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Why doesn't aviation use standard (metric) units?

Peter wrote:

The US system of AN and MS parts is absolutely wonderful. You get access to a vast range of bits, at competitive prices which don’t resemble aviation at all, and you know that if something needs doing again 10 or 20 years from now the part will still exist.

Nah, I can get quality DIN / ISO parts in every larger town at a local industry supplier for not much money. All our products are built using those parts. That’s not an argument. (Of course, if you choose to use 12,7mm bolts, you’ll pay extra. But so do you for a 15/32" bolt.

You can’t blame overpricing parts on the SI system.

Peter wrote:

The gotcha is that you can’t easily buy such parts in metric.

I don’t have a picture displays, but you have tons of different DIN / ISO parts readily available. Allmost all worlds industry uses metric normed hardware, so I find there is very little impact on this argument.

BTW: Moranes are mostly metric for the airframe and only US for the engine.

Last Edited by mh at 09 May 08:16
mh
Inside the sky.
EDXE, EDXF, Germany

mh wrote:

BTW: Moranes are mostly metric for the airframe and only US for the engine.

With my Europa it’s the other way around

EDLE

I can get quality DIN / ISO parts in every larger town at a local industry supplier for not much money

Well, yes, which is where the other bit of the US system comes in: the “standard parts” are unconditionally acceptable for certified aircraft.

Personally when I am making something I use metric bolts. I buy them in big assorted kits on Ebay, in stainless steel (why use anything less than the best?)

Usually they are made in China. But, in Europe, you can’t use “retail” parts (in general) on certified aircraft and that is what has created the restrictive practices in the supply chain.

Again, not a direct supporting argument for the US non-metric aviation parts system, but it is an explanation for why we have what we have.

Administrator
Shoreham EGKA, United Kingdom

Silvaire wrote:

None of the old UK-style Whitworth, BA, Cycle Thread etc stuff was ever used on US aircraft, and isn’t in use on anything else today

Unless you own an Auster. Mine’s a maddening mix of Whitworth (on the airframe) and AN on the engine (Lycoming engine), except the fuel lines upstream of the carb which are all BSP pipe fittings…

Last Edited by alioth at 09 May 09:32
Andreas IOM

Peter wrote:

But, in Europe, you can’t use “retail” parts (in general) on certified aircraft and that is what has created the restrictive practices in the supply chain.

Sure you can.

mh
Inside the sky.
EDXE, EDXF, Germany

I know you can, mh, but that one-liner is about 1% of the story, as you know

Yeah; I must make a mental note to get my next set of wing bolts from here

Administrator
Shoreham EGKA, United Kingdom

Always been somewhat puzzled as watching the SpaceX launches everything is in metric, altitude in Km, speed in km/h, range Km, yet reading their literature they build in Imperial – they seem to have no problem in production but I assume it is just easier as all the tooling and materials will come from the US. They will obviously do some careful checking with payloads from the rest of the world!

Seem to remember when working on the older HS125’s that I needed A/F, Metric, BA, and even some Whitworth!

M

EGHH, Near EGVO

I did say industrial supplier, not home improvement store. OBI-Aerospace doesn’t sell high quality.

Last Edited by mh at 09 May 10:33
mh
Inside the sky.
EDXE, EDXF, Germany

Seem to remember when working on the older HS125’s that I needed A/F, Metric, BA, and even some Whitworth!

If I was doing a new certified plane design I would make it “fully US” i.e. AN, MS etc. Then the owner can bypass the whole aviation supply chain and buy parts from stockists such as LAS Aero whose main business is ostensibly homebuilders because you don’t get an EASA-1. You just get a CofC. The drawback is that my new aircraft manufacturing operation would have a smaller value to sell to somebody because the parts business turnover would probably halve

I did say industrial supplier,

That’s not anywhere near the whole story either, though, is it, if you look at the sum total of the bits which make up a certified plane?

However we are now debating European restrictive practices / moneymaking schemes, rather than the merits of the different measurement systems.

Administrator
Shoreham EGKA, United Kingdom

Of course if one is trying to sell an aircraft (especially a new minted one) it is perhaps a human perception that 120 knots is somewhat less attractive that 222kph!

UK, United Kingdom
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