Moderators: richierich, ua900, PanAm_DC10, hOMSaR

 
vedatil4
Topic Author
Posts: 270
Joined: Thu Sep 17, 2015 4:38 pm

why no metric system in aviation?

Sun Mar 14, 2021 10:50 pm

Hello A.net friends,

Something that has always caught my attention is the continued use of knots, feet, pounds, etc. (USA customary units) and not the metric system in aviation. Being from an engineering background, I know firsthand the problems that can arise using customary units and not the metric system.

One very obscure aviation and/or civil engineering example I learned about recently is the use of the international foot in avionics but the US survey foot was used for surveying at some airports. So pilots were unknowingly entering an incorrect runway elevation sometimes. This is just the story I heard at a seminar. Please fill me in on this problem if you know more details.

But an easier example I can give you is trying to join a 2 inch plastic pipe to a 50mm pipe. The difference is just .04 inches or 1.016mm along the circumference. This is almost invisible to the naked eye but water, or hydraulic fluid I imagine, will notice the difference instantly.

95% of the world is using the metric system. Airlines fly all over the world. Airbus based out of Europe and is doing good business. Why, oh why, continue using customary units? There have been accidents before; the Gimli Glider comes to mind. Please explain the reason for sticking it with a measurement system that just invites problems. Example: rounding tape measurements to 1/16th of an inch which add up over long distances.

I leave you with a funny quote: "My car gets 40 rods to the hogshead and that's the way I likes it". ;-)
 
Kent350787
Posts: 2040
Joined: Wed May 28, 2008 12:06 am

Re: why no metric system in aviation?

Mon Mar 15, 2021 1:35 am

Although the USA is now the only country where the Imperial system is in widespread use, the former British Empire also used it up until the 1970s. I'd expect that bloc cemented its use and that change to the dominant system is now considered to hard for international aviation.
 
Bostrom
Posts: 1116
Joined: Tue Feb 02, 2016 7:11 pm

Re: why no metric system in aviation?

Mon Mar 15, 2021 1:51 am

vedatil4 wrote:
95% of the world is using the metric system. Airlines fly all over the world. Airbus based out of Europe and is doing good business. Why, oh why, continue using customary units? There have been accidents before; the Gimli Glider comes to mind. Please explain the reason for sticking it with a measurement system that just invites problems. Example: rounding tape measurements to 1/16th of an inch which add up over long distances.


International aviation uses SI-units, with a few exceptions; the foot for altitude and vertical speed, the knot for horisontal speed and nautical mile for longer horisontal distances. Those units are "permitted for temporary use", but no end date has been set as far as I know. Apart from that it is SI units, temperature in Celsius, runway length in metres, fuel in kilos, air pressure in hPa etc. You can read more in this document: https://www.icao.int/publications/Documents/9958_en.pdf
 
johns624
Posts: 4067
Joined: Mon Jul 07, 2008 11:09 pm

Re: why no metric system in aviation?

Mon Mar 15, 2021 2:19 am

The UK still does some stuff non-metric. Distances and speeds are still in miles. Ales are still in pints. :D
 
Kent350787
Posts: 2040
Joined: Wed May 28, 2008 12:06 am

Re: why no metric system in aviation?

Mon Mar 15, 2021 2:38 am

johns624 wrote:
The UK still does some stuff non-metric. Distances and speeds are still in miles. Ales are still in pints. :D


But it's a weird by exception thing. ANd currency only decimalised around the time the former Empire was adopting the metric system in full.

But a pint is far preferred to 330ml of beer
 
GalaxyFlyer
Posts: 8359
Joined: Fri Jan 01, 2016 4:44 am

Re: why no metric system in aviation?

Mon Mar 15, 2021 3:21 am

Knots and nautical miles is best for navigation, easy to use minutes of latitude to measure distance. But, lots of aviation does use metric and any international pilot needs to be conversant in both.
 
User avatar
T18
Posts: 828
Joined: Sat Jan 31, 2009 12:28 am

Re: why no metric system in aviation?

Mon Mar 15, 2021 1:59 pm

GalaxyFlyer wrote:
Knots and nautical miles is best for navigation, easy to use minutes of latitude to measure distance. But, lots of aviation does use metric and any international pilot needs to be conversant in both.


Eh, my e6B con do the conversion ;)
 
dfwjim1
Posts: 2591
Joined: Fri Sep 09, 2011 8:46 pm

Re: why no metric system in aviation?

Mon Mar 15, 2021 2:17 pm

Back in the 70s the United States Government proposed a plan for the U.S. to completely switch to the metric system but there was such a public outcry against its implementation that the swichover was dropped.

Always thought it was a shame that it didn't happen.
 
User avatar
CrewBunk
Posts: 284
Joined: Sat Nov 18, 2017 3:12 am

Re: why no metric system in aviation?

Mon Mar 15, 2021 4:29 pm

Canada started conversion to metric in the early 1970s. Aviation was one of the areas that resisted the conversion.

In the early 1980s, Air Canada was 100% government owned, and subject to the political whims and policies of the era. The delivery of the new 767 therefore, was the first of AC’s aircraft (and the first in Canada) that was required to be in metric where feasible.

The 767 being metric in Canada was the start of a long chain of events that ended up with one landing in Gimli, Manitoba with no fuel in 1983.

Fuel ordered in kilos, delivered in pounds. With a malfunctioning fuel total system, a drip was required. Boeing charts in inches, drip sticks in cms. Each roughly 2:1. As a result of being metric, that 767 took off with less than half the required fuel.

None of this would have occurred had the aircraft been delivered in Imperial units instead of Metric.
 
johns624
Posts: 4067
Joined: Mon Jul 07, 2008 11:09 pm

Re: why no metric system in aviation?

