I think the issue is quality control and I find it hard to believe the QA requirements for commercial hardware, supplied by any commercial vendor without airframe manufacturer involvement, would be acceptable to aircraft certifying authorities.
It depends on what the part was designed/certified with, doesn’t it? I still fail to see why a design based on DIN EN ISO hardware would be unsafer than one based on AN hardware. For critical parts you still would be able to (despite the minimum strength norm) test prior to installation.
I think we’re going around in circles here. The spare part being supplied and discussed is the bolt, not the assembly it’s installed into, because the aircraft owner wants to buy the bolt only, not the assembly. This also has nothing to do with calculated safety of the assembly or the theoretical strength and fatigue resistance of commercial hardware. The (obvious) point is that if the aircraft is certified with commercial hardware, which is certainly possible, those nuts and bolts when supplied as individual spare aircraft parts must be processed through an aircraft qualified QA system prior to being sold to the end user. That means you buy them from the airframe manufacturer under its part number, not like certified AN hardware which you can buy from anybody, anywhere in an open market that supports many different types of aircraft specifying common, aircraft certified, standardized hardware in their parts lists.
This is the same certification issue that prevents you from legally installing an identical automotive Facet electric fuel pump to replace the one supplied on a certified plane originally, installing identical alternator brushes bought at auto parts store and all the rest, as is no doubt very well understood by virtually everybody reading this. This applies equally to commercial nuts and bolts if used by a manufacturer as parts within a certified airframe assembly.
Circumventing all of the above is one benefit of the FAA Experimental category, including both Experimental Amateur Built and also Light Sport aircraft that have been moved by their owners into FAA Experimental Light Sport. Obviously and as you point out, those Experimental Category aircraft rarely fall out of the sky as a result of the lack of an aviation qualified production quality program for their hardware, possibly because standardized, aircraft qualified AN hardware is so cheap most individually built Experimentals use it anyway!
Gliders in Europe use metric system. km/h and meters. That can be rather confusing at times when towing gliders. They report altitude in meters AGL, I “see” altitude in feet MSL (QNH). The airport I tow from is at 1840 ft alt. A gliding instructor could ask me to do horizontal eights from 400 m, which makes me think for a short while 
Silvaire wrote:
possibly because standardized, aircraft qualified AN hardware is so cheap most individually built Experimentals use it anyway!
AN bolts are OK. They are designed to hold together soft metal (aluminium). They are not really bolts though, not in classical engineering terms. They are more like low tensioned shear pins with questionable precision. But, they are perfect for the job. Solid rivets are also OK, perfect for the job and cheap, and so are AN fittings (not exactly cheap though). As for the rest, I use metric whenever I can. Higher precision, higher quality.
Taking the £3.20 item above, that is roughly similar to the 10-32 one higher up which is £0.50. That is a 6.4x price difference! Or 740% in Daily Junk speak.
The “£POA” next to the others tells a story too: just about very nearly nobody buys the metric ones
Just one data point, but this took me just 5 mins, and anyone who can be bothered to do some searching can check this out for themselves, and you get this sort of multiple all the way down the line of small aircraft parts.
And this is before you get onto the “scam” of airframe manufacturer de-facto mandated sourcing whereby the mfg lists the (metric) part under a custom P/N, e.g. Socata listing an M5 nut under Z00894623542178438943 with the sole objective of preventing it being easily sourced directly. Especially as the maint co. has a zero incentive to look for a lower cost part.
Obviously the above P/N scam is also done with US parts, because every airframe manufacturer has an interest in maximising their parts revenue, especially if [insert your favourite GA aircraft brand] doesn’t make any/many airframes anymore. But there you get three key differences which greatly favour the AN/MS system:
I have some M4 floating nutplates in the TB20 wingtips and there is no way to find the manufacturer, which is probably some obscure French company, which, on past record, has quite possibly been taken over by an American company and half of it was shut down, as in e.g. here. So, to replace them, I would have to measure one up and spend time going through data sheets, but (as in the metric example above) there are no dimensions provided so you have to request them… and you waste half a day of your life, whereas the US A&P replacing the MS part just orders…. the MS part
Now multiply the above microcosm 10000 times and you start to see why the US aviation engineering scene has taken over the aviation world, despite a subsection of the European aviation community describing the US stuff as antiquated, dinosaur-age, etc…
vic wrote:
when a manufacturer picked a specific type – for certain reasons (money making?) you are stuck with it. Socata have their ideas , they could have taken AN fasteners just as well but who could have forced them to use the AN system if they chose not to ?
Their customers. You’ll notice that nobody is buying new Socata light aircraft in 2018. This is also one reason that factory built LSA aircraft have a difficult time. As I’ve described, the FAA has provided a way to convert LSA aircraft to E-LSA Experimental, so that as with your non-ICAO certificated Yak you are no longer legally tied to the manufacturer for all parts, as long as you accept new operating limitations. However people buying an aircraft designed partly for training (which can no longer be done) rightly worry about resale value. I think as used LSAs depreciate in market value and are attracting mostly individual pilot owners, almost all in the US will be converted to E-LSA Experimental and their owners will then be able to buy and install commercial grade hardware directly, like the manufacturer did.
