the wings for undetermined reasons." The factual report of the 2008 looks like they're heading the same way (overstress failures with no specific cause). Several accidents were in sight of people on the ground. Witnesses do not report aerobatics or abrupt manuevering.

Many of the Fly Baby accidents involved older aircraft, which you'd expect might harbor some mechanical problems. Fly Babies were dirt cheap in the late '70s, and guys bought near-derelicts in expectation of some cheap flying.

But the Zenith accidents are happening to nearly-new airplanes. N105RH was less than a year old. N158MD had received its airworthiness certificate just five months earlier. The most recent accidents in the US didn't happen to new airplanes, but both were less than three years old.

Admiral Jellicoe's famous comment seems apt, here....

Ron Wanttaja

 

[mcjon77]

To give a little perspective, I compiled a rough statisitcal breakdown of accident rates and fatalities among several of the most popular "fast" LSAs. Notice that I intentially left out the cubs, due to their different mission profile and performance specs. I limited these numbers to ONLY SLSAs (factory produced LSAs), not homebuilt versions. I highlit the 601XL numbers. The 601XL numbers include only SLSAs manufactured by AMD or Czech Aircraft Works.

evector sportstar
fleet size 94
accidents 11
fatalities 1
accident rate: 12%
fatality to accident rate: 9%
fatality to fleet size: 1.1%

Sport Cruiser
fleet size 85
accidents 5
fatalities 1
accident rate: 6%
fatality to accident rate: 20%
fatality to fleet size: 1.2%

Tecnam (all models)
fleet size 116
accidents 2
fatalities 0
accident rate: 2%
fatality to accident rate: 0%
fatality to fleet size: 0%

Flight Design CT (all models)
fleet size 289
accidents 18
fatalities 0
accident rate: 6%
fatality to accident rate: 0%
fatality to fleet size: 0%

Remos
fleet size 97
accidents 2
fatalities 1
accident rate: 2%
fatality to accident rate: 50%
fatality to fleet size: 1%

601XL
fleet size 54-70?
accidents 4
fatalities 3
accident rate: 6-7%
fatality to accident rate:75%
fatality to fleet size: ~5%

Jabiru
fleet size 87
accidents 0
fatalities 0
accident rate:0%
fatality to accident rate:0%
fatality to fleet size: 0%

Fantasy Air Allegro
fleet size 49
accidents 4
fatalities 1
accident rate: 8%
fatality to accident rate: 25%
fatality to fleet size: 2%

One of the things that I noticed in comparing 601XL to both other SLSAs and to Experimentals is that the accident rate itself is not that much higher than average for the class. What is higher is the fatality rate. when 75% of the accidents in the SLSA version are fatal and 42%of the accidents for the Experimental version are fatal (I think the average for all experimentals is ~26%), it may be a cause for concern.

 

[flyingcheesehead]

im afraid that this is going to create a stigma around the type. Unfortunate, but many pilots will know nothing about the Zodiac CH601XL except that the the ailerons flutter and then you die. I hope your group can do something to help eliminate that Jay.

Meh... Didn't do much to the Bonanza, did it?

 

[jmaynard]

29 were ELSAs, 54 were SLSAs, the rest (273) were Experimental Amateur-Built.

Uhm, I don't think so: the Zodiac has never been offered as an E-LSA kit. To build it as an E-LSA, you'd have to build a kit from AMD.

So we've lost approximately 10% of the SLSA fleet, or are the others in the same sampling?

Two of the accidents were SLSAs; the rest were E-AB.

 

[jmaynard]

Only one Experimental Amateur Built CH601 wing-failure in my analysis period, though it was in final year.

Which period was this? As I mentioned, there have been six wing failures, and only two of those (N158MD (AMD) and N357DT (CZAW)) were SLSAs. Of the other four, two were US E-ABs and two were European microlights.