Mon Mar 15, 2021 6:58 pm

I think it's an advantage since many Americans are "bilingual" in measurements, especially those that have to do with speed, length or distance...
 
vedatil4
Topic Author
Posts: 270
Joined: Thu Sep 17, 2015 4:38 pm

Re: why no metric system in aviation?

Tue Mar 16, 2021 1:58 pm

CrewBunk wrote:
Canada started conversion to metric in the early 1970s. Aviation was one of the areas that resisted the conversion.

In the early 1980s, Air Canada was 100% government owned, and subject to the political whims and policies of the era. The delivery of the new 767 therefore, was the first of AC’s aircraft (and the first in Canada) that was required to be in metric where feasible.

The 767 being metric in Canada was the start of a long chain of events that ended up with one landing in Gimli, Manitoba with no fuel in 1983.

Fuel ordered in kilos, delivered in pounds. With a malfunctioning fuel total system, a drip was required. Boeing charts in inches, drip sticks in cms. Each roughly 2:1. As a result of being metric, that 767 took off with less than half the required fuel.

None of this would have occurred had the aircraft been delivered in Imperial units instead of Metric.


Thank you for sharing the backstory of the Gimli glider. I love Canada and always thought they had adopted the metric system whole heartedly. I wish we had the 1-liter milk bags.

But what about technicians in 95% of the world? They're being expected to have a grasp of charts and drip sticks in customary units? That's probably the only time of their day when they'll deal with those units. So the risk of another Gimli-like conversion error event lives on and might be higher now?
 
CowAnon
Posts: 219
Joined: Fri Nov 03, 2017 12:03 am

Re: why no metric system in aviation?

Wed Mar 17, 2021 6:48 pm

AFAIK, ex-Soviet countries still use horsepower instead of kilowatts for engine power and kilograms-force (which is metric but not SI) instead of kilonewtons for thrust.
 
User avatar
WesternDC6B
Posts: 787
Joined: Thu Mar 14, 2013 3:05 pm

Re: why no metric system in aviation?

Wed Mar 17, 2021 8:24 pm

Kent350787 wrote:
Although the USA is now the only country where the Imperial system is in widespread use, the former British Empire also used it up until the 1970s. I'd expect that bloc cemented its use and that change to the dominant system is now considered to hard for international aviation.


Picking a nit here:the USA used the same linear measures, but, our quarts are 32 oz not 40, and our gallon is 128oz not 160. Our system has been called “customary”, while the British one is “Imperial”.

Short anecdote:I recall the first time I drove into a petrol station in England. It was 1969 and most stations still displayed cans of motor oil. I thought to myself that those “quart” cans of oil looked quite tall compared to back home. Then I remembered the difference.

I, for one, prefer metric. I weigh so much less that way! Only 123, and not 271! :duck:
 
Kent350787
Posts: 2040
Joined: Wed May 28, 2008 12:06 am

Re: why no metric system in aviation?

Wed Mar 17, 2021 10:29 pm

WesternDC6B wrote:
Kent350787 wrote:
Although the USA is now the only country where the Imperial system is in widespread use, the former British Empire also used it up until the 1970s. I'd expect that bloc cemented its use and that change to the dominant system is now considered to hard for international aviation.


Picking a nit here:the USA used the same linear measures, but, our quarts are 32 oz not 40, and our gallon is 128oz not 160. Our system has been called “customary”, while the British one is “Imperial”.


A fair nit to pick - I know a US gallon is only 4 litres, rather than the Imperial 5 litres, but I didn't know that the US variant of the Imperial system was "customary". Whichever way, it can be a little strange crossing the border into either Eire or Canada. Speedo in mph and speeds in kmh isn't good unless you know (or remember from childhood) your conversions.
 
UA857
Posts: 718
Joined: Thu Aug 10, 2017 3:41 am

Re: why no metric system in aviation?

Thu Mar 18, 2021 6:35 am

 
vedatil4
Topic Author
Posts: 270
Joined: Thu Sep 17, 2015 4:38 pm

Re: why no metric system in aviation?

Thu Mar 18, 2021 7:54 am

A gallon in the USA is roughly 3.785 liters which is not the same as an imperial gallon. I use the word "roughly" because there are many digits past the number 5 that shouldn't be ignored if the number of gallons is large and is later being multiplied by another number, specific weight, to get to a weight.

Also long tons, short tons, and tonnes (also called metric tons) are not the same; 2240 pounds, 2000 pounds, and 1000 kilograms, respectively. BTW: the last one is a mass not a weight.

The point is that customary units such as gallons and tons are not consistently the same wherever used. Planes fly all over the world. I hope ground crews and pilots are aware of this. I hope they know these minor but sometimes crucial differences.

I figured aviation would've adopted the metric system completely and outlawed any kind of customary units as a way to avoid the possibility of calculation errors after the Gimli Glider.
 
vedatil4
Topic Author
Posts: 270
Joined: Thu Sep 17, 2015 4:38 pm

Re: why no metric system in aviation?

Thu Mar 18, 2021 8:15 am

Bostrom wrote:
vedatil4 wrote:
95% of the world is using the metric system. Airlines fly all over the world. Airbus based out of Europe and is doing good business. Why, oh why, continue using customary units? There have been accidents before; the Gimli Glider comes to mind. Please explain the reason for sticking it with a measurement system that just invites problems. Example: rounding tape measurements to 1/16th of an inch which add up over long distances.


International aviation uses SI-units, with a few exceptions; the foot for altitude and vertical speed, the knot for horisontal speed and nautical mile for longer horisontal distances. Those units are "permitted for temporary use", but no end date has been set as far as I know. Apart from that it is SI units, temperature in Celsius, runway length in metres, fuel in kilos, air pressure in hPa etc. You can read more in this document: https://www.icao.int/publications/Documents/9958_en.pdf


Thanks for sharing. Unfortunately it looks like Appendix F for "units of measurement" was not included.

