Why does aviation still use imperial system?

[Yenn]

The yanks have a great system!

 

Something can be in inches and feet, or maybe it is just inches ie 96" rather that 8'

 

Then they have decimal inches ie inches divided into tenths.

 

No matter what you are using it is better than it was a few years ago. I remember using drawings from Gladstone Power Stn to do some design work. some were in feet, some in inches, some in feet and inches, some in feet inches and tenths of an inch plus they had a few in metric, some in mm and some in M and mm.

 

I built an RV4 and couldn't buy a yard rule. The local tool supplies were getting me one, as far as I know they still are 6 years later. I managed with an imperial tape, metric tapes and rules and a good calculator.

 

 

[Guest ozzie]

Spent so much time measuring airframes in imperial i have trouble with building something in metric. None of my steel rules have metric on them. Just lack of practice i suppose. Even now when driving on trips, see a sign giving klms to destination i still tend to convert it over to miles then work out a time.

 

On a trip to Italy rented some gear to do a jump and the Alti was in metric. Needle still did one and a bit revolutions when at height and the yellow and red bits were the same heights as here so didn't worry about it.People still looked the same at opening height.

 

Later went for a fly and same deal altimeter was metric bit harder with no colored bits so it was a bit of 'that looks about right' for the circuit.

 

 

[old man emu]

The yanks have a great system!Something can be in inches and feet, or maybe it is just inches ie 96" rather that 8'

Then they have decimal inches ie inches divided into tenths.

Using 1/10ths of an inch is a good compromise when dealing with very short lengths, but factors of 1/8ths expressed as decimals are also easy to use once you have learned that 1/8 = 0.125.

 

Halve 1/8 and you get 1/16 = 0.0625. Halve that again and you get 1/32 = 0.03125.

 

What you have to realise is that, apart from close tolerances in engine components, if you can build to an accuracy of 1/8" (3.175 mm) you are as close as dammit to perfection.

 

OME

 

 

[spacesailor]

Has no,one looked up the French metric time yet, I saw it years ago ! Frightening .

 

Metric has 12 steps and not many know all of the names, but I've used most of them over the years. "The metric system is far easier", say,s the french man.

 

Sounds like the word for "Tidal-wave", when it looks like the tide going in or out, today it's "Harbour-wave" in Japanese. tesumi ( not in my spell checker"

 

spacesailor

 

 

[pmccarthy]

Years ago I paid a fortune for a lot of 25mm bar for a construction job. When I complained, the supplier said it is rare and expensive, a pity you didn't order one inch bar.

 

 

[facthunter]

We certainly have created a difficulty. Fractions of an inch will drive you mad. Metric is to the power of ten I still give bearing and piston clearances in ("Thou's) but use both system pretty interchangeably otherwise.Maybe all the effort has fazed my brain and I'm just not aware of it. The way the Imperial system grew up was like topsy. No one realy stood back and designed it. Nev

 

 

[Old Koreelah]

Years ago I paid a fortune for a lot of 25mm bar for a construction job. When I complained, the supplier said it is rare and expensive, a pity you didn't order one inch bar.

A frustrating experience for you PM, but don't blame the metric system. Blame governments with short memories. They should have kept pushing reform until the new system was properly implemented. Instead they lost interest and turned a blind eye as the medieval system drifted back into the shops.

Governments have a duty to make and maintain standards. America's half-hearted approach to measurement reform has cost heaps: the Gimli Glider incident and the Hubble Mirror fiasco are only the top of the iceberg.

 

How can anyone find Metrics too complicated? A child can learn the system easily. In a little rural school in the '50s I was educated on both systems and know plenty of people older than me who readily adapted to the sensible way of measuring.

 

 

[facthunter]

25 mm is not exactly an inch. IF you ordered a 25mm bar it would have to be made to order. Probably from a 1" bar, which is slightly bigger. Nev

 

 

[fly_tornado]

I walked into Repco last year and asked for some 1/8" hose for the Rotax 503 primer and the young bloke took a while to tell me they didn't have it but the old guy overheard him and offered some 3mm.