I know the stock wing of the CH601 uses the skin itself as the aileron hinge, and that Zenair eventually offered a "conventional" hinge arrangement. How were the wing-failure airplanes equipped? I'd heard complaint for years that the "skin-hinge" ailerons were stiff...but maybe that stiffness was helping the flutter situation. Do the E/SLSAs have conventional hinges?

The AMD airplanes use piano hinges, and I believe CZAW did as well. It's not known what the other aircraft used.

The spate of Zenith accidents bothers me, because we AREN'T hearing anything reassuring out of the post-crash investigations. The two US accidents were the investigation has finished both concluded that the accident was due to "structural failure of the wings for undetermined reasons." The factual report of the 2008 looks like they're heading the same way (overstress failures with no specific cause). Several accidents were in sight of people on the ground. Witnesses do not report aerobatics or abrupt manuevering.

It's worth noting that the report on N158MD specifically said that there was no oscillatory overloading of the control surfaces - which, as I understand it, is a fancy way of saying there was no evidence of flutter. The NTSB exhaustively studied this airplane for over two years. Matthieu Heintz saidn in a message to the ZBAG mailing list last year that there was no common factor coming out of the investigations at all.

Admiral Jellicoe's famous comment seems apt, here....

Which is?

 

[Capt. Geoffrey Thorpe]

I do remember a royal metallic blue and white monocoque/stressed skin wings with skins held on by hardware store pop rivets.

I'm not clear on how you determined the source of the rivets.

John Thorp (with no "e") recommended a particular brand of Monel pop rivets for the T-18 as an alternative to A/N rivets. Personally, I suspect that Mr. Thorp generally knew what he was talking about. But not knowing what kind of rivets were used for the example you cite, and not knowing what kind of design analysis was done, I wouldn't attempt to speculate on whether or not the aircraft in question was likely to suffer a catastrophic failure.

Though, as long as we are on the subject of flutter - the T-18 had a problem with tail flutter that triggered a re-design of the counterweights and a doubler in the spar (IIRC).

There were also a few lost to failures (as in the whole blade breaking off) of cut down metal props (moved a prop resonant frequency on top of the engine firing frequency at cruise speed). Not something one would notice in a pre-flight.

 

[jmaynard]

I'm not clear on how you determined the source of the rivets.

Indeed. The Zodiac uses Avex flathead pulled rivets with a special head on the puller to form a rounded head against the skin. Other aircraft use aviation grade rivets as well. Can you tell the source of a rivet after it's installed, especially if it's been painted?

 

[GigG601XL]

But even if you toss that one out, it seems like there are plenty of other accidents that seem to be flutter related. I'd be pretty concerned if I were an owner, especially since these failures are happening below Vne and even Vno.



If FAA doesn't take any action on the NTSB letter, would that really be the case?


Trapper John

The designer has already given what he feels is a proper fix to the problem, proper cable tension.

And no if the FAA doesn't ground the planes the FAA vetting of any fix would not be required.

 

[GigG601XL]

So we've lost approximately 10% of the SLSA fleet, or are the others in the same sampling?

No, all of the accident were not S-LSA, some were erither EXP-HB some were foreign built versions and at least one of those was an un-licensed copy built in Poland.

Jay, there are E-LSA version that were granted their AW certificate under the now passed "Fat-Ultralight" exemption. Sabrina's is one of these.

 

[Henning]

I'm not clear on how you determined the source of the rivets.

John Thorp (with no "e") recommended a particular brand of Monel pop rivets for the T-18 as an alternative to A/N rivets.

Because I have driven and pulled nearly every type of rivet there is, and they all have features which allow you to distinguish what they are. If you have an aluminium rivet with no core in it and the "pop" point where the shank is pulled off is an a-symmetrical peak and when you look down the hole, it is skewed and misshapen like the pull isn't even and appears wedged open and crooked, that's a cheap ass hardware store rivet. There are perfectly acceptable blind rivets, no doubt, no argument. What I was looking at was not an example of one. A proper blind rivet will have a backer for the pulling stud so it pulls down the entire rivet back into a flat sealing mass. A hardware store pop rivet has a ball at the end of the stud that just flares the rivet outward rather than compressing it and exerts little to no clamping force on the skin. It's this clamping force that provides the friction between the skins that takes the loads in a stressed skin structure. So now we have a soft aluminum rivet with no core taking the sheer load of T'd aluminum sheet in sheer. Houston, we have a problem.