If the foot will continue to be allowed for altitude I hope you're all agreeing to the international foot with a metric system definition. The US survey foot is not consistently used even within the USA from one state to another.
 
User avatar
Lingon
Posts: 130
Joined: Sun Sep 03, 2017 4:18 pm

Re: why no metric system in aviation?

Thu Mar 18, 2021 12:43 pm

I believe Russia and China uses metres for altitudes (correct me if I am wrong). So it is partially implemented...

We children of the SI shouldn't point fingers too much. In Sweden, engine power of cars are still in horsepower (kilowatts mentioned too, but it is the horsepower that counts - it's a bigger figure..), diet and health columns still count calories instead of joules, I know of no country where speed limits on the roads are in metres per seconds.
Even if the horsepower is founded in the metric system and speeds in kilometres per hour are too, they will need a conversion factor to the proper SI units.
Changing systems turns your world upside down, but it is for the benefit of generations to come, so I am all for proper SI units. Can't wait to see a "25" speed limit sign (m/s) instead of "90" (km/h). After a decade or so, even us 50+ fossils will adapt and get a feeling for new units.
For you UK people, a pint of beer is more than a 330 ml beer but less than a 500 ml beer (referred to as small or big here).
 
GalaxyFlyer
Posts: 8359
Joined: Fri Jan 01, 2016 4:44 am

Re: why no metric system in aviation?

Thu Mar 18, 2021 1:54 pm

Why would m/s be better than km/hr? Russia recently converted to feet for altitudes/levels, don’t know about China.
 
CowAnon
Posts: 219
Joined: Fri Nov 03, 2017 12:03 am

Re: why no metric system in aviation?

Thu Mar 18, 2021 7:34 pm

^^^Because 1 meter per second is about 2.2 miles per hour, just like 1 kilogram is about 2.2 pounds. So that's one less conversion number you need to remember. :cheeky:

In the U.S., schools teach people that 1 meter is a little more than 3 feet, but they don't bother to tell us (at least in my school) the significance or reasoning of that measurement. It wasn't until a few years ago that I learned that 10,000 kilometers is defined as the distance between the North Pole and the equator. That sort of "legitimized" the SI length standard for me, as before that it was just some miscellaneous measurement forced on the country from the outside world.
 
GalaxyFlyer
Posts: 8359
Joined: Fri Jan 01, 2016 4:44 am

Re: why no metric system in aviation?

Thu Mar 18, 2021 7:47 pm

Okay, if your traveling at 35m/s and the next gas station is 75 km down the road, how long will it take to drive there? Seems far easier using 108 km/hr, so about 45 minutes. The only time m/s works for speed is wind velocity—double it to get knots and at small magnitudes like wind, it’s accurate enough.

A nautical mile is more intuitive—one minute of latitude. 6 degrees equals 360 no.
 
flipdewaf
Posts: 4194
Joined: Thu Jul 20, 2006 6:28 am

Re: why no metric system in aviation?

Thu Mar 18, 2021 11:53 pm

Lingon wrote:
For you UK people, a pint of beer is more than a 330 ml beer but less than a 500 ml beer (referred to as small or big here).

Pint of beer in the US may be less than 500ml but in the UK its 568ml...it's also often not served cold...

Fred
 
Kent350787
Posts: 2040
Joined: Wed May 28, 2008 12:06 am

Re: why no metric system in aviation?

Fri Mar 19, 2021 12:06 am

flipdewaf wrote:
Lingon wrote:
For you UK people, a pint of beer is more than a 330 ml beer but less than a 500 ml beer (referred to as small or big here).

Pint of beer in the US may be less than 500ml but in the UK its 568ml...it's also often not served cold...

Fred


How in the heck is a pint less than 568ml?That is extremely wrong. Also noting that "warm" beer is still around 10degC (I'm not converting) and quite flavoursome and drinkable. A "commercial" lager really does need to be prpoerly cold to hide the fact it tastes of very little.

In Australia we converted pretty much entirely to metric in the 1970s, but not completely in aviation.

As for beer, in my State we have a middy at 285ml (formerly 10floz) and schooner (425ml, formerly 15floz). Unfortunately the bastard "schmiddy" has made an inroad into expensive pubs but is really just and excuse to charge you schooner prices for a little more than a middy.

When served on aircraft, beer is Australia is usually the international 330ml, although sometimes a "stubby" 250ml. Australian beers are generally canned or bottle as 375ml (schooner volume, allowing for head in a glass)
 
User avatar
RyanairGuru
Posts: 8715
Joined: Wed Nov 01, 2006 3:59 am

Re: why no metric system in aviation?

Fri Mar 19, 2021 12:53 am

Kent350787 wrote:
flipdewaf wrote:
Lingon wrote:
For you UK people, a pint of beer is more than a 330 ml beer but less than a 500 ml beer (referred to as small or big here).

Pint of beer in the US may be less than 500ml but in the UK its 568ml...it's also often not served cold...

Fred


How in the heck is a pint less than 568ml?That is extremely wrong. Also noting that "warm" beer is still around 10degC (I'm not converting) and quite flavoursome and drinkable. A "commercial" lager really does need to be prpoerly cold to hide the fact it tastes of very little.

In Australia we converted pretty much entirely to metric in the 1970s, but not completely in aviation.

As for beer, in my State we have a middy at 285ml (formerly 10floz) and schooner (425ml, formerly 15floz). Unfortunately the bastard "schmiddy" has made an inroad into expensive pubs but is really just and excuse to charge you schooner prices for a little more than a middy.

