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Comment by Tankenstein | Hacker News Reader

Just a guess, but aerospace generally works with feet for altitude and knots/mph for airspeed, internationally. I’m doing a PPL in Europe and we, like everybody, use feet and knots/mph. I believe this is because the US have been on the forefront of aerospace regulation (a set of rules called the chicago convention is the basis of all air law) and aircraft manufacturing.

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Not for aerospace no

And knots are not mph, they're "nautical miles per hour" which are a different measure (1nm is 1.8km, not 1.6km as the regular mile")

Sorry, not a native speaker, I was under the impression that aerospace means air and space. I guess i meant aviation.

I didn’t imply knots are mph, I used the slash to signify “or”. They are completely different units, but both are used. Sometimes the airspeed indicator even has two scales, one for kt and one for mph.

Can confirm, all aviation worldwide deals in feet and knots. It's also because it's much easier to do calculations on the fly (literally) - in your head. Metric is precise and logical but harder to use in stressful situations.

Can you please give some real-world example of why it's easier to do calculations? Not disputing what you say, just hard for me to imagine why it would be so.

1 knot is about 100 ft/min which is very convenient for descent at a specific glide slope (i.e. for 100 knots ground speed at 5% slope you want 500 ft/min descent rate). Standard is 3° which is about 5%.

Knots are also handy for navigation as 1 nautical mile equals 1 minute of latitude. And of course a knot is 1 nautical mile per hour. So if you're doing 300 knots, that's 5 degrees of latitude per hour.

The units fit together nicely as a system.

This sums it up. Metric is nice and clean tenths, but the real world is seldomly easily expressed in clean tenths.

Another example: The feet is cleanly divisible in thirds, quarters, and twelfths, which is greatly appreciated in industry and particularly construction.

Also to be bluntly mundane, almost everyone can just look down and have a rough measure of a foot which is good enough for daily use.

Also, the "sterility" of metric doesn't do it any sentimental favours. Japan loves measuring size/volume in Tokyo Domes, for example.

Not really, I have no idea what a foot is. But I can just look at yhe tiles and know they are 1*1 meter

If you're an amputee I truly am sorry for you and hope the handicap hasn't disrupted your life too much.

Jokes(...?) aside though, your absolute deference to precision is an example of why metric flies over people's heads. Feets, Tokyo Domes, arguably even nautical miles and so on are relatable at a human level unlike metric which is too nice and clean.

This sort of argument is odd to someone in a country which uses both, where a yard is intuitively "a bit smaller than a metre", a pint corresponds to a pint glass or "about half a litre" rather than anything meaningful and I'm aware that a rod and a furlong are things but have absolutely no idea what they correspond to. A foot is comfortably bigger than the average foot size, and an inch really isn't an easier unit to approximate than a centimeter

The SI was specially aimed to reduce such meaningless discussions, yet we steel have big endians and little endians comparisons, long after the dust settled.

Now I'm wondering if right-to-left languages (e.g. funnily enough, Arabic) write the least significant digits at the left or the right.

EDIT: numbers in those languages are the same way as in English, the "ones" are at the right. Kinda strange!

One meter is about one long step for an adult. To approximate the length of a field, you just walk along it with big steps and count. It will not be correct, but pretty close. A cm is a little bit smaller than the width of your index finger. It's all bout what you are used to. Metric doesn't "fly over people's head" where metric is the standard way to measure things, but inches, feet, gallons, pounds, miles fly over our head because we are not used to it so don't have any frame of reference.

Who cares? It's what the indicator says, I don't need to visualize feet to do calculations and talk to the tower about them.

If you can see a 1x1m tile from the cockpit, you're dead.

A foot is about 1 sheet of metric A4 paper :)

A meter is _exactly_ square root of area of A0 paper.

Yes and a blank 80 gsm A4 sheet weighs exactly 5g, if you need a weight reference!

I suspect that the math is even easier using meters, meters, and meters per second than nautical miles, feet, and knots. I'll eat my hat if you can tell me the conversion from feet or inches to nautical miles without looking it up

Well if what they say is true then 100ft/min = 1 nautical mile/60min, so one nautical mile is 6000ft. Or I guess I missed the about so not exactly.

It's within about 1%

The calculation in the metric system would not necessarily be more complicated, but it would be different because the reference points in the metric system are not directly aligned with the geography of the Earth.

"1 knot is about 100 ft/min which is very convenient for descent at a specific glide slope (i.e. for 100 knots ground speed at 5% slope you want 500 ft/min descent rate). Standard is 3° which is about 5%."

You are right. It's an easy calculation. But I would say its easy because its historically based on imperial units. Its easy to think about easy calculations like this in metric units like:

A 5% slope means descending 1 meter vertically for every 20 meters horizontally.

The gradient thing would work if ground speed and vertical speed were both in m/s, but km/h is more common in metric for a ground speed. You don't usually think in terms of hours during a climb/descent!

Glide slope of 3.6% would fit nicely though. Then, 100 km/h ground speed goes with vertical speed 1 m/s.

Metric navigation would use the fact 90 degrees of latitude is 10,000 km.

who is flying exactly north/south?

Certainly not "worldwide". China uses metres. Recreational aircraft in Europe often use metres (almost all sailplanes).

Thank you, I wasn't aware of China using metres. It turns out Russia uses them as well, confusingly below the transition level.

No glider I have ever stepped in used metres. It doesn't make any sense, the tower wants to hear feet and knots and will communicate using that.

"Metric is precise and logical but harder to use in stressful situations."

That fully depends on your cultural background. Feet, miles etc. are so foreign to me that I would be unable to calculate with them under stress.

But I am not a pilot nor a navigator, so...

No, it doesn't. I'm European, never used imperial before I became a pilot, and it's easier. Check it out, the formulas are much simpler to do in your head. Intuition doesn't matter, all that matters is that I can do the calculations quickly so I know I'm within parameter limits.

I'm curious which ones you find easier? There or a few thermodynamics equations that are much more practical in SAE. This is because the many units are often developed out of within discipline experiment, whereas metric tries to use fundamental units across disciplines.

Take a look into a pilot handbook, they are all written down there.

Are you saying that every single equation in the book is easier in SAE than metric?

Nope

You can be just as precise with either system.

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