Kirk Stant wrote:

Bill,

GPS altitude gives you absolutely NO more headroom below Class A
airspace, since FL180 is a PRESSURE altitude (referenced to 29.92),
not an absolute altitude above sea level (which is approximately what
GPS altitude indicates). At 17,990 ft you have 9 feet of headroom,
you still can't go above 17,999' without an IFR clearance or wave
window) regardless of what the GPS is telling you.


I disagree. If the flight level at which class A begins has a true
higher altitude that in standart atmosphere, you have mode room. Here
in the vicinity of Paris we are very concerned with this, since in
some places class A begins at FL045. As usually soaring is done in
good weather associated with high pressures (higher than in the standart
atmosphere), FL045 is usually significantly higher than 4500ft AMSL.