"David Brooks" wrote in message
...
I have occasionally idly wondered about one little detail. When it's a
significantly low or high pressure day, when do you change the altimeter
setting while climbing or descending? Maybe as you pass through FL180
descending, and as you pass through 18,000ft climbing?

Most of the planes flying in the flight levels, they climb quickly enough
that asking "when do you change the altimeter" is like asking "should I have
my dinner delivered at 5 minutes, 31 seconds past the hour, or 5 minutes 37
seconds past the hour".

For the rest of "us" (my plane is certified to 20,000', though it's not
worth the trouble to fly that high), I believe that you're supposed to
switch over to 29.92 during the climb when you actually reach FL180, and
switch back to the local altimeter during the descent when you leave FL180.

When you actually do switch is, of course, more a matter of practicality
than anything, since you won't be given vertical separation during the climb
or descent that might depend on your altimeter setting (not counting
separation at the top or bottom of the climb or descent, of course).

Pete