Q&A FAA Special Review Team Report - Zodiac 601XL/650 - FAA clarification letter

This Q&A is about the FAA's clarification letter of March 8th, 2010.

Like previous Q&A's, post all your questions / concerns at the bottom of the last
page of this Q&A.  If I do not have the answer, I will consult with
Chris Heintz and or other engineers.
If you have a very technical question or just want to vent, call me at 705-526-2871 or email me at newplane@gmail.com

Thank you for your continued support.
Mathieu Heintz
Zenair Ltd

Views: 917


Replies are closed for this discussion.

Replies to This Discussion

I for one am glad to be a customer of Zenith Aircraft. im also one of the slowest builders on the net
i DO NOT object to strengthening my 601xl i flew in the factory demo in 2001 and was impressed then.
unlike jake my extra work will be double his my wife says ,some guys are slow some guys are fast im sort of
half-fast . zenith keep up the great work !!!!!
601XL-B Experimental Boys,
Congrads, but W/ completions of all Stages of the upgrades or / and any major modifications, we all must know the following... Your FSDO requires you to test fly (Phase I) again your a/c in the orginal 25nm area for at least five hours w/ documentation noted. Flying the a/c in another area requires verification & premission from FSDO. I have restated this because it seems to me that some of our builder/pilots are forgetting this step of the upgrade or I am maybe misunderstanding their blogs.
It seems to me to be very important that all of us follow the required procedures w/ our FSDO in order to maintain respect in the experimental community. At this stage, it would be a real setback for all of us if one of us skipped a safety step in the process. So, rebuild it, refly it and rewrite it within the 25 nm orginal Phase I area. The FSDO reps are good about this and will clarify test flying points of order. Safe sky all...
Very good point. The FAA has been very consistent about following the AMD Safety Directive / Safety Alert.

The SAIB states:

In order to prevent potential catastrophic structural failure, we strongly recommend that all owners
and operators of Zodiac CH601XL/CH650 comply with actions outlined in a forthcoming Aircraft
Manufacturing & Design, LLC (AMD) Safety Directive / Safety Alert to address the abovereferenced
concerns before further flight.

In the FAA Zodiac report 5.2 Summary of Recommendations:
1) According to 14 CFR Part 21 § 21.190(c)(5), owners and operators of S-LSA CH 601 XL and CH 650 must adhere to the requirements of the AMD safety directive, dated November 7, 2009 to address the potential safety issues listed in this report. In a related FAA SAIB CE-10-08, the FAA also recommended that owners and operators of the kit built versions of these aircraft review the SAIB and follow the AMD safety directive. The FAA will continue to monitor the situation to verify the SAIB, Airworthiness Memo, and Safety Directive are having the desired impact on the CH 601 XL and CH 650 fleet.

Here is the link to the AMD Safaty Directive / Safety Alert.

In the Corrective Action section of the AMD Safaty Directive / Safety Alert, it calls for flight testing and ASI calibration.

This information is in the AMD Service Manual

For ASI calibration, we are recommending that you follow the procedure outlined in the National Pilot Test Flying School web site.

There is a huge amount of information out there, so please use it, and please let me know if you cannot find a document or need more information.
Regarding flutter and the need for flutter testing of the upgraded 601XL, the FAA Report is very clear on the occurrence of flutter in the accident aircraft. From the report (p. 22),

“The FAA evaluated the photographic evidence of several accident aircraft and observed the physical evidence first hand of the Antelope Island accident aircraft. The evidence indicates the presence of compression failures in both the upper and lower aft spar caps (or skins). In some cases the evidence shows a complete wing failure in one direction, yet exhibits compression buckling consistent with bending in the other direction as well. This combined condition is indicative of complete load reversal and provides consistent evidence that flutter occurred in these cases.”

And, on the occurrence of flutter in non-accident aircraft (p. 23)

“Pilots have reported to the NTSB that they experienced what can only be described as flutter. These reports have been documented and pertinent portions of those reports are available on the NTSB web site”

If you can read those paragraphs and believe that flutter does not occur in the unmodified 601XL, I don’t know what to say.

The photographic evidence referred to in the first paragraph has been available on the Yahoo ZBAG Group file section since 10/24/09,


You may have to join the Yahoo Group (it’s free and open to all Zenair builders/owners) to access the files. If that offends you, send me a private message and I’ll get the photos to you. In any case, you should examine the photos. The above paragraph from the FAA provides the narrative that was missing when the NTSB added those photos to the Polk City docket. The FAA narrative clearly tells why and how those photos prove the occurrence of flutter in the accident aircraft. For me, personally, the evidence conclusively shows that flutter caused these aircraft to lose their wings. Form your own conclusions.

As noted in Ryan’s letter, the FAA Report says,

“The manufacturer needs to reevaluate the modified CH 601 XL wing structure with the balanced ailerons to determine the flutter characteristics of the modified design. This should include a complete flutter investigation (ground vibration test (GVT), flutter analysis, and flight test) accomplished by a noted flutter expert.”

In my previous post (to Mat’s Q&A on the FAA report), see p. 8 of:


I stated my belief that FAA testing requirements only apply to new S-LSA’s. Ryan’s letter confirms this. The modified design is a new design, and Zenair must show that it meets ASTM 2245 before continuing sales. If AMD choose to walk away from the Zodiac S-LSA, then, in my opinion, the FAA would require no further testing. However, if Zenair wish to continue marketing the 650 S-LSA, I believe that they must test, given the language of ASTM 2245. And, as I noted in my previous post, “simply rapping the stick with a mallet would not reliably excite the vibrational frequencies and modes that were predicted by flutter analysis of the 601XL/650. In consequence, some sort of remotely controlled, electro-mechanical device would be needed to induce the vibrations in the appropriate frequencies and modes.” For the modified aircraft, the vibrational frequencies are unknown. So, GVT would be needed to determine the frequencies that need to be tested. Once the GVT is complete, the method of excitation could be decided. For additional info on this subject, see AC 23.629-1B in general, and, for flight testing, see appx. 2, in particular.