When served on aircraft, beer is Australia is usually the international 330ml, although sometimes a "stubby" 250ml. Australian beers are generally canned or bottle as 375ml (schooner volume, allowing for head in a glass)


In the US a pint is 16oz rather than 20oz in the Imperial system. That’s why their gallons are smaller, there are 8 pints in a gallon in both the US and Imperial systems.
 
889091
Posts: 366
Joined: Sat Apr 06, 2019 7:56 pm

Re: why no metric system in aviation?

Fri Mar 19, 2021 1:13 am

 
N1120A
Posts: 26859
Joined: Sun Dec 14, 2003 5:40 pm

Re: why no metric system in aviation?

Fri Mar 19, 2021 2:52 am

RyanairGuru wrote:
Kent350787 wrote:
flipdewaf wrote:
Pint of beer in the US may be less than 500ml but in the UK its 568ml...it's also often not served cold...

Fred


How in the heck is a pint less than 568ml?That is extremely wrong. Also noting that "warm" beer is still around 10degC (I'm not converting) and quite flavoursome and drinkable. A "commercial" lager really does need to be prpoerly cold to hide the fact it tastes of very little.

In Australia we converted pretty much entirely to metric in the 1970s, but not completely in aviation.

As for beer, in my State we have a middy at 285ml (formerly 10floz) and schooner (425ml, formerly 15floz). Unfortunately the bastard "schmiddy" has made an inroad into expensive pubs but is really just and excuse to charge you schooner prices for a little more than a middy.

When served on aircraft, beer is Australia is usually the international 330ml, although sometimes a "stubby" 250ml. Australian beers are generally canned or bottle as 375ml (schooner volume, allowing for head in a glass)


In the US a pint is 16oz rather than 20oz in the Imperial system. That’s why their gallons are smaller, there are 8 pints in a gallon in both the US and Imperial systems.


Imperial pints are actually 19.2 oz.
 
User avatar
RyanairGuru
Posts: 8715
Joined: Wed Nov 01, 2006 3:59 am

Re: why no metric system in aviation?

Fri Mar 19, 2021 5:31 am

N1120A wrote:
RyanairGuru wrote:
Kent350787 wrote:

How in the heck is a pint less than 568ml?That is extremely wrong. Also noting that "warm" beer is still around 10degC (I'm not converting) and quite flavoursome and drinkable. A "commercial" lager really does need to be prpoerly cold to hide the fact it tastes of very little.

In Australia we converted pretty much entirely to metric in the 1970s, but not completely in aviation.

As for beer, in my State we have a middy at 285ml (formerly 10floz) and schooner (425ml, formerly 15floz). Unfortunately the bastard "schmiddy" has made an inroad into expensive pubs but is really just and excuse to charge you schooner prices for a little more than a middy.

When served on aircraft, beer is Australia is usually the international 330ml, although sometimes a "stubby" 250ml. Australian beers are generally canned or bottle as 375ml (schooner volume, allowing for head in a glass)


In the US a pint is 16oz rather than 20oz in the Imperial system. That’s why their gallons are smaller, there are 8 pints in a gallon in both the US and Imperial systems.


Imperial pints are actually 19.2 oz.


Indirectly you just taught me something I never knew, US and Imperial fluid ounces are different. I honestly thought that fluid ounces were constant between both.

An Imperial pint in 20 Imperial fluid ounces, or (as you said) 19.2 US fluid ounces.

An Imperial fluid ounce is 1/20 of an Imperial pint or 28.4 ml, while a US fluid ounce is 1/16 of a US pint or 29.6 ml.
 
User avatar
Lingon
Posts: 130
Joined: Sun Sep 03, 2017 4:18 pm

Re: why no metric system in aviation?

Fri Mar 19, 2021 1:20 pm

GalaxyFlyer wrote:
Okay, if your traveling at 35m/s and the next gas station is 75 km down the road, how long will it take to drive there? Seems far easier using 108 km/hr, so about 45 minutes. The only time m/s works for speed is wind velocity—double it to get knots and at small magnitudes like wind, it’s accurate enough.

A nautical mile is more intuitive—one minute of latitude. 6 degrees equals 360 no.


I didn't intend to say m/s is always better, but it is the SI unit of speed. km/h is derived from SI units, as is the metric horsepower, but they are not the SI units for speed and power.
So my point was when we still use such units in the "metric world" we should not complain about feet or nautical miles. But if we were to go "all SI", I could very well live with it.

flipdewaf wrote:
Lingon wrote:
For you UK people, a pint of beer is more than a 330 ml beer but less than a 500 ml beer (referred to as small or big here).

Pint of beer in the US may be less than 500ml but in the UK its 568ml...it's also often not served cold...

Fred


Mea culpa. I had no clue about the differences between UK and US - it's a little bit like the old feet where every country had its own.
 
N1120A
Posts: 26859
Joined: Sun Dec 14, 2003 5:40 pm

Re: why no metric system in aviation?

Sun Mar 21, 2021 5:42 am

RyanairGuru wrote:
N1120A wrote:
RyanairGuru wrote:

In the US a pint is 16oz rather than 20oz in the Imperial system. That’s why their gallons are smaller, there are 8 pints in a gallon in both the US and Imperial systems.


Imperial pints are actually 19.2 oz.


Indirectly you just taught me something I never knew, US and Imperial fluid ounces are different. I honestly thought that fluid ounces were constant between both.

An Imperial pint in 20 Imperial fluid ounces, or (as you said) 19.2 US fluid ounces.

An Imperial fluid ounce is 1/20 of an Imperial pint or 28.4 ml, while a US fluid ounce is 1/16 of a US pint or 29.6 ml.


Imperial gallons are larger than US gallons. It is all very weird.
 
vedatil4
Topic Author
Posts: 270
Joined: Thu Sep 17, 2015 4:38 pm

Re: why no metric system in aviation?

