Serge: As a former "Liaison Engineer' for a big aerospace company that was bought out by Boeing I will give you the SOP (standard operating procedure) for your aircrafts 'non conformance' that occurred during 'manufacturing' or 'depot level overhaul". (My text book answer) FOR THE; a. original WAFs with the oversize 3/8 inch hole---are they a standard oversize? Let us assume they are not a standard size.
Looking at FWD WING ATTACH FITTINGS, drawing no. 18 in my RR book pg.23 dated january 1990 the WAF is 1 1/2 inch wide and has a 3/8 dia. hole drilled 3/4 inch from the end. In my working days I would go to a stress engineer, show him the part drawing, identify the 3/8 hole is oversize and tell him what my method of repair was to be. The stress engineer typically went to his books and evaluated the over size hole condition as a 'lug analysis' with reduced edge distance (tear out) and 100% of the time went along with the installation of a larger diameter fastener. For really critical lugs, with really screwed up damage, I actually had inspection record the tear out as well as the actual part thickness and provided this data to the stress engineer. The hole could be drilled to an oversize such as 1/64 oversize, 1/32 oversize or even a next full size standard size as was required by the part condition.. Any hole increased in size must have the same fastener/hole assembly tolerances as the rest of the WAFs. If there was a possibility of the oversize bolt being removed in the future the area was marked to denote the hole and special size fastener. For those parts that would undergo constant bolt removal and replacement then a minimum 0.016 inch wall thickness bushing would be pressed fit into the discrepant part. The bushing would have a 0.0005 to 0.001 inch press fit and be of the same material. This would bring the discrepant part back to blueprint. b. for your new WAFS-- my drawing referenced above shows a series of 3/16 inch diameter holes located 1/4 inch from the long edge of the part. If I take half of the 3/16 inch and subtract it from the 1/4 inch we have 5/32 (0.0156inch) of an inch for tear out. That is not much edge distance. As an intelligent guess, if you are looking at tear out of less than 0.140 inch I would not use them. I am going to ignore the fact that the WAF drawings used are not to aircraft standards. I have never seen fractions used on such a critical machined part. Standard sheet metal tolerances are +- 0.030 inch. Can you see what would happen on the WAF if that tolerance was used indiscriminately during manufacture? How about checking your WAF holes to see what kind of assembly tolerance your fittings have. I never did get involved in reliability engineering during my work career but I do love redundancy. I hope the above info gives you a feel for what you have on your hands Do you have any friends that are stress engineers that you can pass this by? Regards Bob Morrissey, New Bern NC ----- Original Message ----- From: "Serge VIDAL" <serge.vi...@sagem.com> To: "KRnet" <kr...@mylist.net> Sent: Wednesday, September 14, 2005 5:16 AM Subject: Réf. : RE: Réf. : KR> RE: Dual Ignition Systems, it Saved My Bacon - CORRECTION Well, as a former reliability expert, I can give you the text book answer: - Reliability is risk management; - The universally accepted method of managing that risk is what gives birth to aircraft safety standards; - The authorities (like ICAO) are supposed to define what is your acceptable level of risk for the activity (roughly, basically, deaths per X flight hours). - At designer level, you then find what is likely to cause these risks, and define "unwanted events" (example: power loss, flight controls failure, etc.) - If failure of one component that is likely to cause an unwanted event is so unlikely that it is less than the acceptable risk level, then you don't have to do anything about it. You assess that by estimating the probability of failure (occurrence) and its consequences (severity), to define the level of CRITICITY. A part can be non critical because it is so unlikely to fail, or because .. If it is not the case, then you have to improve the reliability. You have many ways to do that, (can be better technology, better part design, monitoring, maintenance, you name it.). Of course, one method is redundancy. It is seldom the correct answer, but it is generally the easiest, and that's why it is so popular in aviation. Now, this is the rule for expensive designs (airliners, jet fighters, nuclear power plants, space shuttles or whatever). In general aviation, the trouble is it is not affordable to calculate exactly all the risks attached to all the parts and equipment. So, we apply a rule of thumb, which is: no single failure may lead to an unwanted event. That is what you do with your ignition or fuel system. But we also apply criticity, through return of experience of 100 years of designing and flying. We know that single engine is an acceptable solution, so we don't make the engine redundant. But we make the engine's most critical components redundant. The ignition, but not the carb... Likewise, we know that control cables are unlikely to fail, provided you rig them properly and inspect them regularly, so we don't make them redundant either. In my opinion, the statistics of engine failure in aviation are an absolute shame, and the ignition is the main culprit. So, I go for a better technology, and ultimately, as soon as it will become practical, I will go for an engine technology without ignition: the Diesel engine. Serge Vidal KR2 "Kilimanjaro Cloud" Paris, FranceSent: Tuesday, September 13, 2005 9:13 AM To: Corvair engines for homebuilt aircraft Subject: Re: CorvAircraft> Dual Ignition Systems, it Saved My Bacon