 

 

[facthunter]

SEE . You NEED OLD guys ft. Nev

 

 

[fly_tornado]

no i just need 3mm hose, rotax originally specced metric but the aviation industry fixed it.

 

 

[facthunter]

Yes , I agree. You can't replace fuel hose with an old guy. It's just a matter of being on the right wavelength. Nev

 

 

[Yenn]

How do you go adding fractions of an inch. I used to have a calculator just for that. One of those card things with a rotating wheel. In the end I found it easier to convert to mm and do the addition.

 

 

[Happyflyer]

You need a common denominator to add fractions. 1/3 plus 1/4 becomes 4/12 plus 3/12 = 7/12. Amazing the rubbish you remember from school!

 

 

[Bernie]

I'm 68 this month I have never understood the metric system I'm numbers dyslexic .

 

I only have imperial tape measures at least I some what understand the old system as it was bashed into me .

 

It was also a challenge for my CFI's but as the instruments where in imperial I could cope .

 

To this day I cant use a wiz wheel . But I can fly a plane and read a map .

 

Bernie .

 

 

[Oscar]

When Australia bought the Macchi trainers, there was the usual 'local manufacturing' offset deal. Hawker deHavilland at Bankstown got the contract for machining the Oleo legs. Some genius in Defence Procurement ( or whatever it was called then), drew up the contract, changing the metric measurement for tolerance to imperial - to an order of accuracy of something like six or seven decimal places!.

 

HdH had quoted on the basis of the original (metric) drawings, and had the devil's own job trying to make the Defence dummy understand that to produce them to the 'new' specification, would cost something like ten times the quoted price, and for absolutely NO sensible reason. The Defence wallahs (wallies..) insisted that they had it right, so HdH countered with a requirement that the temperature at which they would be measured be specified to extreme precision, since even a minor variation in temp. would cause dimensional change at over the required tolerance. The Defence people seemed to think that if it wasn't 'imperial', then it couldn't be right!

 

 

[dutchroll]

I have only recently started in the aviation game and I had expected that if any industry would have switched over to metric, it would have been aviation. Currently only Myanmar, Liberia and USA are the only countries clinging to imperial (Even the USA military started using metric years ago). A few countries do use metric for aviation, do you think Australia (or other countries) will eventually switch to metric for aviation?

Sounds to me like you were talking about the "feet", "nautical miles" and "knots" used worldwide in aviation before the discussion quickly drifted to metric versus imperial generally speaking.

To understand this you have to go back to the end of World War 2 and the dominance of the USA around the world, supported by the British.

 

The adoption of standardised units in aviation was a consequence of the Chicago Convention in 1944 and subsequent post-war years. The general thrust was to adopt the metric system universally for aviation. However the nautical mile was included due to it's relevance in map usage (being one minute of latitude on maps and charts). Hence also "knots" or "nautical miles per hour" for speed. Even so, there's no specific reason it couldn't be adapted to kilometres but it remains as it is for convenience. The foot for altitude was purely a result of American and British insistence. There's absolutely no reason it should be used for altitude, but he who wins the war writes the post-war rules.

 

As to metric versus imperial measurements for other purposes, again you have to look at the aircraft and general manufacturing juggernaut of the USA after the war. There's not actually a lot about the old imperial measurements which makes logical sense in the modern day, but trying to get a powerful nation with significant world influence to change is....well....not all that easy. Technically the USA is supposed to have changed to metric (Congress authorised it back in 1975), but practically the country has largely refused to do so mainly due to the sheer cost and effort it would require. Put simply, it's "too hard".

 

It hasn't been without consequential trauma either. NASA lost a $125 million Mars probe in 1999 as a direct result of confusion between imperial and metric measurements. In 2006 a remote NASA spacecraft rammed into a military satellite it was meant to dock with for the same reason.