If Zenair choose to test, I believe that the FAA has the right idea. Hire a “noted flutter expert.” BUT, do not tie his hands with a non-disclosure agreement (NDA). If Zenair believe in the modified design, let the expert do his tests and report everything he learns, with his personal attest that he is not encumbered by any NDA from Zenair. In my opinion that is how Zenair can regain credibility on the 601XL/650 design.

I think that the mods will eliminate the wing loss accidents, because I believe that the counterbalance will eliminate flutter. I am working to finish my 601XL kit and I plan to fly my airplane. However, I’d be happier if a “noted flutter expert” published his report confirming that the new design is free from flutter. I’ve sent Zenair my $100 check to help pay for new flutter tests. If you believe in the 601XL/650, I encourage you to do likewise. Let’s put these problems behind us and enjoy our airplanes.
You make some good points and I do not think that a lot of people are in disagreement with them. Where things get hazy is flutter and or vibration in the original XL. However, there is no point arguing that as I hope all the aircraft will be upgraded.

I agree that it would be a good idea to complete a more details flutter test than what the ASTM LSA rule calls for. In an earlier Q&A, Doug Norman accurately stated what is required when flutter testing to the LSA rule, and believe that AMD has already done with with an upgraded aircraft. However, the FAA would like to see a lot more than that.

So this brings us back to one of my earlier Q&A's where I was asking what type of flutter testing you all want. The more complex, the more expensive.
I beleive that a comprehensive test including GVT of the modified airplane is needed to clear the cloud--- I don't even own a 601/650, mine is a 701, but my son is building a 650. If you will do the comprehensive test, My $100. will be sent to help with the 50k you said it would cost and I would hope everyone else who would consider flying a 601/650 would do the same. Thats only about the cost of 3-4 hours fuel!!! Peace of mind is priceless. Jerry
Doug Norman completed a very impressive independent analysis and interpretation of the Sept 09 load test of the Zodiac 601XL/650. Please see

Regarding flutter, I am in contact with a few test pilot specialists where we are trying to schedule the flight test. I am also hoping that the German flutter expert will want to do a new report with the upgrade.
Good on all aspects. My encouragement to those who can continue the evaluation process, please carry on!

A problem I have with the upgrade is keeping up with changes to the drawings and the photo guides.

E.g., the photoguide that shipped with my upgrade kit is Rev 1.0, dated 12/10/09, while the last 6-ZU-1 photo guide that I downloaded from the Zenair website is Rev 1.1 dated 2/18/10. Presumably, some changes were made between Rev 1.0 and Rev 1.1. However, after going to all the trouble of printing out a copy of Rev 1.1, and then comparing page by page, I did not pick up any difference other than the Rev No. and the date.

Rather than waste more hours poring over the documents and more paper printing unchanged pages, would it be too much to ask that Zenair not put a new version of the drawings or photoguides without simultaneously publishing a list of changes between the versions?

Zenair has done an admirable job of listing changes made with each new release of the 601XL drawings. It would be customer friendly if Zenair would, similarly, list the changes between all the new versions of the Upgrade drawings and photo guides since 12/9/09. Thanks.

Good point. I will see if we can do an update sheet and add it to the website rev 1.1
In response to the update of the upgrade drawings.

The following link is a summary of the changes made to the drawings for Rev 1.1 2/16/2010:

It was updated to have the both 6ZU1-3 and 6ZU1-4 on all the nose ribs before the tank. The first revision of drawings used different number of angles depending on what nose rib it was. It was a very minor change to the photo guides. Likely only a change to a couple words that unless it was read word for word with a rev 1.0 you would never be able to tell the difference between them.

In the event that a major change would be made such as material thickness etc. everyone would be updated immediately.
Thanks, Mat

That helps a lot. I would request that they list all changes when they are made, so that builders out in the field don't have to waste time looking for a needle in a haystack--both to the dwgs and to the photoguides. It would sure make it easier to do the upgrade.



New from Zenith:

Zenith Planes For Sale 

Classified listing for buying or selling your Zenith building or flying related stuff...


Weather Maps

Custom Instrument Panels
for your Zenith

Custom instrument panels are now available directly from Zenith Aircraft Company exclusively for Zenith builders and owners. Pre-cut panel, power distribution panel, Approach Fast Stack harnesses, Dynon and Garmin avionics, and more.

Custom Upholstery Kits for your Zenith Aircraft:

Zenith Vinyl Upholstery Kits

Zenith Apparel from EAA:

Zenair Floats

Flying On Your Own Wings:
A Complete Guide to Understanding Light Airplane Design, by Chris Heintz

Builder & Pilot Supplies:

How to videos from HomebuiltHELP.com

Developed specifically for Zenith builders (by a builder) these videos on DVD are a great help in building your own kit plane by providing practical hands-on construction information. Visit HomebuiltHelp.com for the latest DVD titles.

Aircraft Insurance:


West Coast USA:

Pro Builder Assistance

Pro Builder Assistance

Aircraft Spruce & Specialty for all your building and pilot supplies!

© 2020   Created by Zenith.Aero.   Powered by

Badges  |  Report an Issue  |  Terms of Service