"NY" wrote in message
o.uk...
"Rod Speed" wrote in message
...
On sunday I was trying to work out why they say a plane is
flying at 30,000ft (about 5.6 miles) why not use miles or km ?

Because aviation works like that. You need to specify what height
they have to fly at so they dont run into each other and you need
a lot more levels than every mile or km vertically.

So use metres, but I noticed that flight will level off at 31,000 ft
rather than 9448.8 metres. so why not just fly at 10k meters
or 10km. or perhaps 9km if they can't make 10km.

Because that doesnt give enough flight levels.

I presume flight levels measured in feet and 1000 feet apart are a
worldwide standard,

Yes they are and 1000 metres apart would give only a third of them.

and that countries which measure altitude in metres (on their altimeters)
convert from the round number of feet.

Those altimeters have dual markings. And those
planes dont fly outside their own country anyway,

It would be *very* prone to errors if air traffic control gave round
numbers of metres (and therefore non-round number of feet) when you
entered "metric airspace"

No such animal.

because you now had to fly at a round number of metres, so I presume it's
not done like that.

Correct.

(But I may be wrong...)

No you arent.

Its a bit like the language used. It sposed to be universally english
but in the wilds of Tadzhikistan etc they dont bother with that.

I did notice on a little turboprop plane that took me from Schipol to a
small airport (Paderborn) in Germany that the pilot announced (in German)
"we will be flying at 5000 m" or whatever (he certainly mentioned "hoch"
(height) and "four-and-twenty-blackbirds" style German numbers and
"metres"). But I presume he was still flying at a round number of
thousands of feet, which he rounded to the nearest whole number of metres
for the benefit of the passengers.

Correct.

I wonder if the world will ever redefine airspace to use metres (maybe
have flight levels ever 300 m which is *roughly* 1000 feet),

Unlikely because its much harder to say. Flight level 10.3 doesnt work.

since metres are the international scientific/engineering standard. I
suppose they won't change unless the advantage of doing so outweighed the
problems during transition.

And there is no advantage in doing that. Flight Level 320 is just a unitless
number.

Do air-traffic control throughout the world specific air-pressure settings
(*) in inches of mercury,

No, only the USA and Canada.

or do metric-speaking countries use mmHg or millibars?

Yes, and altimeters display both so you can set using either.

But with light aircraft, particularly when flying outside air
traffic control just set the height to the height of the airport
while still on the ground before takeoff so no pressure involved.

(*) For calibrating the altimeter to today's sea-level air pressure, so
the altimeter reads a consistent height above ground irrespective of
changes in air pressure.