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Comment by paulannesley | Hacker News Reader

> one thousand feet above

Are the imperial units of measurement an aviation thing, or an American thing, or a bit of both?

Mostly an aviation thing but descended from the fact that many early airplanes were American/British. IIRC only China, Russia and the DPRK use metric units.

EDIT: ahahaha <3 HN commenters

I believe Russia uses FL measurements since a few years ago.

Containers are also sadly measured in TEU (twenty foot equivalent units) stemming from their American design.

Global, impossible to upgrade standards that mean we are stuck with imperial units for a hundred years yet or more.

Aviation thing (unfortunately, but it is what it is).

You'll hear them referred to as Flight Levels or Angels as well (Angel 15 == 15,000feet).

Technically, the lowest flight level is FL200 or 20000'. Below that, altitudes are reported in feet. I'm not really familiar with the "angels" thing. I suspect that is specifically military.

EDIT: there was a reply (since deleted) that indicated I may not be completely correct on stating FL200 as the lowest number used. I contend that I am still basically correct for North America, but there is some subtlety:

https://en.m.wikipedia.org/wiki/Flight_level

Technically, a flight level isn't a distance measurement at all. It's defined by atmospheric pressure, so that all planes use the same "ground" barometer by definition. The feet are expressed nominally relative to a standard sea level pressure.

That is absolutely correct. I thought about mentioning it, but did not. Flight levels are definitely expressed as "pressure altitudes" rather than absolute altitudes. The air pressure (or rather density) is the more important number when it comes to keeping an aircraft in the air, anyway.

It used to vary in the UK, but I believe they've settled on a Transition Altitude of 18,000ft now.

Could you clarify why you cite 20,000ft as the transition altitude? I've always been under the impression it is FL180.

From your wiki link:

> In the United States and Canada, the transition altitude is 18,000 ft.

Depending on conditions, FL180 could be below the transition altitude at 18000'. I doubt, therefore, that you would ever be given anything below FL200. That is the subtlety to which I'm referring (well that and the fact that different transition altitudes are used in other countries). I'll admit that this is stretching my knowledge. Prior to this thread, I merely "had it on good authority" that FL200 was the minimum.

Angels is a Nato standard brevity code for "thousands of feet". There's also Cherubs which is height of a friendly aircraft in hundreds of feet (https://en.wikipedia.org/wiki/Multiservice_tactical_brevity_...).

Generally, you'll use flight levels until either you hit the transition level specified in the approach plate or ATC clears you beneath that level, at which point they'll switch to using feet and give you the QNH (atmospheric pressure at sea level) to calibrate your altimeter.

For example, the ILS/DME approach for 26L at Gatwick has a transition of 6000 ft (http://www.ead.eurocontrol.int/eadbasic/pamslight-48EDD962FD...)

In aviation, altitudes are in feet, distances in nautical miles, speed in knots (nautical miles per hour).

To be fair:

1. The nautical mile is an SI derived unit, now fixed at 1852 meters.

2. It's a more useful unit than the kilometer for long-distance navigation due to its historical definition in terms of arc along a line of latitude making it easy to work with on charts.

The foot is nowadays SI-derived in that sense also, fixed at 0.3048 meters.

To be specific: horizontal distance is given in nm, vertical distance is given in feet.

An aviation thing. Flight levels are measured in feet across the vast majority of the world.

Aviation uses feet... and centigrade. The standard lapse rate that pilots use in the troposphere is -2C/1000'.

Is it also called centrigrade, rather than Celsius? Or a mix?

It's an aviation thing.

In aviation, feet are exclusively used, except in Russia and China.

So, not exclusively.

Aviation. 500 & 1000 ft intervals are very convenient discrete altitude levels for coordinating traffic, and that's probably a key reason why it's held up universally. In meters, the interval will either awkward and hard to communicate numbers or spaced too widely to be efficient.

I think the reasons are historical, because the U.S. pretty much invented the industry.

I believe the eastern block used to use meters, I doubt 300m was much more difficult to deal with mentally than 1000 feet!

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