On Sun, 09 Jan 2005 17:36:19 GMT, "Julian Scarfe"
wrote:

For air traffic control purposes, this is a Good Thing. It doesn't matter
if you fly around with 10000 ft on your altimeter when the true altitude is
9700 ft, as long as everyone flying at that level agrees to use the same
standard!

That's what I meant by 'flying at a pressure altitude'.

To enlarge, but you probably know this, above transition level all
aircraft set their altimeters to 29.92 or 1013. Thus, they are all
using the same standard and their true altitudes matter not. It's
their altitudes relative to one another that matters. Two aircraft,
one flying at an indicated 30,000 feet and the other flying at an
indicated 31,000 feet will maintain 1,000 feet separation even though
their true absolute altitudes could be more or less than 30,000 feet
and 31,000 feet, depending on the atmospheric conditions, i.e.
temperature and pressure.

The true altitudes of our two aircraft could vary considerably flying
through areas of widely different atmospheric pressure but relative to
one another their altitudes remain constant provide they both maintain
an indicated 30,000 feet and 31,000feet. Autopilots handle this quite
well.

When flying below transition level, the altimeters will have to be
adjusted at the behest of ATC otherwise things get tricky.

Jack can explain this better than me.

--
Alan White
Twenty-eight miles NW of Glasgow.
Overlooking Loch Goil and Loch Long in Argyll, Scotland.
Web cam at http://www.ufcnet.net/~alanlesley1/kabcam.htm