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.

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...)

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.

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