Sun Mar 21, 2021 12:16 pm

In a similar way, a nautical mile, an international mile, and a survey mile aren't the same distance. So what's the advantage of using the nautical mile? Most people around the world don't have a grasp of the concept of a mile measured on the ground. I don't think they'd know there's a slight difference when nautical miles are used. Also, why miles and not furlongs? Couldn't we just use kilometers to avoid the possibility of any measurement errors?

Someone explain what the advantage is of usung nautical miles in navigation when the meter is based on the size of the earth.

When aviation first started did the US set all the standards so that everyone else around the world was forced to use a customary unit? In a way similar to forcing someone to buy an extra set of wrenches (or spanners)? See, even for an English word for a thing we're not all in agreement.
 
Bostrom
Posts: 1116
Joined: Tue Feb 02, 2016 7:11 pm

Re: why no metric system in aviation?

Sun Mar 21, 2021 12:34 pm

vedatil4 wrote:
Bostrom wrote:
vedatil4 wrote:
95% of the world is using the metric system. Airlines fly all over the world. Airbus based out of Europe and is doing good business. Why, oh why, continue using customary units? There have been accidents before; the Gimli Glider comes to mind. Please explain the reason for sticking it with a measurement system that just invites problems. Example: rounding tape measurements to 1/16th of an inch which add up over long distances.


International aviation uses SI-units, with a few exceptions; the foot for altitude and vertical speed, the knot for horisontal speed and nautical mile for longer horisontal distances. Those units are "permitted for temporary use", but no end date has been set as far as I know. Apart from that it is SI units, temperature in Celsius, runway length in metres, fuel in kilos, air pressure in hPa etc. You can read more in this document: https://www.icao.int/publications/Documents/9958_en.pdf


Thanks for sharing. Unfortunately it looks like Appendix F for "units of measurement" was not included.

If the foot will continue to be allowed for altitude I hope you're all agreeing to the international foot with a metric system definition. The US survey foot is not consistently used even within the USA from one state to another.


It seems like appendix F is not available as a download, but you can find parts of it with a bit of googling. But as a I mentioned, the standard is SI units with a few exceptions. Including the foot which is defined as 304.8 mm exactly.

vedatil4 wrote:
In a similar way, a nautical mile, an international mile, and a survey mile aren't the same distance. So what's the advantage of using the nautical mile? Most people around the world don't have a grasp of the concept of a mile measured on the ground. I don't think they'd know there's a slight difference when nautical miles are used. Also, why miles and not furlongs? Couldn't we just use kilometers to avoid the possibility of any measurement errors?

Someone explain what the advantage is of usung nautical miles in navigation when the meter is based on the size of the earth.

When aviation first started did the US set all the standards so that everyone else around the world was forced to use a customary unit? In a way similar to forcing someone to buy an extra set of wrenches (or spanners)? See, even for an English word for a thing we're not all in agreement.


Wild guess: When aviation started, skilled navigators where needed and those came from the naval sector and where used to working in knots and nautical miles and brought those units with them.
 
vedatil4
Topic Author
Posts: 270
Joined: Thu Sep 17, 2015 4:38 pm

Re: why no metric system in aviation?

Sun Mar 21, 2021 1:18 pm

It's good that SI units are the official international standard or define the remaiming customary terms such as the foot. (at least for distance).

My engineering experience has been that it's when a metric size thing has to join up with a customary size thing that there's slop, there's a chance of stripping something, or the math gets intense. This is the worst when it comes to drill bits and screw threads.

That being said, and since SI is the standard for aviation, I shouldn't expect to see a 9/16inch or 5/8inch spanner inside an aviation mechanic's toolbox when only 14 or 16mm should be there right? Is there a chance I see one, let's say in, Chicago but definitely not in Stockholm or Buenos Aires?

Is it fair to say there should be no customary size mechanical parts in a modern airplane?
 
Chemist
Posts: 946
Joined: Tue Oct 20, 2015 4:46 am

Re: why no metric system in aviation?

Sun Mar 21, 2021 6:04 pm

Let's be thankful that we all use seconds, minutes, hours, weeks, and months consistently.
For time, you could certainly envision a metric system of metric seconds, hours - where 100 metric seconds was a metric minute, 100 metric minutes were a metric hour, and 10 metric hours were a metric day.
 
Wednesdayite
Posts: 272
Joined: Sun Feb 19, 2017 8:28 pm

Re: why no metric system in aviation?

Sun Mar 21, 2021 8:38 pm

Chemist wrote:
Let's be thankful that we all use seconds, minutes, hours, weeks, and months consistently.
For time, you could certainly envision a metric system of metric seconds, hours - where 100 metric seconds was a metric minute, 100 metric minutes were a metric hour, and 10 metric hours were a metric day.


Tested in France in the 1790’s.

They still have a few decimal clock faces from that time.
 
m1m2
Posts: 261
Joined: Sat Dec 31, 2011 3:39 am

Re: why no metric system in aviation?

Sun Mar 21, 2021 8:52 pm

I work on mainly Canadian built aircraft and I've never seen a metric fastener on any of them (Dash 8 and CRJ's). In my tool box is all standard size wrenches and sockets 1/4", 1/2", 3/4", etc. I've worked on Cessna and Piper in the past, all standard bolts/nuts, that sort of thing. I also don't remember seeing any metric fasteners on the few Dassault aircraft I've worked on either, and every altimeter I have ever seen is in feet as well as every airspeed indicator being in knots.
 
GalaxyFlyer
Posts: 8359
Joined: Fri Jan 01, 2016 4:44 am

Re: why no metric system in aviation?