 

Interestingly I witnessed a discussion on metric versus imperial between two American engineers not long ago. One designed and built aircraft, the other was involved in precision CAD manufacturing. The aircraft guy was arguing that he found it easier to think in terms of the old fashioned imperial units (probably because that's just what he grew up with). The precision manufacturing guy argued the complete opposite, saying that in his line of business, nothing about the old system made any sense and metric was far better. When you think about it, metric measurement is a natural "base 10" system just like we use in mathematics everywhere in the world (including the USA). I personally can't see why they hang onto imperial, other than the old "it's just too hard to change" reason.

 

 

[kasper]

From my experience and perspective it matters very little what a unit is called so long as you have an actual appreciation of what it means and what the names are within use of the unit system.

 

In flying and manufacture I cross from imperial UK to to imperial US and metric without a great deal of problems because I understand what a metre is, and a foot and a kilogram and even a pound. The appreciation an ability to move between the systems is based on the core appreciation and an understanding of the major divisions and aggregations within the systems ... I have no idea what a gill or a chain is but I don't need those and equally a nanometre is a bit too fine for practical application in flying.

 

I would be equally happy using 'wombats' as a base measure of mass and 'koalas' as the base measure of length - its simply a name given to a known base unit. The argument then comes down to how to divide and aggregate the units.

 

Imperial uses a system that is nominal to division across many low units of measure

 

Metric uses 10 as the base aggregation and division for simplicity of mathematical calculation

 

Imperial ends up with even division and aggregation across small units that are useful for everyday human operations - particularly 3 and 6 ... but in engineering we ended up using thousandths of inch anyway ... which is a metric divisional split across the imperial base unit

 

No system is perfect but whatever base you use all that is really needed is an understanding of what the base unit looks/feels like and the way things are divided/aggregated.

 

And personally I am one of those that crossed from imperial to metric in school and find that the metric divisional system is my preferred system - base units make bugger all difference but the base 10 divisional/aggregation is more widely usable across everyday and detailed engineering management.

 

 

[Head in the clouds]

Interestingly I witnessed a discussion on metric versus imperial between two American engineers not long ago. One designed and built aircraft, the other was involved in precision CAD manufacturing. The aircraft guy was arguing that he found it easier to think in terms of the old fashioned imperial units (probably because that's just what he grew up with). The precision manufacturing guy argued the complete opposite, saying that in his line of business, nothing about the old system made any sense and metric was far better. When you think about it, metric measurement is a natural "base 10" system just like we use in mathematics everywhere in the world (including the USA). I personally can't see why they hang onto imperial, other than the old "it's just too hard to change" reason.

That's interesting - I've experienced almost the opposite.

 

I provide a drafting service for both construction industry structural steel, which is fabricated by boilermakers, and precision machining drawings for toolmakers and general engineers.

 

The standard overall tolerance requirement for structural steel is plus or minus 2mm on any individual member or fitting of a structure, so the metric system works very well for steel fabricators and many other welded, riveted and/or bonded structures, most automotive coachbuilders and cabinetmakers, for example, work to plus or minus 1mm, rather than 2mm.

 

Machining is quite a different matter and just about all the old fellas use imperial and I know many of the young brigade, who were only taught in metric at college, who also use imperial wherever possible. The reason behind that is that the metric system doesn't work well with required fits for most medium and high precision machining. A tenth of a millimetre is very close to 4 thousands of an inch and that is not nearly a close enough fit for almost anything except some of the cheap Chinese gear some may buy. A hundredth of a millimetre is 0.4 thousandths, or "four tenths" as a toolmaker would say (that's four tenths of a thousandth of an inch).

 

Given that the most precise machining requirements I ever detail is for an accuracy of two tenths (with a tiny tolerance) and the usual range of fits is between half a thou and two thou, you can see that in metric we need to be specifying figures like a two hundredth of a millimetre (rather than a hundredth - or 0.005 rather than 0.01) and then the range of an eightieth of a millimetre to a twentieth of a millimetre (0.0125 to 0.05) - so, regardless of being a metric system machinists still have to work in bases that are rarely ten ...