 

[Henning]

Indeed. The Zodiac uses Avex flathead pulled rivets with a special head on the puller to form a rounded head against the skin. Other aircraft use aviation grade rivets as well. Can you tell the source of a rivet after it's installed, especially if it's been painted?

To an extent yes. First off, most aviation blind rivets are cored in that the pin breaks off flush with the head and remains inside the rivet for strength against sheer. There is also a difference in the way the pin is built and pulls on the rivet. A rivet acceptable to me in this use will have a pin with a "T" head that pulls down on the entire mass of the backside and squeezes it all flat down against it's clamping surface creating a much larger contact surface. A pop rivet has a little conical type pull that pulls through the rivet body spreading it out until it wedges itself against the surface. This leaved the contact pressure at a small ring around the hole. So, if you know what you're looking for/at, yes, you can tell if they are of an acceptable type by looking at them after installation and even painting.

 

[wanttaja]

29 were ELSAs, 54 were SLSAs, the rest (273) were Experimental Amateur-Built.

Uhm, I don't think so: the Zodiac has never been offered as an E-LSA kit. To build it as an E-LSA, you'd have to build a kit from AMD.

The E-LSAs all have a "48A" airworthiness code. "4" is Experimental, "8" is "Operating Light Sport Aircraft." The "A" means the aircraft was registered prior to the "Fat Ultralight" conversion date of 1/30/2008. So the builders took advantage of the grandfather clause to register their planes as ELSAs rather than Experimental Amateur-Built.

Pretty smart move, I'da done it, too.

Ron Wanttaja

 

[wanttaja]

Which period was this? As I mentioned, there have been six wing failures, and only two of those (N158MD (AMD) and N357DT (CZAW)) were SLSAs. Of the other four, two were US E-ABs and two were European microlights.

My analysis period was 1998 to 2006, inclusive. N105RH is included in my database, N158MD was not due to it being an SLSA.

It's worth noting that the report on N158MD specifically said that there was no oscillatory overloading of the control surfaces - which, as I understand it, is a fancy way of saying there was no evidence of flutter. The NTSB exhaustively studied this airplane for over two years. Matthieu Heintz saidn in a message to the ZBAG mailing list last year that there was no common factor coming out of the investigations at all.

The difficulty is that the wings of *other* SLSAs aren't failing, which either tends to minimize the amount one can blame the pilot in the failures of the 601s, or indicates that the Zenair line has lower design margins.

While I'm reassured by the NTSB results of investigations of most the wing-failure Fly Babies, some of them still bug me. There's one several years ago where the conclusion was that the parallel wing bracing wires didn't have equal tension, leading to one to fail first. Pete Bowers at one point had actually *flown* the prototype with only single bracing wires. But it's certainly true that the G-limit would be reduced. This one was complicated by some amount of evasiveness on the part of the aircraft owner (another person had been piloting) as to who had assembled the aircraft (it had been bought as parts), who had connected the wires, etc.

The most recent one was another example. The airplane had a modified wing-bracing system, which had been a factor in past accidents. But there was no failures in that part of the system...in fact, the owner had pulled the metal parts off at the last annual and had them magnafluxed. No one witnessed the accident, but it occurred at a location where the pilot often flew aerobatics, and the plane did not have the modifications Bowers recommended (RECOMMENDED, not required) for acro. At one point during the earlier series of wing-failures, the FAA banned Fly Babies from aerobatics (my operating limitations ban them). But it sure would have been better to have found what failed first, and why.

It's nice that flutter is contraindicated in one of the Zenith accidents, but unfortunately it leaves folks with six fatal wing-failure accidents WITHOUT a cause. While I'm not comfortable about a couple of the Fly Baby failures, I, at least, do have NTSB conclusions to fall back on. But Probable Causes saying "The structural failure of the wings for undetermined reasons" are NOT reassuring.

