The discussion here is directed specifically toward concerned builder/owners of 601XL aircraft already certified in the Experimental-Amateur Built category. This discussion does not apply to S-LSA or E-LSA aircraft, and is probably irrelevant to not-yet-certified E-AB aircraft unless the FAA changes its directions to the DAR community. It also is not meant for second or later owners of Experimental 601XL’s, who may have much less knowledge about their specific aircraft’s construction.

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My name is Andy Elliott. EAA #687140. I have a flying Corvair-powered 601XL taildragger constructed from a quick-built kit (from Zenair in Canada). My plane is certified Experimental – Amateur Built. My first flight was 24 Nov 2008 and the plane now has about 145 hrs on it, including a summer trip to Oshkosh from my home base in Mesa, AZ.

I have an ATP and something over 4000 hours flying time, more than half of it military, in both fixed-wing and helicopters. I did my B.S. at MIT (1974) and my Masters and Ph.D. (1987) at the Univ. of Maryland, all in Aerospace Engineering.

And I am really upset by the way the FAA, NTSB and Zenith have handled the hypothetical problem with the 601XL design, and am trying to figure out what to do, if anything, to my aircraft.

I have uploaded here an open (long) letter to the builders/flyers of already-certified E-AB 601XL's (Open_Letter.doc). In addition, I have also made this available on-line at I would like to invite builders/flyers of already-certified E-AB 601XL's to read this letter and add comments here.
Andy (or should I call you Dr. Elliott?), Excellent letter! With your credentials, it's even better. Thank you for clearing up some questions I had about our E-AB's. I'd really like Zenith to comment on this.
Thanks, Tim. If there are specific areas you have additional questions about, be sure to raise them. We need as much open interchange as possible.
Well defined thoughts and reasoning Andy. Just today I was speaking to another flying XL owner, who like you tested his experimental airframe and set the limits in his POH. He continues to fly and inspect his airframe on a regular basis and sees no need to make these changes. His experience and instinct tells him it's not necessary if he stays within the limits. I have flown in his plane and believe he is a reasonable man. As you pointed out even the designer himself agrees there is not a problem with the structure, just bending to the pressure of the legal team and governing bodies.

My experience is metal working and testing structures. I am scratch building and noticed as I was building the wings and center spar, they are strong. That is one of the things I liked about this airplane. Could they be stronger? Yes, but for the type of flying I do that won't be necessary.

I have witnessed the hysteria over these unexplainable crashes on the internet and noticed much of it in the NTSB's letter to the FAA. The loudest noise is for an answer as to why these things happened. Sometimes there is no answer.

Many of these planes are flying and have accumulated many hours of flight time. I would like to know what that total number is. With that information and the total count of planes we could at least make some statistical insight as to the 1st question "Is anything really wrong with your plane?".

What I have seen so far is the result of no science but much emotion.

Now here I sit with a completed set of wings that have been inspected by an A&P/I and an EAA Tech. both of them and many others have commented on how good the work is. From what I've read so far I still have a choice, name it anything other than a 601XL, plan to self insure. The naming is not a problem but I would like to get this thing in the air with an AW and insurance.

Just to hedge my bets, I have ordered the upgrade, really not that much $, but it will be a lot of careful work required. I have time, and hope we can make a difference by pointing out the total lack of science in what this bureaucracy is declaring we must do.

I had not thought of this option for still-in-progress aircraft. Assuming that the aircraft was scratch-built , could a person name the aircraft something other than "601XL" and get a DAR to do an inspection based on the standard criteria, i.e. that he in fact built the plane, it is not a glaring death trap, and and that his paperwork is in order? The insurance would probably be the high rate that associated with a one-off custom design, and hull coverage might not be available, but liability should be.

Dr. Elliot,

Given your extensive military and academic experience you no doubt are trained to thoroughly research any position you take. So, I assume that you have carefully studied all of the public documents available on these crashes. Yes? I am interested in your explanation of what could have caused the compression buckling identified by the NTSB in three documents posted this October to the Polk City, FL crash public docket. While you obviously have seen them, I post them here for others who may not follow as rigorous a regimen as you.

The documents show a consistent pattern of compression buckling on the top and bottom of the rear spar in the area where the flap and aileron meet.

In your opinion, since you believe flutter is a hypothetical problem, what force could have caused this buckling?

