"BUFDRVR" wrote in message
...

Interesting. I did not know that.


Probably few do. 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.