People might remember the wing-failure accident of the first RV-8. Van went through a lot of work to show that the design of the wing was adequate, that the only way it could have failed was for the pilot to have overstressed the airframe. But it still left doubts. IIRC, the production kits used a different spar system.

Ron Wanttaja

 

[jmaynard]

The E-LSAs all have a "48A" airworthiness code. "4" is Experimental, "8" is "Operating Light Sport Aircraft." The "A" means the aircraft was registered prior to the "Fat Ultralight" conversion date of 1/30/2008. So the builders took advantage of the grandfather clause to register their planes as ELSAs rather than Experimental Amateur-Built.

Okkay, I stand corrected.

Pretty smart move, I'da done it, too.

Why so?

 

[wanttaja]

... So the builders took advantage of the grandfather clause to register their planes as ELSAs rather than Experimental Amateur-Built.

Pretty smart move, I'da done it, too.

Why so?

You picked *exactly* the right time of year to ask me that question. The annual condition inspection of my Fly Baby is due next month. Since I'm not the builder, I have to hire an A$P to do the inspection.

If the builder of my Fly Baby had been able to use the Grandfather clause to license it as ELSA instead, I could take a 16-hour Light Sport-Inspection course and do the annuals myself....

Ron Wanttaja

 

[Dave Krall CFII]

My analysis period was 1998 to 2006, inclusive. N105RH is included in my database, N158MD was not due to it being an SLSA.



The difficulty is that the wings of *other* SLSAs aren't failing, which either tends to minimize the amount one can blame the pilot in the failures of the 601s, or indicates that the Zenair line has lower design margins.

<snip> Ron Wanttaja

This has been what I've wondered about in this scenario too. It seems like a wing should be able to take quite a bit of flutter without falling completly off, even with harmonics entered in, unless it's just too flimsy. This model has been flying a long time it seems.

Maybe Jay will tell us how its wings and carry throughs constructed?

 

[wabower]

As a guy with no dog in the fight but who spends a lot of time around MX shops, I frequently hear comments about cable tension adjustments. None of them are regarded as life-or-death issues for the occupants, but that's the impression I'm getting for this airplane. Is that an incorrect assumption on my part? If so, who the hell wants an airplane that comes apart if cable tensions are bad?

The designer has already given what he feels is a proper fix to the problem, proper cable tension.

And no if the FAA doesn't ground the planes the FAA vetting of any fix would not be required.

 

[mcjon77]

...If so, who the hell wants an airplane that comes apart if cable tensions are bad?

Someone who already has $40K-$100K+ invested in it with little hope of recouping the cost if no other solution is found.

 

[wanttaja]

Someone who already has $40K-$100K+ invested in it with little hope of recouping the cost if no other solution is found.

*If* the problem is aileron flutter (and Jay says at least one case probably wasn't) I suspect the solution will be relatively straightforward. Not necessarily cheap, but it probably won't involve any changes to the wings themselves. I have a lot of respect for Chris Heinz.

There are a lot of emotional and legal factors involved in something like this. If there truly is a problem, I'm sure folks want it fixed right on the first try, rather than rushing a "solution" through that doesn't fix the problem.

Ron Wanttaja

 

[wanttaja]

This has been what I've wondered about in this scenario too. It seems like a wing should be able to take quite a bit of flutter without falling completly off, even with harmonics entered in, unless it's just too flimsy. This model has been flying a long time it seems.

Flutter is just plain bad stuff; I don't think there's really such a thing as "minor flutter."

I'm no structural engineer, but my understanding is that flutter is essentially self-amplification of movement...each swing results in an even larger excursion in the opposite direction. The amplitude increases until either the external forces causing the motion are reduced or the resonant frequency of the structure changes (usually because it breaks...).

In other words, one doesn't design the airframe to *tolerate* flutter, but one tries to ensure that flutter doesn't have a chance to start in the first place.