Sun Mar 21, 2021 9:26 pm

vedatil4 wrote:
In a similar way, a nautical mile, an international mile, and a survey mile aren't the same distance. So what's the advantage of using the nautical mile? Most people around the world don't have a grasp of the concept of a mile measured on the ground. I don't think they'd know there's a slight difference when nautical miles are used. Also, why miles and not furlongs? Couldn't we just use kilometers to avoid the possibility of any measurement errors?

Someone explain what the advantage is of usung nautical miles in navigation when the meter is based on the size of the earth.

When aviation first started did the US set all the standards so that everyone else around the world was forced to use a customary unit? In a way similar to forcing someone to buy an extra set of wrenches (or spanners)? See, even for an English word for a thing we're not all in agreement.


Then, you don’t know what a nautical mile is—one minute of latitude. Last I knew, latitude on Earth was Earth-based. It is a very useful measurement if you’ve ever used a......chart.
 
GalaxyFlyer
Posts: 8359
Joined: Fri Jan 01, 2016 4:44 am

Re: why no metric system in aviation?

Sun Mar 21, 2021 9:29 pm

Bostrom wrote:
vedatil4 wrote:
Bostrom wrote:

International aviation uses SI-units, with a few exceptions; the foot for altitude and vertical speed, the knot for horisontal speed and nautical mile for longer horisontal distances. Those units are "permitted for temporary use", but no end date has been set as far as I know. Apart from that it is SI units, temperature in Celsius, runway length in metres, fuel in kilos, air pressure in hPa etc. You can read more in this document: https://www.icao.int/publications/Documents/9958_en.pdf


Thanks for sharing. Unfortunately it looks like Appendix F for "units of measurement" was not included.

If the foot will continue to be allowed for altitude I hope you're all agreeing to the international foot with a metric system definition. The US survey foot is not consistently used even within the USA from one state to another.


It seems like appendix F is not available as a download, but you can find parts of it with a bit of googling. But as a I mentioned, the standard is SI units with a few exceptions. Including the foot which is defined as 304.8 mm exactly.

vedatil4 wrote:
In a similar way, a nautical mile, an international mile, and a survey mile aren't the same distance. So what's the advantage of using the nautical mile? Most people around the world don't have a grasp of the concept of a mile measured on the ground. I don't think they'd know there's a slight difference when nautical miles are used. Also, why miles and not furlongs? Couldn't we just use kilometers to avoid the possibility of any measurement errors?

Someone explain what the advantage is of usung nautical miles in navigation when the meter is based on the size of the earth.

When aviation first started did the US set all the standards so that everyone else around the world was forced to use a customary unit? In a way similar to forcing someone to buy an extra set of wrenches (or spanners)? See, even for an English word for a thing we're not all in agreement.


Wild guess: When aviation started, skilled navigators where needed and those came from the naval sector and where used to working in knots and nautical miles and brought those units with them.


Actually, it was in the post-WW II era that we changed from mph using statute miles to nautical miles and knots which had a navigational advantage.
 
vedatil4
Topic Author
Posts: 270
Joined: Thu Sep 17, 2015 4:38 pm

Re: why no metric system in aviation?

Sun Mar 21, 2021 10:45 pm

m1m2 wrote:
I work on mainly Canadian built aircraft and I've never seen a metric fastener on any of them (Dash 8 and CRJ's). In my tool box is all standard size wrenches and sockets 1/4", 1/2", 3/4", etc. I've worked on Cessna and Piper in the past, all standard bolts/nuts, that sort of thing. I also don't remember seeing any metric fasteners on the few Dassault aircraft I've worked on either, and every altimeter I have ever seen is in feet as well as every airspeed indicator being in knots.


What about the clearly superior Robertson screws? Are those used on Canadian aircraft?
 
vedatil4
Topic Author
Posts: 270
Joined: Thu Sep 17, 2015 4:38 pm

Re: why no metric system in aviation?

Sun Mar 21, 2021 11:00 pm

GalaxyFlyer wrote:
vedatil4 wrote:
In a similar way, a nautical mile, an international mile, and a survey mile aren't the same distance. So what's the advantage of using the nautical mile? Most people around the world don't have a grasp of the concept of a mile measured on the ground. I don't think they'd know there's a slight difference when nautical miles are used. Also, why miles and not furlongs? Couldn't we just use kilometers to avoid the possibility of any measurement errors?

Someone explain what the advantage is of usung nautical miles in navigation when the meter is based on the size of the earth.

When aviation first started did the US set all the standards so that everyone else around the world was forced to use a customary unit? In a way similar to forcing someone to buy an extra set of wrenches (or spanners)? See, even for an English word for a thing we're not all in agreement.


Then, you don’t know what a nautical mile is—one minute of latitude. Last I knew, latitude on Earth was Earth-based. It is a very useful measurement if you’ve ever used a......chart.


True, I haven't used a navigation chart. I'm approaching this from a civil or mechanical engineering perspective.

The position of an aircraft in earth's atmosphere doesn't seem as critical as the manufacture and servicing of that same aircraft. Planes normally don't fly that close to each other. A screw coming loose at high speed and altitude because of stripped threads seems more serious. Now I'm glad there's safety wire between bolt heads on engines I've seen. That's good insurance. :-)

I admit it's too late for metric system navigation charts using gradians instead of degrees. But had that idea taken hold, been developed, and made the standard, latitudes and degrees would've seemed as old-fashioned as using "leagues" to measure distance.
 
User avatar
DrPaul
Posts: 157
Joined: Sun Dec 04, 2016 7:21 pm

Re: why no metric system in aviation?

