On Jun 2, 6:39*am, "
wrote:
On Jun 2, 7:16*am, Cats wrote:

....Also MSL is not the
"datum" used in collision aviodance.

Cookie

Uh, wrong. When you report your altitude over the radio, you are
going to read it off the altimeter, and if you are below 18,000ft (in
the US) it should be set to QFE for the closest reporting point.

So when someone calls out that he is "Eastbound over Littletown at
7300ft" on a hazy afternoon, and you are westbound over Littletown,
staring into the sun, at 7400ft, you had better hope he is using the
correct altimeter setting! Cuz that's how you are going to check to
see if you have altitude deconfliction (since I doubt you have TCAS in
your glider).

You are correct that transponders use pressure altitude when
reporting, but that is a different issue - you don't normally use raw
Mode C altitude data in the cockpit for altitude deconfliction - and
ATC applies a correction when reporting traffic altitude over the
radio.

My .02$: QFE can be useful for low altitude aerobatics - for an
airshow pilot who performs at a lot of different locations. That's
about it, since the advent of radio altimeters and GPS. Otherwise,
QNH is what should be used (and it's arguably required by the FARs),
from the very beginning. I don't want to share airspace with someone
who can't do the math and needs the altimeter to know when to turn
Base and Final! (Hint - if you are really math-in-the-cockpit
challenged, write the darn pattern altitude on the back of your
hand!).

Kirk
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