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how high must one climb to remain within gliding distance?
Except possibly in a motorglider, a helluva lot higher than you can go without an IR
I made some quick calcs on the back of an envelope and, from Dover to the St Inglevert NDB, with a 1:9 glideslope, came out at 6000 feet - which would be FL060 in Belgium. So in Belgium one would need a transponder to ever get a hope of being allowed that high, but to my knowledge an IR would not be really required - but what about the French and English airspace?
(sorry for diverting from the original topic)