Mon Mar 22, 2021 12:12 am

johns624 wrote:
The UK still does some stuff non-metric. Distances and speeds are still in miles. Ales are still in pints.D


I scared a bunch schoolkids a couple of years back by telling them that now Britain was leaving the European Union, everything, including currency, would be reverting to imperial measures.
 
m1m2
Posts: 261
Joined: Sat Dec 31, 2011 3:39 am

Re: why no metric system in aviation?

Mon Mar 22, 2021 1:35 am

vedatil4 wrote:
m1m2 wrote:
I work on mainly Canadian built aircraft and I've never seen a metric fastener on any of them (Dash 8 and CRJ's). In my tool box is all standard size wrenches and sockets 1/4", 1/2", 3/4", etc. I've worked on Cessna and Piper in the past, all standard bolts/nuts, that sort of thing. I also don't remember seeing any metric fasteners on the few Dassault aircraft I've worked on either, and every altimeter I have ever seen is in feet as well as every airspeed indicator being in knots.


What about the clearly superior Robertson screws? Are those used on Canadian aircraft?


No, Robertson screws are not used on Canadian aircraft although I wish they were. You will find phillips, quad-wing (I think the proper term is apex), or the dreaded tri-wing used. You can probably find examples of them in an aviation hardware catalog.
 
vedatil4
Topic Author
Posts: 270
Joined: Thu Sep 17, 2015 4:38 pm

Re: why no metric system in aviation?

Mon Mar 22, 2021 2:32 am

So now I'm wondering if aircraft are being built with metric electrical wires or American Wire Gauge wires. The fun happens when you take a metric wire stripper to an american wire. I hope mechanics around the world know AWG wires get smaller the higher the number.
 
User avatar
Grizzly410
Posts: 535
Joined: Sun May 10, 2015 8:38 pm

Re: why no metric system in aviation?

Mon Mar 22, 2021 9:03 am

vedatil4 wrote:
So now I'm wondering if aircraft are being built with metric electrical wires or American Wire Gauge wires. The fun happens when you take a metric wire stripper to an american wire. I hope mechanics around the world know AWG wires get smaller the higher the number.


Airbus are AWG.
Any quick explanation on why it gets smaller the higher the number ?
 
vedatil4
Topic Author
Posts: 270
Joined: Thu Sep 17, 2015 4:38 pm

Re: why no metric system in aviation?

Mon Mar 22, 2021 11:21 am

Grizzly410 wrote:
vedatil4 wrote:
So now I'm wondering if aircraft are being built with metric electrical wires or American Wire Gauge wires. The fun happens when you take a metric wire stripper to an american wire. I hope mechanics around the world know AWG wires get smaller the higher the number.


Airbus are AWG.
Any quick explanation on why it gets smaller the higher the number ?


The AWG number was based on how many times a wire had been passed through a die. The more passes, the thinner the wire would get.

I'm surprised Airbus adopted that system of wire measurement since they're based in Europe. The metric system wire numbers are much easier to understand. I know that mechanics around the world would get used to AWG but why even expose them to this idiosyncrasy?

Now I wonder if Airbus also used inch-based micrometers during manufacture. Or how about american D-size drafting paper? (BTW: there's two types and it's easy to mistake one for the other). Or inch-based drafting scales? I shudder to think they'd take metric scales to inch papers or vice-versa.

Again it's when one measurement system meets or is forced to mingle with another that the chance of error increases. I figured aviation would have put out an aircraft that was absolutely and completely metric from concept drawing all the way to maintenance by now.

In civil engineering we have goofy, pseudo-science tests for materials; example: concrete slump. Are there some of those in aviation too?
 
User avatar
Grizzly410
Posts: 535
Joined: Sun May 10, 2015 8:38 pm

Re: why no metric system in aviation?

Mon Mar 22, 2021 12:12 pm

vedatil4 wrote:
Grizzly410 wrote:
vedatil4 wrote:
So now I'm wondering if aircraft are being built with metric electrical wires or American Wire Gauge wires. The fun happens when you take a metric wire stripper to an american wire. I hope mechanics around the world know AWG wires get smaller the higher the number.


Airbus are AWG.
Any quick explanation on why it gets smaller the higher the number ?


The AWG number was based on how many times a wire had been passed through a die. The more passes, the thinner the wire would get.

I'm surprised Airbus adopted that system of wire measurement since they're based in Europe. The metric system wire numbers are much easier to understand. I know that mechanics around the world would get used to AWG but why even expose them to this idiosyncrasy?

Now I wonder if Airbus also used inch-based micrometers during manufacture. Or how about american D-size drafting paper? (BTW: there's two types and it's easy to mistake one for the other). Or inch-based drafting scales? I shudder to think they'd take metric scales to inch papers or vice-versa.

Again it's when one measurement system meets or is forced to mingle with another that the chance of error increases. I figured aviation would have put out an aircraft that was absolutely and completely metric from concept drawing all the way to maintenance by now.

In civil engineering we have goofy, pseudo-science tests for materials; example: concrete slump. Are there some of those in aviation too?


Thanks for the information.

Not using AWG would entail having specific pins for non AWG size, and would make things more messy than anything else.
I'm not even sure aerospace cable manufacturer have products not AWG based !

An Airbus is buld in metric system, it's basically only standards (like cables, fasteners, pipes...) that you'll find quoted in inch-based dimensions (length/diameter), for basically everything else I'm aware of it's directly translate in mm.
 
CowAnon
Posts: 219
Joined: Fri Nov 03, 2017 12:03 am

Re: why no metric system in aviation?

Mon Mar 22, 2021 5:54 pm

CowAnon wrote:
AFAIK, ex-Soviet countries still use horsepower instead of kilowatts for engine power and kilograms-force (which is metric but not SI) instead of kilonewtons for thrust.

