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. Thank you for your continued support. Mathieu Heintz Zenair Ltd
"The FAA’s investigation did not indicate a single root cause for the in-flight failures." Okay... So, I didn't learn a thing I didn't know. Nothing new. Nothing changed. No questions, Mat. But, thanks for the interactive approach!
I am continuing my build with confidence that Zenith has done everything within reason to ensure that the 601/650 remains a quality kit. I appreciate the expense and effort put forth by Zenith and your patient and detailed response to our questions, Matthew. We, as builders, need to be just as diligent in our construction and flying techniques, for the quality of the final product rests in our hands. The design is proven, the variables are construction technique and the way the final product is piloted. If I didn't have faith in all of these, I would not finish my build.
The first thing that we need to remember is that the SAIB supports the upgrade kit. If there was any doubt from the FAA that the aircraft was not safe, the SAIB would have been very different and all SLSA aircraft would be grounded until we could prove to the FAA that the aircraft was safe.
Yes, the report is not clear in many ways. For instance, the date of the report is January and it was released yesterday. All information was released to the FAA before the report.
Why is this important? If you remember, the original upgrade drawings were "DRAFT" and did not include a lot of parts. This "DRAFT" was submitted to the FAA before the SAIB, which is what the FAA used as a basis to comply the aircraft to the LSA rule.
After the release of the "DRAFT" to the public, we got a huge amount of input for different CAA's around the world and from a lot of builders and engineers.
So, the final upgrade drawings were issued which were significantly different than the "DRAFT". So how significant?
Lets take a look at a few items.
- Spar bolts from AN type to NAS
- Wing root doubler at spar cap bolts from 0.032" to 1.125" (6-ZU-1-1)
- Fuselage rear spar attachment plate from 0.093" to 1.125" (6-ZU-3-3)
- Rear wing root doubler. Added a 0.040" plate (6-ZU-1-7)
So the upgrade adds a margin to the wing/fuselage area that the FAA was saying is up to 5% less than the LSA rule.
Because of all this, there is absolutely no point in redoing a very expensive test, especially since the FAA calculations are extremely conservative and beyond FAR 23 requirements.
Regarding flutter, the German flutter expert did confirm that the upgrade kit had no negative implications to flutter. We feel that this is sufficient, especially since we added the aileron balance weights, tested by the UK authorities.
We feel that we proved to the FAA that the aircraft does meet the ASTM standard at the time the SAIB was issued. Again, the FAA would not be allowing the upgraded SLSA aircraft to fly if they were not 100% satisfied with the upgrade kit and supporting information.
Yes, it is a long report. What I find frustrating is how the report does not properly deal with the actual aircraft. It is not a factual report and deals more with wanting to make changes to the ASTM rule.
Remember that the SAIB states "For SLSA owners and operators: We remind all owners and operators of their regulatory obligation to comply with Safety Directive / Safety Alert issued by AMD..."
The SAIB also states "For amateur-built and E-LSA owners and operators: Due to shared design characteristics that amateur-build and E-LSA aircraft have with S-LSA, we strongly recommend compliance with the drawings and instructions contained in the AMD Safety Directive/Safety Alert"
In the FAA's review, the FAA did not request that additional flutter testing must be done. They did state that it would be a good idea to get confirmation that the upgrade kit had no negative implications on flutter. The German expert confirmed this.
I am checking with Chris Heintz on the speeds. However, we will probably go to the lower Va as a safety precaution, even though the design calculations were done to the higher speed.
Thanks for opening up the discussion on the FAA report. It's a long read, but it does help you understand the FAA's thinking.
The FAA referenced tests that Zenith was doing in Sept of 2009 regarding loads on a modified 601XL wing. The report said the test were stopped when some buckling was noticed (para-phrasing here) Additional mods were made and the tests resumed.
Could you share what mods were in at that point and what mods were made after that point? I'm hoping to better understand the loads you were encountering and how they were affecting the wing. The FAA mentioned torsion quite a bit, so i'm looking for the thoughts of the Zenith guys at the testing.
Do you have plans to test a modified wing to past Ultimate to failure?? And in the loads test conducted after the mods of Sept 09 did any bolt stretching or hole elongation occur??
Thanks for keeping the lines of communication open!!
When doing the calculations and not using extremely conservative methods, the aircraft easily complies to the +6/-3g's at the reduced limitations in Chris's letter. However, the upgrade kit does significantly increase the margins, stiffen the aircraft, addresses flutter etc.
The static load test was repeated after the centre spar and outer spars were beefed up as per the static load test "extra material" chart.
"In response to these findings, the manufacturer reevaluated the wing design loads,
modified the structure, and tested the modified structure to the new design loads.
However, the manufacturer stopped their tests about five percent below the ultimate
load estimated by the FAA for compliance to the ASTM design standard."
So the modifications are still non-compliance to the ASTM design standard?