Doug Sire

Your question is posed in an interesting manner. It is fairly clear that these wings failed due to gross overstress. The question should not be "what force" caused the failure. Any aircraft structure can be bent if excessive loads are applied. The question should be, "Is there any reason to think that these structures failed under loads within the allowable flight envelope, assuming the structures were built and maintained properly?" The NTSB was not able to establish a consistent failure more or cause, so the answer to that question is still, IMHO, "No."

That being said, I did not say that flutter was a hypothetical problem. Flutter is a real problem and every aircraft designere has to be concerned about it. Again, nearly any aircraft wing can be made to flutter at a high enough speed. (Although some will diverge before that - a different kind of instability.) What I did say, and what the extensive German study found, was that for the 601XL, if the cables are tensioned properly, flutter is not possible within the allowable flight envelope.

FInally, before all these studies were done, I personally felt that the location of the aileron pushrod exit hole in my aircraft was kind of close to the rear spar flange. I have 1-piece rear spars without a splice in that area. Because of this concern, I added a small strap to the bottom spar flange in that area. While this strap does provide some additional strength, it's main purpose is to be a tell-tale for any indication of high loads in that area. I check the strap every pre-flight for any indications of working rivets or cracking, hypothetically giving me some advance warning if there should be any concern.

This strap is shown (and has been there for more than 2 years!) at:

Well actually, I think the question really still should be "what force". Saying that it is overstress is painting with a pretty broad stroke. Let's narrow it down and look at just one photo. Please refer to the Markermeer (Dutch Antelope Island document) accident photos. Look at the top photo on page 4. This is taken from the inside looking aft on the left wing at the aileron rod pass through hole. To my poor, pathetic brain, I can only imagine that the buckling damage could have occurred by a force exerting downward force on the end of the wing. This aircraft was reported to be in straight and level flight over a lake at the time of the crash. Please walk me (slowly please) through a detailed and plausible set of actions which could have caused this.

It's not possible, looking at just photographs of a crumpled airframe, to determine what damage was caused in what sequence, nor what occurred in the air and what upon ground impact. Even for investigators on the site, this is very difficult if the plane parts are together. Usually, it requires careful metallurgical analysis and even then it sometimes is simply not possible.

Ground impact would be my guess, but if you want hypothetical scenarios for level flight (Which is only a supposed condition.Ground observers are notoriously unreliable and there is no flight data recorder) - (a) 30 deg rapid down elevator at high airspeed? (b) wake turbulence encounter? (c) loss or failureof rear spar attachment bolt? (d) control cable failure? (e) extreme gust at high airspeed? (once ripped the vertical tail off a B-52) (f) attempted suicide? (g) previously damaged due to ground winds without control lock?

I could go on and on if you expand this from "events" to "scenarios". That's the problem! Not enough data for anyone to make a determination, not even the actual investigators. And that turns out to be the case for a lot of accidents, even airliners. They are never attributed to a specific cause, and don't lead to AD's or SB's or anything except hand-wringing.


You are doing a great job of dancing around this; your money with Fred Murray was well spent.
I ask you to again look at the photos, and tell me whether or not my theory of flutter might be plausible.

Look again at the document posted earlier showing the compression buckling on the Dutch accident plane. Earlier I pointed out the left wing bottom rear spar compression buckling on page 4. Now go up to the top of page three and note the upper left wing buckling. Note that both the buckling on the upper and lower wing has an outward buckling of the skin. I can only envision that occurring from a bending force from the end of the wing in the direction of the buckle (wingtip moving up to create the outward bulge on the top skin and down to create the outward bulge on the bottom skin).

Both of the bulges are at the intersection of the flap and aileron. This is the weakest spot on the rear spar. There is a steel hinge and box structure (flap) inboard of this, and a steel hinge and box structure (aileron) outboard of this spot. And of course add in a hole in the rear spar for the aileron control at the same spot. The tension of the control cables also adds the rigidity of the inboard section as opposed to the outboard section.

So my question is, could flutter which began in the outboard section of the wing (either aileron or entire wing tip) have caused an up and down oscillation of the outboard section of the rear spar against the rigid inboard section of the rear spar and created the compression buckling damage shown? A simple yes or no would do.

