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credibiltiy
"The OTHER Kevin in San Diego" skiddz "AT" adelphia "DOT" net wrote in message ... On Mon, 8 Feb 2010 16:16:59 -0800, "Stu Fields" wrote: It seems that if there are two possible causes for a helicopter accident, the preferred one for the manufacturer is the one for which they have no fault. That certainly is the best for them. However, lets say that a low hour helicopter suffers a failure directly attibutable to fatigue. Further lets assume that the helicopter had had a prior series of hard landings or other beyond normal stress loadings. Now lets further assume that the fatigue failure occurred at a point in the helicopter drive system where a diameter change was machined into the shaft without any radius or attempt at a proper fillet which yielded a strong stress riser. Lets say that the kit manufacturer is very aware that a number of kits have been sold with the same machining flaw. Should the kit manufacturer issue a service advisory statement advising all owners of those ships of a potential safety issue caused by those parts? You'd hope so. What should their action be? Recall and supply exchange parts for no charge? If it's a machining/design defect, absolutely. Since you mention in a later post the machine shop was given drawings that don't indicate any type of fillet where the diameter of the shaft changes, I'd lean towards a design defect and expect the manufacturer to replace the shaft on their dime. Recall and supply exchange parts for their cost? At a minimum. Change the machining process and ignore the other parts out there? If I'm following what you said, it's not a machining process issue. It's that the designer (or CAD guy) didn't call out that fillet. How about sell the business to someone else and just duck and hope that nothing bad ever comes from the above? Would that "absolve" the previous owners of any liability, especially if they were aware of the issue prior to peddling the biz? Common sense (and the litigous nature of the States these days) says no but we all know how short in supply THAT is. This wouldn't have anything to do with your upcoming project, would it? Hi Kevin. Things have moved on. I have found a jagged groove in the main rotor shaft that is approx 0.020 deep with jagged bottom. I've got a photo from an optical comparator for more detail. This groove occurs just at the top of a roller bearing that is the last point of support of the shaft. There is a rubber seal at this point. Manufacturers instructions were to remove the spring from the inside of this seal to avoid possible gouging of the shaft. Well evidently the rubber seal is capable without the spring of generating this jagged groove. The manufacturer has said that they have seen these before and they do not constitute a hazardous condition!! This is a soft (non-heat treated) TI shaft and all my books say that TI doesn't handle fatigue well if the surface of the shaft is rough with imperfections. I could be wrong, but I don't believe that the manufacturer has done a detailed fatigue analysis of the shaft with this kind of groove. Hell with what I've read about fatigue, it would take a world famous structural engineer with a bunch of fatigue design experience to convince me to fly that shaft. Even then I would want to try to borrow a bunch of money from him before my flight to test his surety. Apparently, based on written statements, the manufacturer doesn't believe that any of the accidents that occured were the fault of the poor fillets. Even in spite of the accident investigating agencies statement that the fatigue failure had occurred where the fillet was a sharp corner. There were reportedly other events of exceptional stresses that possibly could have started a crack in these highly stressed points. The mfr. has issued drawings that specify radiuses at the flillet locations and have offered a "Speedy-Sleeve" to protect the shaft from seal wear. All new ships are supposed to have these mods. The manufacturer has offered to provide these parts to owners of older ships. However, there has been no indication that the owners will receive any cost breaks. Aah the world of experimental aviation!!! I have sure got a new appreciation for the depth of the education available to the people who build, or buy kit aircraft. Upcoming project has had a closer look at the serious points and so far it looks good. That isn't to say that I'm not finding some warts on it, but so far, with the exception of adequate room for a helmet, the warts look easily handleable. |
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