Before releasing the final upgrade kit, Chris did reevaluate the wing design as to make sure that it exceeded the VERY conservative requirements of the FAA. So this is already done. As for doing another test, there is no need as the last test was done with only a few items added to the original airframe. The upgrade kit increases the loads at the wing root area by a lot more than 5%.
Also, the costs associated with another test is quite significant and we just do not have the funds for this. A slow down in sales, litigation and testing is really drained us. However, our customers continued support is really helping us through this.
Chris' evaluation of the original 601XL design was deficient as apparent in the FAA's report that "[t]hose [static] tests revealed that the wing structure could not sustain the original design loads developed by the manufacturer for compliance to the ASTM standards for a 1,320 lb airplane." Chris' re-evaluation of the re-designed (upgrade) "exceeded the VERY conservative requirements of the FAA" and the upgrade kit increases the loads at the wing root area by a lot more than 5%." 5% relative to what? The original design or 5% below ultimate load? Are there quantifiable data to support this claim? The question still remained, why did the testing stop at 5% below ultimate load and why not go all the way if Chris was confident in his re-evaluation in the re-design?
After the FAA did its initial review of the design and noted that Chris had not accounted for the no fuel etc. Chris redid the calculations and we did the load test. After Ultimate, we added more weight as to make sure we had a nice margin. If you look at the static load test table 3A & 3B you will see that what we added quite a bit past Ultimate. 78 + 78 kg to be precise. Additionally, note that we added more weight at the tip. http://zenithair.com/zodiac/xl/data/doc369.pdf
Chris is presently updating his load analysis to incorporate the upgrade kit. The issue is not structural but which way to calculate as he is not interested in using unknown assumptions and certain load practices that are just not used in this type of aircraft.
So ideally, the FAA and Chris need to discuss the load methods so that everyone is on the same page.
So here we start again after a relaxing week in this Q&A. Please see the FAA response letter attached here.
So I think that the FAA letter does clarify the type of tests and reports they would like us to do.
"...it is acceptable for AMD to use stress analysis to resolve the five percent difference between the maximum loads sustained in the company’s September 2009 static test and FAA estimates of the maximum loads".
Chris Heintz already completed the load calculations showing that with the latest upgrade kit, the structural areas of concern exceeds the 5%.
Chris's report states:
The 1.25" aluminum plate at the root increases the strength in that area by 16.5%,
The AN5 bolts were replaced by NAS increasing the bolt strength and stiffness by some 25%.
More reinforcements as per drawing 6-ZU-2 were also incorporated.
All these reinforcements increases the possible loads in bending from 2,084 to 2,500 kg m (+20%), in shear and torsion from 1,252 to 1,560 kg (25%), -300 to -375 kg m (25%).
It is my opinion that the airframe structure with the upgrade is more than what the FAA is looking for. I believe that any reputable aeronautical engineer will have a hard time proving that the Zodiac 601XL & 650 structure does not meet the LSA ASTM rule.
However, if a lot of existing owners are still not comfortable with this, I recommend that they hire a reputable aeronautical engineer to redo the complete stress analysis of the aircraft.
The FAA clarification letter states:
Regarding flutter: Based on the service history of the aircraft, the original aircraft design
did not meet paragraph 4.6 Vibrations, which states:
4.6 Vibrations—Flight testing shall not reveal, by pilot observation, heavy
buffeting (except as associated with a stall), excessive airframe or control
vibrations, flutter (with proper attempts to induce it), or control divergence, at
any speed from VSO to VDF.
If the FAA is talking about the AMD SLSA aircraft, I do not know of anyone stating that their aircraft had heavy buffeting (except as associated with a stall), excessive airframe or control
vibrations, flutter (with proper attempts to induce it), or control divergence, at any speed from VSO to VDF.
If we are talking about the Experimental aircraft, yes, we have had customers stating some of the above. However, all of them confirmed that their control cable tensions were not to spec or control stops were not installed etc. Bottom line is that I do not know of a single Experimental XL or 650 that has heavy buffeting (except as associated with a stall), excessive airframe or control
vibrations, flutter (with proper attempts to induce it), or control divergence, at any speed from VSO to VDF, when built and rigged properly.
The large FAA report does not state flutter as the cause of the accidents but they do recommend that "you go beyond the basic ASTM flutter requirements and perform a complete flutter investigation (GVT, flutter analysis, and flight test) accomplished by a noted flutter expert".
I agree that its maybe good idea to do some type of additional flutter testing as this issue will not go away until we do. If we do a GVT, flutter analysis, and flight test, the costs will exceed $50K. Can we pool that much or do we settle for a flight test only. Also, the German GVT and flutter analysis (one GVT and flutter analysis was done on the 601XL and another separate one for the 650). Both reports were done without the aileron balance weights and passed. So if we redo some of the tests, do we do them without the balance weights?
When responding to the FAA clarification letter, please remember that this is a very emotional issue, so lets respect each other when posting responses on this site. For those who want to release some steam, call me at 705-526-2871 or email me at firstname.lastname@example.org
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