"Kirk Stant" wrote...
No, what he said was he would have more headroom "under the Class A
airspace." Since the Class A would start when his altimeter reads
18'000ft (when set to 2992) regardless of what his GPS says, he would
not have any more room. He could very well be higher than 18,000ft
above MSL, which is a different thing altogether, and is really what
we care about for final glides, etc.

It's been a long time since I've done any IFR aviating, guess I need
to get back into the AIM again!

Yes, since the AIM is quite clear that Class A starts when the altimeter reads
18,000 feet MSL, set to the nearest reporting station, *not* set to 29.92 (which
would be FL180). ATC does not assign FLs to aircraft in Class A which would be
low enough to conflict with aircraft flying at 18,000 feet MSL, given the
ambient pressure.

The "headroom" argument is based on the fact that 18,000 feet MSL pressure
altitude (again. set to the nearest reporting station) is almost always more
than 18,000 geometric feet above mean sea level on hot summer days, due to the
fact that the ambient pressure gradient does not match the International
Standard Atmosphere, to which altimeters are calibrated. I often start my final
glides into Truckee with the altimeter (set to local pressure) reading 17,900
feet, while the GPS (more closely approximating actual elevation above mean sea
level) is reading in the range of 19,000 feet. Since my final glide range is
determined by my actual elevation, rather than by what the altimeter happens to
be reading, that extra 1000 feet or so is often what makes it possible to get
home. That meets the definition of "more headroom below Class A" as far as I'm
concerned...

Marc