Flutter testing is usually performed by diving the aircraft, then pulling it up into a steep climb and "batting" the stick one way or the other. If flutter then starts, you hope that the fact that the plane is decelerating in the climb will smother the flutter before the structure yields.

Ron Wanttaja

 

[Henning]

*If* the problem is aileron flutter (and Jay says at least one case probably wasn't) I suspect the solution will be relatively straightforward. Not necessarily cheap, but it probably won't involve any changes to the wings themselves. I have a lot of respect for Chris Heinz.

There are a lot of emotional and legal factors involved in something like this. If there truly is a problem, I'm sure folks want it fixed right on the first try, rather than rushing a "solution" through that doesn't fix the problem.

Ron Wanttaja

I read through the NTSBs on the 6, and I'm a bit dubious on the flutter being a singular issue. Sounded a lot like some may have been excessive control force above Va due to the tail balance/stick force issues. I'm wondering about out of spec materials from the mill as well.

 

[jmaynard]

I read through the NTSBs on the 6, and I'm a bit dubious on the flutter being a singular issue. Sounded a lot like some may have been excessive control force above Va due to the tail balance/stick force issues. I'm wondering about out of spec materials from the mill as well.

Exactly. The three European-sourced aircraft (both European accidents were in airplanes built from kits supplied by the Czech Air Works (CZAW), as well as one American SLSA) might be due to materials changes from the Heintz design. After all, how do you shrink an aircraft designed for a 600 kg max gross down to a 450 kg limit? You have to make it lighter.

I'm not sure we'll ever know about the others. If the NTSB didn't find evidence of flutter on N158MD despite studying the accident for over two years, then I'd be greatly surprised if flutter was involved. OTOH, since every aeronautical engineer I've heard comment on the subject says that cable tension alone isn't sufficient to prevent flutter, I fully expect to have a balance weight design come out of Zenair, and to have to install it on my airplane by way of a mandatory AMD safety alert. Beyond that, I'm not expecting anything else.

 

[jmaynard]

*If* the problem is aileron flutter (and Jay says at least one case probably wasn't) I suspect the solution will be relatively straightforward. Not necessarily cheap, but it probably won't involve any changes to the wings themselves.

I expect the main change to the wings will be whatever is required to accommodate aileron balance weights. I haven't seen the design of the UK mod, but that will likely be what's used for the rest of the fleet as well.

I have a lot of respect for Chris Heinz.

So do a lot of folks. Indeed, that's the source of nearly all of the resistance to the idea that there needs to be a modification to the design within the Zodiac community: many folks can't accept that Heintz may not have gotten something quite right.

There are a lot of emotional and legal factors involved in something like this.

Oh man, are you ever right about that. The Zenith and Zenith601 mailing lists are war zones right now, with several folks hurling epithets at ZBAG for what they see as meddling where they don't belong and sending reports to NTSB instead of keeping the information within the Zodiac builders' community. This is exacerbated by the engineers (two separate groups) who have done the analyses keeping their work closely held rather than release it to the community; one ZBAG opponent has filed a Freedom of Information Act request to get copies of everything the NTSB got from ZBAG.

If there truly is a problem, I'm sure folks want it fixed right on the first try, rather than rushing a "solution" through that doesn't fix the problem.

Exactly. I only want to modify my airplane once. Until then, I'm comfortable flying it as I always have: conservatively.

 

[Henning]

Exactly. The three European-sourced aircraft (both European accidents were in airplanes built from kits supplied by the Czech Air Works (CZAW), as well as one American SLSA) might be due to materials changes from the Heintz design. After all, how do you shrink an aircraft designed for a 600 kg max gross down to a 450 kg limit? You have to make it lighter.

I'm not sure we'll ever know about the others. If the NTSB didn't find evidence of flutter on N158MD despite studying the accident for over two years, then I'd be greatly surprised if flutter was involved. OTOH, since every aeronautical engineer I've heard comment on the subject says that cable tension alone isn't sufficient to prevent flutter, I fully expect to have a balance weight design come out of Zenair, and to have to install it on my airplane by way of a mandatory AMD safety alert. Beyond that, I'm not expecting anything else.

