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I've been studying the SB cover letter, SB, Accident Report, and Probable Cause report and I guess I've revised my thinking on this whole affair after doing some additional studying. Here's what I'm thinking now. Comments - agreeing or disagreeing - are welcome.
1. The NTSB Probable Cause report came out on 1/31/06. This is very recent. It is more recent than the LAX04FA150 accident report. I would have to assume that the NTSB had at their disposal all of the info contained in the Accident Report, plus additional info that might not have been contained in the Accident Report. Their probable cause statement was that the crash was "due to corrosion in the wing center section not detectable by inspection methods described in manufacturer's service bulletins and FAA Airworthiness Directives". (emphasis added) The NTSB did not even mention the aerobatics or overstress in the Probable Cause report. This would imply to me, that their thinking is that had the rivets not been corroded, then the failure would not have happened, despite the aerobatics. I also find it interesting that they did not say "due to corrosion in the wing center section rivets". In the report they blamed the rivets, and the SB is only inspecting the rivets, so why didn't they say "rivets" in the last line of the Probable Cause statement?
2. It should be remembered when considering a failure like this that one failed or cracked rivet causes the adjacent rivets to be subjected to more stress, which can cause those rivets to fail, etc, etc. So having a bad rivet or two is a bad thing. If the only way to find them is to ultrasonically inspect them, and if there is no question that corrosion and failure of the rivets caused the crash, and if the rivets can be corroded without any external signs of corrosion, then as far as I am concerned the inspection needs to be done. That is a lot of "if's" though. Personally, if the NTSB says do it, and the FAA says do it, then based upon what I know from reading the reports, that's good enough justification for me.
3. There seem to be some statements in the SB cover letter, SB, and accident reports that just don't quite jive with each other. Hopefully this is just due to less than perfect wording or information being left out, and not due to conflicting opinions as to what happened in the accident aircraft. For instance, I have to wonder why only one rivet was examined with an electron microscope. Based on the Probable Cause document, it looks like they are saying that approximately 20 rivets had this same problem (corrosion), but they don't say how or why they arrived at that conclusion. They also mentioned in the Accident Report that <<At these five stiffener locations, the rivets through the upper spar cap flange also fractured at the transition between the bucked tail and shank. The fractured shank portion adjacent to the tails contained elongation deformation, and the fracture faces in this area showed a matter appearance typical of a ductile fracture. The ductile fractures were not covered with aluminum oxide.>> Why weren't these rivets examined with a SEM as well? Or maybe they were. It is odd that they said the tails came off, there was no visible corrosion, and yet the rivet faces weren't examined with a SEM to see if corrosion stress cracking was evident. Did these rivets fail just because the rest of the structure had already failed and was tearing apart, or were they part of the cause? May examining only one rivet with the SEM makes sense to the examiners and scientists, but it seems strange to me. There are just a lot of unanswered questions after reading the report and SB.
4. It is interesting that in the Statement Of Difficulty in the SB, they say " If corrosion has been detected anywhere in the wing center section additional hidden corrosion may be present in the main center section spar cap rivets." This statement in itself would imply that if the center section does not have any detectable corrosion, then the rivets should not be corroded. Too bad aluminum starts to corrode as soon as it hits air. Some corrosion will always be present, and they didn't qualify the statement by saying " If significant corrosion has been detected ...."
5. The SB Statement Of Difficulty also says that "If the aircraft has experienced hard landings the main spar rivets may be damaged." Um, yeah, and what else is new? If the aircraft has experienced hard landings then the main landing gear legs may be bent too. I don't see why this statement about the hard landings is in this SB.
6. I think that if there are a significant number of airplanes found with bad rivets, then there are going to be some hefty repair bills, and there are going to be some spar caps that are damaged during the repair process. There will be some hefty repair bills because some of the rivets will probably require removing the center section from the airplane in order to drill them and replace them. The damaged spars will be caused by people trying to drill out the rivets, getting the drill crooked, and running the drill bit into the spar cap. And I'll bet that more than one of these incidents will not be repaired properly because repairing it properly will involve replacing the spar cap. ($$$$$) Drilling out the 5/16ths inch diameter rivets is very hard to do properly even with the spar jigged up in a drill press. It took me 2 days and 8 tries before I perfected a way to do it on a junk wing that I had. If I had tried drilling out the first rivet on my real wings, I would have ruined the spar cap. Doing it in the airplane with a hand drill without damaging anything is going to be more luck than anything else. And trying to drive these 5/16ths inch rivets with a bucking bar and a rivet gun in such close proximity to the cap strip is risky business too. The only good, safe way to replace these large rivets (in my opinion, of course) is to partially drill them with the spar jigged up in a drill press, drive them out with a punch or press, and reinstall them with a hydraulic press or rivet squeezer.
7. It is interesting that the SB specifically states not to install bolts instead of rivets because rivets are a "hole filling fastener" and the bolts can't fit the hole as well. There was some discussion on the list a while back debating the merits of installing bolts vs rivets. Looks like Univair agrees with the rivet method, and in my opinion they are exactly right.
8. It is a relief that the inspections will only run in the hundreds of dollars instead of in the thousands of dollars that I was imagining. $400 or so for that type of inspection doesn't sound too bad. Of course that doesn't include opening the plane up so the inspection can be done, and closing it back up.
9. One final thought, that I hesitate to add, but I will anyway, because I'm working on trying not to be so politically correct and sensitive. ;>)
I'd say that one lesson here, even if the failure was caused by corrosion, is "Don't fly aerobatics in a plane that is not approved for aerobatics." Most of these airplanes are 60 years old. They weren't stressed for aerobatics when they were new, and they haven't been getting any stronger over the years. And if one just has to do something so irresponsible in an airplane, they should go out over the desert or the ocean by themselves and kill themselves where no innocent passengers or spectators will get killed. People who do this kind of stuff are the reason that all of us have to live and fly under an ever increasing list of rules, regulations, and restrictions. Every time someone pulls a bone headed stunt like this they are a threat to my flying privileges and your flying privileges. I don't know about anyone else, but I take that personally. There are a lot of good aerobatic airplanes out there, and the Coupe is not one of them.
OK, I'm climbing down off my soap box now.... But I reserve the right to climb back on at any time!
And of course, all or most of this is just my opinion. (Especially No. 9 above!)
Best Regards,
Wayne DelRossi
Alon N5618F
Hours logged since restoration: 91.5
Alon N5618F
Hours logged since restoration: 91.5
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