I forgot to mention that the PD-14 turbofan for the upcoming Irkut MC-21 aircraft is so named because the engine has a nominal thrust of 14 metric tons of force (14,000 kgf or 31,000 pounds). The PD-35 turbofan for the planned Russian-Chinese C929 widebody is similar in that it will have a thrust of 35 tonnes-force. I prefer this system for thrust, since there's no conversion required to calculate thrust-specific fuel consumption: one lb-fuel / lbf-thrust / hour is equal to one kg-fuel / kgf-thrust / hour. The alternate metric TSFC unit is grams / kilonewtons / second, which is a pain to convert between.
 
vedatil4
Topic Author
Posts: 270
Joined: Thu Sep 17, 2015 4:38 pm

Re: why no metric system in aviation?

Tue Mar 23, 2021 1:57 am

Grizzly410 wrote:
vedatil4 wrote:
Grizzly410 wrote:

Airbus are AWG.
Any quick explanation on why it gets smaller the higher the number ?


The AWG number was based on how many times a wire had been passed through a die. The more passes, the thinner the wire would get.

I'm surprised Airbus adopted that system of wire measurement since they're based in Europe. The metric system wire numbers are much easier to understand. I know that mechanics around the world would get used to AWG but why even expose them to this idiosyncrasy?

Now I wonder if Airbus also used inch-based micrometers during manufacture. Or how about american D-size drafting paper? (BTW: there's two types and it's easy to mistake one for the other). Or inch-based drafting scales? I shudder to think they'd take metric scales to inch papers or vice-versa.

Again it's when one measurement system meets or is forced to mingle with another that the chance of error increases. I figured aviation would have put out an aircraft that was absolutely and completely metric from concept drawing all the way to maintenance by now.

In civil engineering we have goofy, pseudo-science tests for materials; example: concrete slump. Are there some of those in aviation too?


Thanks for the information.

Not using AWG would entail having specific pins for non AWG size, and would make things more messy than anything else.
I'm not even sure aerospace cable manufacturer have products not AWG based !

An Airbus is buld in metric system, it's basically only standards (like cables, fasteners, pipes...) that you'll find quoted in inch-based dimensions (length/diameter), for basically everything else I'm aware of it's directly translate in mm.


You make a solid point about the connectors.

I guess as long as it's clear to everyone which components on aircraft model are non-metric, chances of a small error or a critical piece wiggling out should be kept at a minimum.

I'm sort of bashing the US customary system here so I'll point out where I'll be happy to make an exception; military aircraft. There I do want inches, feet, pounds, slugs, and fractions. Why? In case an adversary gets their grubby hands on an aircraft. I don't want it to be easy for them to reverse engineer or fly it. Make the air speed indicators show football fields per hour on those. :-)

But for the commercial aircraft I get to fly on around the world, metric system as much as possible please.
 
vedatil4
Topic Author
Posts: 270
Joined: Thu Sep 17, 2015 4:38 pm

Re: why no metric system in aviation?

Tue Mar 23, 2021 2:10 am

CowAnon wrote:
CowAnon wrote:
AFAIK, ex-Soviet countries still use horsepower instead of kilowatts for engine power and kilograms-force (which is metric but not SI) instead of kilonewtons for thrust.

I forgot to mention that the PD-14 turbofan for the upcoming Irkut MC-21 aircraft is so named because the engine has a nominal thrust of 14 metric tons of force (14,000 kgf or 31,000 pounds). The PD-35 turbofan for the planned Russian-Chinese C929 widebody is similar in that it will have a thrust of 35 tonnes-force. I prefer this system for thrust, since there's no conversion required to calculate thrust-specific fuel consumption: one lb-fuel / lbf-thrust / hour is equal to one kg-fuel / kgf-thrust / hour. The alternate metric TSFC unit is grams / kilonewtons / second, which is a pain to convert between.


It's interesting how kilograms or metric tons (tonnes) are being used in a way to mimic pound-force. Both are mass while pounds is already a force. Thanks for sharing.

I hope to see an Irkut MC-21 someday. I like the big front windows on those planes. :-)
 
User avatar
A333MSPtoAMS
Posts: 364
Joined: Wed Sep 25, 2013 3:18 pm

Re: why no metric system in aviation?

Tue Mar 23, 2021 7:07 pm

johns624 wrote:
The UK still does some stuff non-metric. Distances and speeds are still in miles. Ales are still in pints. :D


Yes, and weight is in stones
 
GalaxyFlyer
Posts: 8359
Joined: Fri Jan 01, 2016 4:44 am

Re: why no metric system in aviation?

Tue Mar 23, 2021 7:24 pm

vedatil4 wrote:
CowAnon wrote:
CowAnon wrote:
AFAIK, ex-Soviet countries still use horsepower instead of kilowatts for engine power and kilograms-force (which is metric but not SI) instead of kilonewtons for thrust.

I forgot to mention that the PD-14 turbofan for the upcoming Irkut MC-21 aircraft is so named because the engine has a nominal thrust of 14 metric tons of force (14,000 kgf or 31,000 pounds). The PD-35 turbofan for the planned Russian-Chinese C929 widebody is similar in that it will have a thrust of 35 tonnes-force. I prefer this system for thrust, since there's no conversion required to calculate thrust-specific fuel consumption: one lb-fuel / lbf-thrust / hour is equal to one kg-fuel / kgf-thrust / hour. The alternate metric TSFC unit is grams / kilonewtons / second, which is a pain to convert between.


It's interesting how kilograms or metric tons (tonnes) are being used in a way to mimic pound-force. Both are mass while pounds is already a force. Thanks for sharing.

I hope to see an Irkut MC-21 someday. I like the big front windows on those planes. :-)


Unless you plan on operating on the moon, what practical difference is there between mass in kg and weight in pounds?

Who is online

Users browsing this forum: aklrno, louA340 and 16 guests

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos