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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There has already been quite some discussion of this in the original Zenith e-forum. I'd prefer to keep this focused on the existing situation. Unless you think that somehow they are related?

148 hours...

Thats absolutely fine with me and probably even better. I tried to locate that original Zenith e-forum, but couldn't find it.
Could you point me to it?


Great "letter". I have not stopped enjoying the sky in my XL and now have over 547 hrs. I will not be giving any more Young Eagle rides unless I mod the airplane, not though that I feel the airplane is unsafe.
I added a special supplement to my POH lowering my Gs (I did have it at +/- 4Gs), pending any incorp of the SAIB /Zenith structural beef-ups.
I still feel that improper stick input, helped by the light stick forces, have been the root cause of people removing their wings. I am especially alert around this part of Tennessee to keep my airspeed to Va below 2000ft AGL because of the birds.
One of the accidents had the wings failing in a negative direction, along with a Horiz Stab failure. This, type of wing failure, as you would know, is typical when you lose your horiz stab. The horiz stab may be the next candidate for beef up. Not a criticism of the XL - a great design would have everything failing at once at the design limit load. (hey, in the future, maybe, unlimited acro).

Tony Graziano
Aerospace Engineer - retired

Gee, Tony being a Aerospace Engineer, did you not see that the main spar was not
attached and or directly tied any of the longerons or anything besides the floor? Dang!
Some interesting data for us to consider: My airplane (148 hrs) is currently down for the annual condition inspection. Since I am a careful maintainer, I didn't expect to find anything of note, but I did find a two things others might want to look at:

[1] I used Torque Seal liberally during the build to make loosening fasteners for visible. All twelve of the main wing bolts had Torque Seal spots that are clearly visible during preflight inspection. During the condition inspection, I found these to all be intact, i.e. the bolts and nuts had not loosened during the year. But since my checklist says check the torques, I got out the torque wrench, opened the seat pans, and found the body position where I could get to the bolts and nuts.

BUT, despite the Torque Seal spots, all 12 of the bolts took additional tightening to get the wrench to click. Some as little as 1/4 turn, and some more than 1/2 turn! I am not sure if this is important or not, since none of the bolts were actually loose. But definitely things had relaxed a bit during the year.. On the other hand, the single bolts for the rear spar attachment were all tight and OK.

Put new TS spots on everything (different color for each inspection year) and closed things up.

Folks - You may want to check these bolts in your plane if you have more than 50 hours on it and have not done so.

[2] Cable tensions on the elevator were unchanged. Cable tensions on the aileron cables had dropped from 30 to 25 pounds. Not a real concern, since my ailerons are counterbalanced already, but interesting and I'll check them again in another 50 hours. Maybe due to the same "settling" that effected the spar bolts?

[3] No working rivets, cracks or any other indications of fatigue in the rest of the plane.

Andy Elliott
601XL/TD/Corvair, 148 hrs.

For those not familiar with Torque Seal.
Andy 1/2 turn is not a small amount! I presume you are using a quality torque wrench...

Last annual my 12 bolts were loose also. It seems unlikely the metal in the spar is compressing so that to me suggests the bolts are stretching. Sure Andy, I know you hate jumping to conclusions but the options here are a bit limited.

Because the bolts provide much needed clamping force this is a bit worrisome. I would say check them again in a month or two. If you find they are "loose" again, replace them. Repeated tightening of a stretching bolt is a loosing game.

Those of us who are upgrading are getting stronger bolts. Perhaps those who are not upgrading should also get stronger bolts.

Agreed! I'll make that check on the next oil change (30 hours) and report the results. I hope everyone that does check their bolts will also report the results.

In the search for an identifiable cause for some of the accidents, I think it is critically important that owner/flyers share this kind of information, as you did with your smoking rivet problem. If we keep the information flowing, we may finally discover something useful.

Even hypothetical scenarios which lead to reasonable risk reduction strategies are useful to the group. From reading lots of accident reports over the years, we all know that most accidents are not the result of some single, unique event, but are instead the result of a series of events leading the to critical one. The big difficulty for us, IMHO, is identifying which phenomena are causes and which are effects, without lots of good data.

This is a HYPOTHETICAL scenario for folks to ponder:

[1] Given - The main spar bolts are often a bitch to get in, even with "match drilled" factory spars. This can involve a lot of jiggling, twisting and hammering, even when done carefully by experienced mechanics. (In my own case, one wing went on in 15 minutes, the other took 1.5 hours!)

[2] Given - Some of the main spar bolts are very difficult to install in the "desired" direction (per plans) and some are hard to get a torque wrench on the nut.

[3] Reasonable inference - Some builders (including the factory) may not have pristine bolts and holes when they finish installing the wings, and some bolts may not be properly torqued.

[4] Given - Some operators (how many?), including those who are careful and meticulous, have discovered on inspection that spar bolts have lost torque.

