"Tony Cox" wrote in message
ink.net...

Yes, but surely an IFR flight plan would be just as valid when ascending
through FL600 as when descending below FL180.


Absolutely, in each case you're leaving Class A airspace and entering Class
E airspace.



Assuming the mode C is properly calibrated, that is.


The IFR flight plan is just as valid with an improperly calibrated encoder.

I worked a flight above FL600 just once. About fifteen years ago I was a
controller at Chicago Center working traffic in the Madison, WI, area. A
NASA ER-1 departed MSN requesting an unrestricted climb to FL610. I
coordinated with the high altitude sector and issued the climb. He went up
like a rocket! The Mode C readout couldn't keep up with him. Shortly after
he reported out of FL600 and cancelled IFR, the Mode C read FL600. I asked
a Data Systems Specialist about it and he told me that FL600 and higher will
read out as FL600.