Let’s take a closer look at some of the possibilities that you have proposed for this damage occurring to the Dutch aircraft:
1.Ground impact – Actually it was water impact for this airplane as it crashed into a lake, the Markermeer, just south of the village of Hoorn. The initial safety board report on this, copy attached, stated that the right wing folded back and the aircraft crashed into the water. As you can see from the photo on page 2 of the NTSB report, the front D cell of the left wing is crushed pretty evenly across the span of the wing. Maybe if it had impacted solid ground more on the front wing tip it might have caused some type of buckling like this, but we would also expect that it would bend the outboard section of the wing back and cause the aileron to hit the flap at the back. But the photo at the bottom of page 3 doesn’t show that to be the case. And possibly if the wing landed just perfectly directly at the tip of the rear spar it might have caused that type of buckling, but that obviously did not happen. And if a wing did land on the wingtip perfectly like that, the aileron would be pushed into the flap.
2.Ground observers are notoriously unreliable and there is no flight data recorder. Actually there was a flight recorder (GPS), and the safety board report indicated “After read out of the GPS unit it appeared that the aircraft did not execute any extreme flight manoeuvre and, shortly before the instance of the accident, conducted a straight and level flight.”
3.30 deg rapid down elevator at high airspeed. See above
4.Wake turbulence encounter. There was no report of any other aircraft in the vicinity
5.Loss or failure of rear spar attachment bolt. As you can see from the photo on page 2, the left wing is still attached even after crashing into the water and taking the full force of the impact and being hauled ashore.
6.Control cable failure. There is no indication of that and the photo at the top of page 3 shows the cables still attached.
7.Extreme gust at high airspeed. From the Dutch report, “Information provided by the KNMI established that the wind at 1000 ft was 070 degrees at 11 knots. The visibility was more then 10 kilometers. No turbulence was reported.” Barometer was 30.26in. A nice day for flying. A complete historical weather report for Amsterdam (20 – 30 miles from the accident site) on that day can be found at:
8.Attempted suicide. Well, in this case it would be murder/suicide. I find it rather arrogant that anyone would speak so ill of the dead and propose that in order to defend their own argument. Shame on you.
9.Previously damaged due to ground winds without control lock. Ah yes, a variation on the “bigger fool” theory. That compression buckling looks pretty obvious to me. Do you really think the pilot would have missed it on preflight? Remember, this flight lasted all of 17 minutes.

So, of all the above theories, do you think the flutter theory is a plausible explanation for this accident?

Some other comments on your letter:

“The FAA has found no common factors in these accidents that they can identify as a cause.” Well, actually the NTSB did find a common factor, compression buckling along the rear spar, as is evidenced in the three NTSB documents posted earlier.

“We also know, because they say so publicly on the Web and even publish videos on Youtube (!), that many people are flying the plane way outside the designer’s intent.” The obvious inference you are making is that maybe the aircraft involved in the accidents may have performed these type of dangerous maneuvers. Could you please direct me to the Youtube videos which show that? I haven’t been able to find them.

“Zenith’s primary concern is with the S-LSA version of the 601XL. In order to offer the plane as an S-LSA aircraft, they have to meet the ASTM Consensus Standard for Light Sport Aircraft. In this standard, the requirement is that the aircraft have operating limits of +4/-2 G’s (+6/-3 ultimate) at the design gross weight, disregarding fuel in the wings.” Do you have proof that Zenith complied with the ASTM requirements before making their recent modifications? Because we haven’t been able to find that.

Time to defend your theories…..

Doug Sire
This is last round from me here, because it is getting to be too much like the other flaming discussions.

OK, first here's a video link:
Any web search for "loop 601XL" will return it.

Second, I will state my opinion that if the aircraft was constructed and maintained properly, and if it was operated within the flight envelope, and if it had control continuity and spec control cable tension, it is not possible to flutter the ailerons. That's a "No."

Third, I said "common factors that they can identify as a cause", not just common factors. In any crash, for example, a common factor would be failure to maintain adequate terrain clearance. Important not to confuse cause and effect.

Fourth, I did not say that Zenith complied with the ASTM consensus standard. I only said that they were supposed to have done so. And compliance might not necessarily have to be done by testing. I don't have a copy of the expensive standard, so I don't know. For sure I, as the manufacturer of an E-AB aircraft, do not have to comply.

Fifth, while investigating accidents in the military, I personally did do one that was apparent suicide and another that in the civilian world might have been called negligent homicide. These were professional aviators, and these things do happen. The theory is at least plausible.

Finally, it is pretty darn clear, even to a prevaricator like me, that the aircraft executed at least one extreme maneuver. GPS low-rate recording is not a flight data record.

Anyway, enough of this rampant conjecture. Anyone got any facts?
Dr. Elliot,
I'm left to conclude that facts would only get in the way of your convictions.


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