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

What standard ? The metric system IS the standard. I still have to see an “aviation” bolt in metric, there is no I guess. Instead you see the stamps on quality screws that clearly tell the tensile strength, 4.6 the weakest type for simple jobs, goes up to 8.8 , 10.9 and 12.9 , means 120 kg per square mm tensile strength – or 1200 N . No counting lines or dots whatever on bolt heads in US aviation bolts that say what ? So when you need a new bolt – get one from your stockist with same class. Precision machined bolts is a different breed, only available as spare part – like for Toyotas, Opels BMWs . Vic


Actually – does it matter ? IMHO the units might as well be apples, oranges and tomatoes – you fly by the numbers anyway. I grew up metric but have never had the slightest issue dealing with imperial measurements. The only exception is Fahrenheit for temperature which must be the least intuitive unit – ever. Thankfully, aviation (also in the US) is in C anyway, so no real issue there.

IME what takes a little getting used to is switching between an airplane with an ASI in kts and one that’s in statute miles. But again – just fly the POH numbers and all’s well.

172driver wrote:

The only exception is Fahrenheit for temperature which must be the least intuitive unit – ever.

I believe it was designed to be intuitive, no? 0-100 degrees spanning the range of reasonable human tolerance, with increments of a reasonable granularity to match human perception. The first thing I do in any car is change the inside temperature readout to Fahrenheit so I don’t have to deal with half degrees.

However, if you do happen to have an inordinate interest in the phase changes of water at sea level that may take you in another direction I think that’s why, when and where people are not legally constrained, they often use degrees Celsius in relation to low ambient temperatures when freezing water is an important consideration and 0 degrees C is easy to understand, switching to Fahrenheit once the temperature is a little higher and 100 degrees F represents an understood, intuitive standard for a hot day. Since I very rarely have to deal with freezing water outside of my refrigerator, I prefer Fahrenheit in daily use. If I lived in Alaska I might see it differently, and either way I’d use what works for me.

Last Edited by Silvaire at 09 May 02:50

vic wrote:

What standard ? The metric system IS the standard. I still have to see an “aviation” bolt in metric

The definition of standard for aviation means that the aircraft parts manual calls out an aviation approved part you can buy from multiple suppliers. I’m not aware of any metric hardware that meets that definition, and with that in mind if it’s metric, that means it’ll be an airframe specific part number supplied by one company. This lack of certified, aviation approved standard parts is a real issue for many Light Sport aircraft, and the FAA solution for owners not wanting this ridiculous maintenance constraint is to allow those factory built aircraft to be moved to experimental (E-LSA) category… Which is fine if your operations can be performed with an Experimental aircraft. At least there is a (partial) FAA solution, and you don’t have to break the law to buy airframe bolts once the manufacturer of your aircraft is long gone – which is a highly probable event for LSAs.

Last Edited by Silvaire at 09 May 03:00

One difference between units like altitude and measurements used for hardware is that screw pitches are not arbitary. I’m open to the idea which I’ve heard expressed, that steel bolts in metric sizes may be less strong than equivalent Imperial sizes.

vic wrote:

But speed etc. is nothing I´d tell on the radio or anybody will ask for.

Maybe not when you’re flying VFR, but certainly when IFR.

ESKC (Uppsala/Sundbro), Sweden

kwlf wrote:

Don’t Russians measure altitude in metres?
During Soviet times, the east bloc used metric units in aviation. (Actually the ICAO standard at the time!) Today AFAIU they use the feet for altitude.

ESKC (Uppsala/Sundbro), Sweden

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.

The lack of such a system in Europe is what has created mad customer fleecing stuff like $6 screws from Socata and that one is just the tip of the vast iceberg of proprietary parts, with hundreds of trivial $0.10 parts costing €50 (yes somebody has just paid that for an M5 nut from Socata, because it is published in the IPC as an obscure P/N and the maintenance company demands a Form 1; of course it suits them because a 25% margin on €50 is better than a 25% margin on €0.10, as one owner of an EASA145 company explained to me not long ago).

You get the same with hoses. A short fireproof metric (ISO thread) hose is currently about €1000 whereas the US version is about €70.

Of course the above is not a good argument for or against metric in isolation. Had the USA gone metric, we would have had AN and MS parts in metric. But they haven’t because they started in aviation and engineering 100+ years ago and there are far bigger fish to fry in “life” than changing stuff like this, and it would have created the above European-style ripoff practices which the USA is culturally strongly against.

The one thing I would say however is that you have a much bigger choice of easily available threads once you go away from the metric system. You can in principle have lots of different types of metric threads. I can cut almost any thread on almost any diameter, on a lathe with a leadscrew. This is a standard M6 thread which I cut with a 1mm pitch

but I could have just as easily cut a 0.5mm pitch thread. The gotcha is that you can’t easily buy such parts in metric.

Shoreham EGKA, United Kingdom

Regarding ft and NM, while I am very fond of the metric units in general, I often use formulae such as:
GS x slope% = VS
distance x slope° = number of FL to lose/gain

They get a bit funky when you try to adapt them to usual metric units. Just my two cents.

LFPL, France

Airborne_Again wrote:

During Soviet times, the east bloc used metric units in aviation. (Actually the ICAO standard at the time!) Today AFAIU they use the feet for altitude.

Correct. They use meters, QFE and mmHg in terminal areas. Charts say hPa on request, but me experience is that they provide it without asking. So one gets “descend height 500 meters QFE 1005”.

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