[5] Reasonable inference - Some operators probably have (or had) spar bolts with improper torque, and have (had) such for various, possibly long, periods of time.

[6] Hypothesis - Improperly torqued spar bolts could lead to relative motion between the wings and the center box. IF not corrected, this situation could get progressively worse over time (How much?, How fast?) and change the load path from the wings to the center box. IF this happens, the overall structural integrity is compromised, since the designer's calculations are made invalid. In that case, and without answers to the "How much?" and "How fast?" questions, anything could happen.

[7] Risk reduction strategy - Check the bolts now and retorque if necessary. Re-check on a regular basis. If the bolts will not hold their torques, replace with stronger ones. If replacing the bolts, be sure to do it correctly, to get proper fit. Continue to inspect wings and fuselage for any indication of any working rivets, a primary indicator of relative motions.

[8] Information request - If anyone does replace the bolts, take pictures and measurements of the old bolts and let everyone know if you find anything important, like cracking, damaged body, damaged threads, etc.


Physics and Torque Seal win, and Andy gets one "dumb mechanic" point. The reason the bolts turned so much, despite the original Torque Seal being undamaged, was me setting the torque wrench incorrectly. Many thanks to Steve Smith for reminding me to check the wrench, which should have set off the bells in my head, and to Jim Timm, who reminded me to use my glasses to read the torque table, instead of my memory.

Anyway, I apologize for the hub-bub. (Checking the bolt torques is still a good idea, though.) New bolts and nuts are on their way from AS&S.

Glad to have friends!

Thanks for that update. Its good to know the bolts are probably OK and the mechanic was "faulty" :) which in this case is better than the opposite being true.

As to why my bolts were loose I can never know because I did not put the wings on my own airplane and no Torque Seal was used.

I enter this discussion with some trepadation. I previously owned a 601HD and now a 601XL with the UK modifications.
To those who are still 'in denial', I can only state the respective accident rates of the HD versus the XL.

Careless pilots are only attracted to the XL, giving the HD a clear berth? Like it or not the XL has an unfortunate record. I have been watching the various forums from a little before I bought my XL. I noted the comments about the rear spar aileron area bt I was re-assured that the UK 's LAA would only approve the design if the calculations had ben checked - it is a very different environment to that in the US.

Well they had not, they accepted the checks that the LBA, the German airworthiness authority had carried out. When this all flared up in Europe with the Dutch crah the LAA did a full analysis. The result? The airframe did not meet the strength requirements of JAR CS-VLA, the design standard (very similar to FAR 23) which the LAA have to check the design against before they are allowed to let them fly in the UK.

So the strengthening of the centre section, the aileron balance and the removal of the flap / elevator spring assist. And a restriction on the maximum fuselage load - to ensure the code requirements are met. Even with all that the LAA were waiting on the results of the recent Canadian load tests - if the airframe had not passed that test we would probably have remained grounded.

So I know know my XL meets the VLA requirements.

Given that virtually the only common feature of the wreckage photos I have seen is the buckling of the rear spar I still have some concerns over this area - as does Chris Heintz - not only do the US modications beef up this area, but the European modifications - intended for the microlight versions of the XL, also beef up this area (and the aileron balance and some strengthening of the centre section.

The LAA's theory is that flutter could give this damage and so eliminating the possibility of flutter addresses the problem - they are probably right but....

As I said in the earlier note on the Matronics site we now have a situation of multiple populations of XLs, the UK ones with LAA mods - which are supported by a detailed engineering analysis, the US LSA fleet - doubtless also supported by CH's calculations and possibily some extra conservatism, the European 'microlights' with balanaced ailerons and some strengthening ad the US experimentals where people can pick and choose.

The ultimate proof will be the accident rates of the differing populations - perhaps then we will start to see some proof.

Great comments, especially on the very wide variety of aircraft we are dealing with here, all under the name 601XL. One of my biggest gripes is that some of the regulating agencies are acting as though all the aircraft are the same and attempting to apply blanket changes to them all, regardless of type and provenance.

This is drastically different from the certified world, where each would have a completely different designation and type certificate, and even within a type, any AD's would be assigned according to serial numbers and installed equipment.

I have never, in any of my messages, meant to imply that there is not a specific design, construction, maintenance or operation problem. There may be one or some combination of such. But I remain unconvinced that anyone has clearly identified a root cause or event or sequence of events that would lead to the failures we have seen.

The rear spar buckling seen in some of the post-crash photos, is, IMHO, an effect, not a cause. If such had occurred in flight, the damage to the aircraft and the types of crashes would be drastically different. Again, just my opinion based on education and experience.

And I remain convinced that until a probable and plausible cause, event or sequence of events is identified, that the regulatory agencies are still "shooting in the dark" and that we, as operators, are being poorly served. All of the various "fixes" may give operators a false sense of security and make no difference at all to our risk level.



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