I'm expecting a tail mod myself. The stick force issue is a major thing that will make it easy to pull excessive force accidentally.

 

[jmaynard]

I'm expecting a tail mod myself. The stick force issue is a major thing that will make it easy to pull excessive force accidentally.

The UK mod includes a bob weight on the elevator to address this issue. I'm not so sure it's a problem in the real world, though, as the breakups tend to happen in straight and level flight.

The Zodiac's elevator forces are lighter than other aircraft, especially when compared to its ailerons, which tend to be on the heavy side. That it has a negative stick force gradient above 2G doesn't concern me as much, simply because I, at least, have never pulled 2G in the airplane to begin with.

 

[Henning]

The UK mod includes a bob weight on the elevator to address this issue. I'm not so sure it's a problem in the real world, though, as the breakups tend to happen in straight and level flight.

The Zodiac's elevator forces are lighter than other aircraft, especially when compared to its ailerons, which tend to be on the heavy side. That it has a negative stick force gradient above 2G doesn't concern me as much, simply because I, at least, have never pulled 2G in the airplane to begin with.

That is not what I read. The witnessed ones all reported changes in altitude, coming into the pattern and maneuvering.

As for negative stick force progression, here's where that is going to bite you... You're going to be slow on take off it's going to be a bouncy and windy, you're going to clear the tree line and WHAM! you're at 2.6 gs on the gust and all of a sudden the stick is throwing itself at your gut and you're stalled at 50' trying to push the stick through center as a wing is trying to wrap itself around your ear in a spin and then all of a sudden the windshield will be full of the ground... and it will be close..... and then you will be dead.

Normal category aircraft are not required to be rated to 3.8gs for no reason. That is a real world number for conditions that may be environmentally induced as well as manually.

Thinking, "I will be ok because I never pull 2gs, well, that's just wrong on several levels including one of the "Dangerous Traits", rationalization. <2gs is an unacceptable limitation.

 

[jmaynard]

Normal category aircraft are not required to be rated to 3.8gs for no reason. That is a real world number for conditions that may be environmentally induced as well as manually.

The Zodiac is rated at +6/-3 G.

Thinking, "I will be ok because I never pull 2gs, well, that's just wrong on several levels including one of the "Dangerous Traits", rationalization. <2gs is an unacceptable limitation.

The aircraft isn't limited to that; that's merely a maneuver limit I stick to - not out of any reason to baby the airframe, but because pulling even 2 G for any length of time is pretty much guaranteed to get me airsick.

 

[Everskyward]

Flutter is just plain bad stuff; I don't think there's really such a thing as "minor flutter."

I'm no structural engineer, but my understanding is that flutter is essentially self-amplification of movement...each swing results in an even larger excursion in the opposite direction. The amplitude increases until either the external forces causing the motion are reduced or the resonant frequency of the structure changes (usually because it breaks...)

 

[wanttaja]

There are a lot of emotional and legal factors involved in something like this.

Oh man, are you ever right about that. The Zenith and Zenith601 mailing lists are war zones right now, with several folks hurling epithets at ZBAG for what they see as meddling where they don't belong and sending reports to NTSB instead of keeping the information within the Zodiac builders' community. This is exacerbated by the engineers (two separate groups) who have done the analyses keeping their work closely held rather than release it to the community; one ZBAG opponent has filed a Freedom of Information Act request to get copies of everything the NTSB got from ZBAG.

You think it's bad now? A while back, a certain homebuilt design had a wing fail during a high-speed low pass with an abrupt pull-up. The company did a load test to failure, and found that the wing withstood the design load with a comfortable margin. However, they found that some minor upgrades could raise the failure point by a half G, and changed the design.

You guessed it: One critic of the company called this an admission of guilt.

Some people say Cessna initially didn't act on the seat-track issue because they were afraid of what would happen in the courts.

Ron Wanttaja