In article ,
Recliner wrote:
On Fri, 15 Jan 2016 13:03:00 +0000, Mike Bristow
wrote:

In article ,
Recliner wrote:
On Fri, 15 Jan 2016 12:29:53 +0000, Mike Bristow
wrote:

["Followup-To:" header set to uk.transport.london.]
In article ,
Recliner wrote:
The Real Doctor wrote:
"Flight level - "A fancy way of telling you how many thousands of feet
you are above sea level. Just add a couple of zeroes. Flight level
three-three zero is 33,000 feet.""

Is that wrong? [Yes, I know it's the barometric altitude, but that's not
something that's normally mentioned.]

The use of FLxxx rather altitude is precisely to avoid having to
work out what your height above sea level is. In other words, by
bringing in "above sea level" they've negated the whole purpose of
Flight levels.

Not for a layman.

This is a piece to explain the jargon. The reason why they're wrong
is Quite Interesting; so why get it wrong?

They're not trying to explain jargon or provide complicated
explanations of the physics behind the jargon. They're simply
providing a quick, simple translation of jargon for ordinary people,
not the pedants who inhabit this group.

I disagree; but neither of can read the mind of the editor or
journalist.

Also, it's hundreds of feet, not thousands.

That's exactly what the article says when it says you need to add a
couple of zeros.

"A fancy way of saying how many thousands of feet you are above sea level".

It's not. It's a fancy way of saying how many hundreds of feet you are
above sea level.

It's a simple way of saying that, and a fancy way of giving the
altitude in thousands of feet.

Nonsense! FL330 is three hundred and thirty hundred feet[1]. It
is not three hundred and thirty thousand feet! There's a big
difference!

Until fairly recently, in the UK it was standard practice to fly
at e.g. FL85 (or 8500 feet; not a multiple of 1000).


Note that they avoiding the jargon
term, 'altitude', saying instead, 'how many thousands of feet you are
above sea level'. They certainly wouldn't want to get into explaining
the difference between barometric, GPS and radar altitudes, nor would
it be reassuring for passengers if the height was described as
approximate.


Also it's not feet everywhere.

But they don't use flight levels, do they?

They do use flight levels. The difference between FLx and an altitude of x
is one is a certain pressure; the other is the gap between you and sea level.

They just quote the
altitude in metres.

They define it in metres (and use metres for altitudes).

And so in those countries the height will simply be quoted in metres,
with no need to translate a flight level.

Nonsense. You translate to a flight level when you want to use a
standard barometric pressure so that everyone at FLxxx is at the
same height. You use the real pressure settings when it is important
you know the gap between you and the ground (e.g. the mountain peak
is at 600', and you're flying at 700').

These two, differing, requirements are independant of the units of
measurement.

In order to make it clear which one is being used, ATC will always
use FLxxx to mean "altitude measured using the standard barometric
pressure of 1013.25hPa" and "xxxx feet" or "yyy metres" to mean
"altitude using the actual local pressure". It's just that in odd
foreign places[2] "FL20" means "600 metres, using 1013.25 hPa as
0", not "2000 feet, using 1013.25 hPa as 0".


[1] In eg. the UK and US
[2] e.g. Russia, eastern Europe, and China. Except, now I look it
up, Russia and some other ex-CIS countries switched to feet in 2011.


--
Mike Bristow