 

CNC machines take the thinking and need for conversion away from the human machinist in many cases but they're still working odd fractions, even when scaled or programmed in millimetres.

 

Not only that but just as some pilots may confuse fuel loading in pounds when it should have been kilos - you should see the number of precision machined parts that end up in the scrap bin when a CNC machine (or human machinist) is set to 0.05 instead of 0.005 - that's when the difference between the imperial "2 thou" and "half a thou" seems to be more readily understood. Perhaps it's easier to perceive the actual size that way, than to recognise the number of zeros in the metric equivalent.

 

Drilling, tapping and reaming is similarly easier in the 'old system' - you seem to need far fewer to get you there if you have a full set of imperials in 1/64th increments and a set of number and letter drills, than if you have metrics, even in 0.1mm increments - even if you can afford them all.

 

 

[dutchroll]

"Four tenths of a thousandth of an inch".

 

Lol.......I've always found it ironic listening to pro-imperial arguments while simultaneously talking about divisors and multiples of ten or a thousand, applied to inches! I think that's a case of the imperial measurement system wanting to have its cake and eat it too.

 

 

[Head in the clouds]

"Four tenths of a thousandth of an inch".Lol.......I've always found it ironic listening to pro-imperial arguments while simultaneously talking about divisors and multiples of ten or a thousand, applied to inches! I think that's a case of the imperial measurement system wanting to have its cake and eat it too.

Sorry dutch, but your irony is misplaced. The inch isn't otherwise divided into anything except by individual choice, you can fraction it into anything you like, tenths, halves, quarters or even sixths if it serves you best. Your argument may apply if you were talking about feet or yards or miles being divided into smaller parts using the base ten, but it's perfectly correct to divide inches decimally where it suits.

 

Similarly, the metre isn't, itself, a metric unit, although it was once defined as a metric division of the distance from here to the sun and later a metric division of the distance around the equator, IIRC, since 1983 it has been "the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second". So - that particular length, itself, isn't metric. The metric aspect only begins when we divide or multiply it into lengths using the base ten. In that sense it's no different to doing the same to the inch, except the inch is far more appropriate a size where machine engineering is concerned.

 

 

[Old Koreelah]

My understanding is that the whole Metric System is based on the metre, which is one ten millionth the distance from the North Pole (thru some place in France) to the equator. That's 10,000km, so it's 40,000 km around the planet.

 

(Interestingly, divide that by the Babylonians' 360 circle and you get 111.11111111. That means each degree on the surface is 111.11111111km or {edit} 60 nautical miles.)

 

What I love about is everything is interrelated- all other measures are defined by the metre: volume, mass, energy..., not the length of some ancient king's thumb.

 

 

[Kamloops]

1 nautical mile is 1.852 kms.

 

 

[SDQDI]

That means each degree on the surface is 111.11111111km or 1 nautical mile.)

You have a totally different measurement for 1nm than what I use old K:what the:

 

 

[dutchroll]

Similarly, the metre isn't, itself, a metric unit, although it was once defined as a metric division of the distance from here to the sun and later a metric division of the distance around the equator, IIRC, since 1983 it has been "the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second". So - that particular length, itself, isn't metric. The metric aspect only begins when we divide or multiply it into lengths using the base ten. In that sense it's no different to doing the same to the inch, except the inch is far more appropriate a size where machine engineering is concerned.

The metre was born of the "International Metre Commission" in 1870-1872, which then led to the creation of the BIPM (International Bureau of Weights and Measures) and in turn, SI units.

It is by definition metric, and an integral and original part of the metric system. How it has been precisely redefined over the years is a matter of convenience more than anything. Redefining the unit for the sake of accuracy doesn't change the whole crux of the metric system.

 

As far as the inch being more appropriate for machine engineering, I'd be interested if you could get support for that contention from the engineering powerhouses of Germany. Or Russia. Or anywhere else except countries which have clung to imperial measurements! Maybe the Germans were never so crash hot at machine engineering after all?