Meanwhile, those of us owning aircraft utilizing standarized AN hardware will continue to buy it relatively inexpensively, and continue without issue to maintain FAA certified non-Experimental aircraft that may not have been supported by manufacturer supplied parts for many years, in my case 47 years and about 71 years.
vic wrote:
So whenever you need screws for certified Rotax or Diamond etc. you get them from your local hardware store, for anything apart from wing bolts that are special parts anyway. As far as I understand this, you are allright for using industrial standardised parts on the aircraft that can be shown to do the job, like bolts , nuts, hose clamps etc. , that are not critical for airworthiness resp. structural strength. Allright, views on that may vary a lot depending on those characters in form of inspectors
What you use to remain legal is what’s listed in the parts manual, and if it is an non-aviation-certified bolt, supplied as part of an aviation certified product it will be listed by manufacturer proprietary part number.
vic wrote:
As far as I understand this, you are allright for using industrial standardised parts on the aircraft that can be shown to do the job, like bolts , nuts, hose clamps etc. , that are not critical for airworthiness resp. structural strength.
But that’s the thing. In principle, you should be able to go to your local hardware store or McMaster-Carr, and order the parts you need, so long as they’re the right quality/spec/dimensions, at least for non-critical parts. Indeed, most home builders joke about the aviation department at Home Depot or Lowes. However, since airplanes in fact fly on paperwork and money, the problem with this route is that you can’t prove the parts are equivalent to what the factory requires, because as Silvaire says, they don’t have a QA system and paperwork generation capacity. With things like interior trim screws, nobody is likely to care or notice, but once you start getting into important parts, people like to have a good paper trail.
Whether you view this as critical or not depends on how good you want your paperwork to be. (Also, moving from approved parts to non-approved parts can have more subtle catches than might be expected. However, if you get the exact same part number, from the exact same manufacturer, then it becomes a question of attitude towards paperwork.)
vic wrote:
Socata have their ideas , they could have taken AN fasteners just as well but who could have forced them to use the AN system if they chose not to ?
As Silvaire says, nobody forced them to use AN or proprietary hardware, the choice was their’s so long as they could meet the FARs. However, while they have the potential to make lots of money by selling proprietary spare parts, the countervailing cost is that an aircraft with a reputation for high maintenance costs is less likely to sell, so it’s a trade-off. The certified piston GA market is so small and so soft that it’s hard to say this was their largest problem, but on the margins it probably boosted their short term profits at the cost of the long term viability of their product. (Or alternately the income stream from the parts outweighs the lack of airframe sales, because in this day and age nobody outside Cirrus is producing pistons in appreciable numbers. Hard to say without looking at the numbers and having access to Socata’s books.)
The problem is that you can have two separate discussions on any aviation topic: doing it legally and doing it illegally. The latter is usually cheaper. But one cannot compare a case from the former with a case from the latter. You cannot compare the cost of a metric bolt which is fitted illegally, with an AN bolt which is fitted legally because the AN system makes the bolt an inherently certified part. It’s a bit like comparing a Lancair IV with a TB20; the Lancair is way better, except for the well documented issues with mostly needing permits, mostly unable to fly IFR, needing a lot more tarmac, etc.
Peter wrote:
because the AN system makes the bolt an inherently certified part
This isn’t the case. Anyone can make AN bolts. You can purchase two types of AN bolts; with and without a manufacturer C of C. Without a C of C, the AN bolt is just a piece of metal with unknown origin and quality. C of C costs US$ 15 at Aircraft Spruce. Buying one bolt at a time, this gets pretty expensive. Also, at professional/industrial hardware stores (not the garden/kitchen type variety) you can purchase metric bolts with paper trail as well. I have never done that myself personally, but it’s standard for large dimension pressure vessels and so on.
For smaller fasteners you have to look at the certifications of the store/manufacturer. If the store is certified to deliver nuts and bolts to the energy sector, or oil and gas industry, you can be pretty sure that the items they sell follow the ISO/DIN standards regarding material properties, shape and finish (genuine parts). Würth is a good source for me, as they are certified to sell fasteners to most industries, and there are Würth stores everywhere. Then I know I get the right quality according to the standard. That wouldn’t help me with a certified airplane, but it’s not clear to me how you can get a standard (aircraft size) metric bolt that is “more certified” than that, or simply of better quality for that matter.
One has to view the AN/MS system in the context of its prime market – the US, and US-registered aircraft.
There, there is no 8130-3 or other “CofC” requirement, for GA.
What Spruce do is make money out of the pilot communities that are living in oppressive regimes such as Europe 
I vaguely recall they charge $100 for an 8130-3, too. I paid an extra $100 for an 8130-3 for the B&C backup alternator so Spruce are not the only outfit doing this. I didn’t need the 8130-3 for my N-reg but I prefer to collect them for the benefit of any registry transfer / future owner.
In reality the invoice from the reputable “aviation” vendor (which is of course free) is a sufficient “CofC” to anybody who just wants to be sure the part came from a proper source and not from a retail hardware store i.e. potentially counterfeit, and this verification is what the FAA A&P is required to do.
You can indeed purchase metric parts with a non-Form-1 CofC, away from the airframe manufacturer, but you can’t use them in the European registry sphere unless they are provably the same or same-spec part as the original (the exact details AIUI vary according to criticality) and that is potentially difficult to establish – a problem in which the metric airframe manufacturers are fully complicit, for revenue maximisation. It also ensures that a maintenance shop will just buy the parts from the airframe mfg because anything else (like working out what the markings on the bolt head mean) is way too much hassle, and there is no money in it for them.
