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Join Date: Jul 2002
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I believe Boeing is designing their current (new) aircraft to be certified to the low-40's. I seem to recall that Concorde went somewhat higher...maybe 50k?
I'm sure there are many complications with going higher - the standard thing I've always heard for commercial airliners is that fuselage development would be much more expensive at higher altitudes. (e.g., it would either need to be heavier or made from completely different materials) The technology exists to do it, but the economics don't work.
Military aircraft, of course, can go much higher, since the plane isn't pressurized - only the pilot's suit is.