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m1m2
Posts: 264
Joined: Sat Dec 31, 2011 3:39 am

### Re: why no metric system in aviation?

GalaxyFlyer wrote:
vedatil4 wrote:
CowAnon wrote:
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?

As far as I can see, the only difference is a conversion of 2.2

CowAnon
Posts: 226
Joined: Fri Nov 03, 2017 12:03 am

### Re: why no metric system in aviation?

vedatil4 wrote:
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.

The spherical coordinate system (degrees or gradians of latitude and longitude) is more versatile and can be applied to every planet or star, but it could've been replaced with metric lengths on Earth. Then you might say that your city is located at something like 4.238 megameters North, 13.936 megameters East.

vedatil4
Topic Author
Posts: 291
Joined: Thu Sep 17, 2015 4:38 pm

### Re: why no metric system in aviation?

CowAnon wrote:
vedatil4 wrote:
GalaxyFlyer wrote:

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.

The spherical coordinate system (degrees or gradians of latitude and longitude) is more versatile and can be applied to every planet or star, but it could've been replaced with metric lengths on Earth. Then you might say that your city is located at something like 4.238 megameters North, 13.936 megameters East.

The thing about measuring in megameters is that the earth is not a perfect sphere nor is it flat between two locations on the surface. So we'd all have to agree to some kind of theoretical "measured through the earth" (a chord) between points or an "idealized surface of the earth" measurement system. I'm with you on the meters and the mega prefix though.

Also, a "zero megameters north, and zero megameters west" origin point would have to be agreed upon if we're going to use them for location on the surface as opposed to distance on the surface. I guess that could be where the equator and the prime meridian meet. Wikipedia says there's a buoy there and it's called "null island".

Again, I'm more curious about any of USA customary distances used on scales, paper sizes, volumes, weights, rulers, threads, wire sizes, measuring tapes, micrometers, pipe sizes, and et cetera used for the manufacture or servicing of any planes flying international. It's when these types of items meet their metric counterparts that crazy things can happen (at least in my civil engineering/architecture experience).

From what I've gathered on this forum I guess there's an expectation of foreign mechanics knowing all of the quirks and idiosyncrasies of the customary system. That's scary to me knowing that weird "nominal" or weird rounding off conventions are being used in the customary system. When it's a game of very small fractions of an inch, a calculation mistake can become critical.

It's difficult enough when you have to live within that system. I imagine those foreign mechanics must gnash their teeth seeing those units. I had figured all the plane manufacturers would've gone full metric, with no exceptions, long ago.

As for a plane's location on the earth itself, I guess it's alright if the plane or the pilot reports a location that's not 100% accurate even in megameters. (Of course that wouldn't apply if we're talking about planes flying in close formation).

Thanks to everyone for letting me indulge in this customary vs metric in aviation conversation.

Now, I'm off to figure out how much I weigh in "stone" in case someone wants to launch me using a catapult.

DocLightning
Posts: 22286
Joined: Wed Nov 16, 2005 8:51 am

### Re: why no metric system in aviation?

vedatil4 wrote:

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.

Which is odd, because kN is a completely legitimate measurement, I agree. Often engine specs list both lbf and kN.

acecrackshot
Posts: 215
Joined: Sun Nov 15, 2020 4:22 am

### Re: why no metric system in aviation?

johns624 wrote:
I think it's an advantage since many Americans are "bilingual" in measurements, especially those that have to do with speed, length or distance...

Its one of the few things we are bilingual at, and I say that as a shamefully monolingual American.

That said, as noted earlier, there is the argument that many of the useful system assets of Imperial measurement are still in use, while SI units, built for their standard, are not always as immediately useful operationally in niche roles.

WesternDC6B
Posts: 911
Joined: Thu Mar 14, 2013 3:05 pm

### Re: why no metric system in aviation?

DrPaul wrote:
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.

Pounds, shillings, and pence!

And as for tools and aircraft, did Armstrong-Whitworth use Whitworth hardware?

vedatil4
Topic Author
Posts: 291
Joined: Thu Sep 17, 2015 4:38 pm

### Re: why no metric system in aviation?

WesternDC6B wrote:
DrPaul wrote:
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.

Pounds, shillings, and pence!

And as for tools and aircraft, did Armstrong-Whitworth use Whitworth hardware?

This is the first I read about Whitworth hardware. It's crazy that two 5/8 inch labeled wrenches (or spanners) could be different sizes.

If I worked on an old machine with those types of threads, I would've stripped them forcing a more common bolt in.

WesternDC6B
Posts: 911
Joined: Thu Mar 14, 2013 3:05 pm

### Re: why no metric system in aviation?

vedatil4 wrote:
WesternDC6B wrote:
DrPaul wrote:

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.

Pounds, shillings, and pence!

And as for tools and aircraft, did Armstrong-Whitworth use Whitworth hardware?

This is the first I read about Whitworth hardware. It's crazy that two 5/8 inch labeled wrenches (or spanners) could be different sizes.

If I worked on an old machine with those types of threads, I would've stripped them forcing a more common bolt in.

I'm old enough to remember Sears, Roebuck offering Whitworth tool sets for sale. Apparently, some English motorcycles and maybe some of their cars used Whitworth hardware. Who knows why.

CarlosSi
Posts: 887
Joined: Sat Jul 08, 2017 8:29 pm

### Re: why no metric system in aviation?

Aviation is in customary (feet, inches, pounds) because it was pioneered in the US . That's my guess. Otherwise we'd probably have it in metric.

Technically it's converted pretty often anyways.
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