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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Mr. Sire:

Seldom in this forum or any other, related to the 601xl, is clear, concise and cogent information made available to the likes of me ... a first time builder and low time pilot. There is much that I have to learn and Mr. Elliot's writing is hugely helpful in this regard ... made all the more valuable for it's lack of sarcasm and personal attacks.

Now, I fear, he has bowed out, not willing to do "battle" as opposed to engaging in constructive discussion. I sincerely hope I am wrong; for as a community we will have lost an educated, metered and civil "voice". I for one will feel cheated out of an opportunity to gain much knowledge, related to the 601xl - design,construction and flying..

I have never learned a thing from sarcasm and "nastiness" ... in fact, it tends to discredit the "speaker" somewhat. You can do better.
A bit premature to show pics of compression buckling and say that is the cause. Did it occur before or after. Its a guess at this point, no matter the credentials of the person taking the photos. EAB means " you built it, you assume the risks" This guy can keep flying, but doesnt want to be confident he built it right? I dont care much about the degrees, its how the plane is built that makes the aircraft sound. The studies show if built correctly, this is a sound , safe aircraft.

Yep, I have decided to place one (1) 6B16-5 on the top of 6B5-2 to support pull and twist. I also
sugested this to Roger at Zenith on Nov 2,2009.....
Its fun to go to a major air show like OSH and look at the many successfully flying airplanes. The variations in design are mindboggling. It is clear there are many ways to create a successful airplane. Really it all comes down to optimizing a design for an intended purpose. The Zodiac XL is one such optimization. I suspect the unmodified Zodiac XL comes very close to what the designer had in mind. So in that regard perhaps the airplane is a success.

I think there is plenty of evidence that if the plane is built and flown as intended by the designer that it is a very safe, reliable and enjoyable airplane. I certainly have found that to be the case, so far anyway. And yes I am still flying my airplane, 637 hours and counting…

I am a firm believer in good science. I have no training in airplane design but have taken my share of science classes in high school and college so I understand the scientific method. The problem is we don’t life in a world dominated by science. It’s filled with chaos, politics and people doing what people do, if you know what I mean. In the real world, I think a person could argue that the Zodiac XL design is less than perfect. I say this because it would appear that the design could have been made much stronger in a few critical areas with a minimal weight penalty. The stronger airplane might better withstand abuse and unexpected conditions. Perhaps the mess we are in could have been avoided with as little as five pounds of strategically placed aluminum. We can’t know that of course, I am speculating. If true, that is an optimization I would have preferred to the one we were given.

It is easy to get upset at the “process” we have been through. It has been far from optimal; lots of wasted time, money and perhaps lives. For those that want to rant about that, I understand. I hope they do it away from me because I see it as a distraction and a waist of my time. We are where we are. The mess is made. It’s now a matter of what to do. That brings us back to optimization which I assert is a personal issue. Each of us is in a unique place with our own unique airplane registered in a particular way. I don’t presume to tell others what they should do. Speaking for myself, even though my airplane is registered such that I can continue to fly it as is, I have decided to upgrade it. Why? Simple. If before I acquired my airplane the factory as offered me a choice: “Steve, would you like an airplane that is 20% stronger in critical areas but weighs 10 pounds more?” my answer would have been “yes”. Why? Because I prefer that optimization. I can afford the time and money to change my airplane so I will.

Now I do realize that some of the modifications are somewhat political and redundant. For example we are strengthening the bell crank bracket, I presume to help keep the cables tight. We are also adding counter-balances to the ailerons. Both of these changes seem to address the same issue. It is not clear that either of these changes is needed. It for sure seems like both changes are not needed at the same time. If the ailerons are balanced, why should we be so worried about the cables being tight? I think there is quite a bit of politics here. To a perfectionist I am sure this is highly annoying. In the world we live in I suspect most of us will just go along for the ride and ignore these non-optimized solutions. The weight penalty is minimal. I think some of this overbuild is necessary because it provides legal cover for the family – hay that’s the world we live in. It is annoying that we may end up flying airplanes that are “overbuilt” in a few ways that may in fact be pointless. On the other hand having spent three days working on Doug’s plane at QSP I am not sure that picking and choosing amongst the modifications has much benefit, therefore I plan to implement them all. Just for the record, the modification I see the least point in is the aileron balance, but I am going to do it anyway “just to be done with it” so to speak.

So Andy, you have an airplane you can literally do anything or nothing to. You are free to optimize as you see fit. You seem to have the training to make choices for yourself and in that regard you are perhaps fortunate. Because you like your airplane as is, why not call the insurance company to learn their point of view. Perhaps they don’t care if you do the modifications or not. If they do, perhaps they can give you a specific cost comparison and you can move forward based on that information.

So here we are, annoyed with the world, but trying to make the best of it.

I propose that without a specific identified structural weakness or accident cause, that all the proposed changes are, in fact, "shooting in the dark". Making one part of the plane stronger may, or may not, address the real, and still unknown issue.

If that issue is poor pilot training, bad material, poor construction or maintenance practices, and/or operating outside the envelope, it is possible that these modifications will fix nothing. They may move the failure point somewhere else.

However, if my airplane had exhibited symptoms of structural problems in the past (like smoking rivets) and if I had operated my plane for some time before addressing the problems, or if I did address them and they later reappeared, then looking carefully at making some mods might be a great idea.

I would say you are being a bit black and white about this. I think the modifications are not quite shooting in the dark. They are perhaps more like shooting in the twilight.

An enormous amount of analysis time and even some honest-to-god science has been thrown at the problem. By and large the modifications do target areas of weakness in the design. That is not to say the original design was too weak, only that the airplane has some points that are weaker than others and those points are being strengthened. Of course even after the upgrades, our planes can be made to fail. It may even be the case that the failure rate will continue in exactly the same way and that some other accident causes will be spotlighted. How else do you propose to learn the cause? Do you have a better plan for the fleet given that lives are at stake? In my experience I have found that it is easy to criticize a plan; it is much harder to come up with one that cannot be criticized.

What is clear is that a lot of airplanes have been grounded and no one knows for sure what is wrong. I for one wish the grounding had not occurred but perhaps lives will indeed be saved – I can’t know that.

The upgrade is apparently being crafted to address all known weaknesses in the design so that builders can resume flying their creations. So in my view, what we have is a near best-effort to correct a largely unknowable problem. It is clear that the airplane will be stronger, so there is some benefit. For example, I see specific changes to address the smoking rivet problem I am aware off. I am glad for that.

Poorly trained pilots will always be with us, likewise inferior construction and maintenance practices. There will always be those who abuse their airplanes. That said, there a few airplane designs which cannot benefit from change and I don’t think the Zodiac XL is an example of such an airplane design. I actually like many of the changes be forced upon us. I just wish they had been put into the original design. After the changes have been made I will know my airplane is stronger and will be better able to cope with some mistake I might make or some random wind shear that I might encounter. I will be happy to have that.

Andy, because of the way your plane is registered, the changes are not being forced on you. If you see no value in them, don’t do them! I will not call you a fool. I see nothing wrong with your personal choice.

Peter Morris, from the UK, provided these corrections. Thanks!
- - - - - - - - - - - - - - - - - - - - - - -
Whilst I would agree with many of the points raised there are some fundamental
inaccuracies in your 'information' regarding the UK aircraft. (I have tried to
sign-up with but for some reason it will not work).

None of the UK aircraft are microlights - they are all experimental, with a mauw
of 1232lbs. The rest of the European population of xls are generally microlights
with 'self declared' weights of 1050lbs. (self declared is important because
there is no way an xl could actually met the microlight empty weight limits!).

None of the UK aircraft fit this category.

The European aircraft which crashed and caused the UK (and Germany, Dutch groundings)
was a microlight version.

The UK aircraft are not built from much lighter materials than the US aircraft
(although the Czech landing gear is much lighter), all the thickness' are the
same as the US version. However the 600kg US version already has a number of
areas where the thickness' have already been increased from the 560kg version
- Zenith's reasons for this are various (and to my mind not convincing!).

The LAA modifications were based upon an independent engineering analysis and the
need to make the mods which could be easily retrofitted. They have resulted
in restrictions to the maximum fuselage weight of 1086lbs. This is to meet
the requirements of CS-VLA at 3.8g - not 6g! So the LAA analysis shows that the
center section did not meet the CS-VLA requirements.

All of the UK aircraft are supposed to meet the requirements of the CS-VLA, this
states that flutter must NOT BE possible and so the LAA insisted upon the counter
balance because the analysis showed that if aileron tension was lost, flutter
could/might result.

To suggest that the CAA and LAA were not under political pressure is naive in the
extreme. The Dutch accident inspectors have yet to report, but the Dutch airworthiness
authority have repeatedly written to the other European authorities
stating the airframe is not strong enough........ The LAA have also been hanging
off all of the reports coming from the US, particularly the recent load
test in Canada.

>From my point of view this has left things as a bit of a shambles. There are
now multiple populations of xls at different stages of modification to differing
design codes (or in the case of US experimental no design code).

Tonight when I get home I should have a new 'permit to fly' for my xl siting in
the postbox.

As a UK homebuilt, built with a rigorous independent inspection process, it has
been grounded for over a year largely on the basis of uncertainty. The checks
of the design have shown it did not meet the CS-VLA code and it now does (apart
from trimming in flight).

So I now have a legal in the UK xl. I can only wish you all well in meeting the
various and now much differing requirements for your aircraft.

Pete Morris
Andy, Thanks for the thoughts.

This is one of my favorite quotes. “Carelessness and overconfidence are more dangerous than deliberately accepted risk” Wilbur Wright, 1901

I will lend my 2 cents. I a retired AF pilot, ATP, and have been an accident investigator, teat pilot, and am currently a System Safety Engineer for one of the Major Aerospace Company's. I think you have given a very thoughtful discussion. We each need to make decisions in this manor. The idea is to manage one's risk. This means a deliberate evaluation of the factors that go into a flight. When you look at accidents during flight test we find that most of the incidents occur flying the mundane points or flying to and from the test area. It is complacency that kills. The fear I have is that we are all focused on this risk. But there are many more out there.


Thanks, Matt.

Risk management and risk mitigation are key concepts in this kind of engineering. Understanding risk is extremely important because risk elimination is just not possible. I don't think these concepts are covered well in GA flight training, although they are of very high interest to the airlines and military.

147 hours...
I agree. In the accidents I have investigated there is rarely a single cause. There are many links in the safety chain. The idea is to understand the chain and reduce the risk. If you want to eliminate the risk then stay in the hangar.

I do feel that the mod has some merit. Especially when I go to sell it some day, and the cost is reasonable. I have not polished the aircraft yet so this will be a good time to do both. However, it is not the only way to mitigate risk, but it is a realistic step for me.

This is a great little aircraft and I have thoroughly enjoyed flying it. However, the pitch response is greater than in a 172. If you do not trim properly, you can get in a PIO fairly easily. I was a Tweet IP at Sheppard (USAF and NATO) for a while and taught many people how to fly. Trimming is one of the skills that I do not see many low time pilots mastering. Trimming is like breathing. You should not have to think to do it. When my students would figure it out, their aircraft control increased dramatically. Where a 172 is very forgiving, I can see how some might get in a bad corner in the Zodie. This is not a bad thing, just different. You can pull the wings off a 172 if you try.

My advice to mitigate risk is to learn how to trim, stay in the envelope, watch the weather, and make sure you have enough gas. These will probably me more effective in reducing risk than this mod.

Interesting points you raise about the flying qualities. I also found the controls to be very poorly harmonized when I test flew other 601XL's. Particularly, I found that the ailerons were far too heavy and the elevator was far too light. PIO seemed to be a possible problem to me also. Risk reduction seemed in order.

So I fixed this when I built my own plane. First, I took advantage of the service note about reducing the elevator travel by 25% (from +/- 30 to +30/-15) to similarly reduce the stick sensitivity by the same amount. With my 2-stick airplane, this was as simple as drilling another hole in the control arm that connects the sticks. BIG improvement in handling, no apparent effect on controlability.

In addition, I added a pure damping mechanism to the elevator control system which reacts against rapid stick movements. When I finished the installation, I had +27/-15 available, which I have found to be more than enough elevator travel.

With respect to the heavy ailerons, I added spades to the ailerons and adjusted the size to reduce the aileron force by about 1/2. (I didn't want to have aerobatic aileron response, just lighten them.) I took advantage of the mounting arms of the spades to add some counterbalance weights which further improved the feel, and increased the flutter margin by some unknown amount. This was all before the big hoo-hah about flutter. Pix of these are on my web pages at

Many people have flown in my aircraft, including Sebastien Heinz, and commented about how nice the airplane felt. I agree! And when you don't have to fight the airplane, you're much less likely to do something dangerous.


Hello Andy

Our High River Group Flying Club built and now fly a Zenith601 XL-B. We are very interested in  your comments about reducing the elevator travel to address the pitch sensitivity . I have tried to find the pictures that you refer to on of the changes you made to the elevator control cable, however have not been able to find them. Is there a measurement that you determined in order to drill an alternate hole in the control arm, which reduced the stick sensitivity? Did you do the same for both cables or just one? Upper or lower?

Hoping to hear back from you.

Debra Dueck 


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