ATC radar's (at least they used to when I was working them) show you on
"assigned altitude" when you are within +/-200ft.. so if you are assigned
9000 it will say "90C"... if you are still climbing to that altitude it will
say 90 "up-arrow" 87, or 8700 climbing to 9000 assigned.

the "traffic alert" on the ATC system is geared to the 90C, if you are at
9300 it will show 90 "arrow" 93 and if there is a traffic conflict at 10,000
it will "alert" the controller.

Then he may "bust you", but if it's a momentary deviation and no other
traffic is involved, the controller may query and ask your altitude to
verify that his read out is correct. If you say, level 90, Then he may offer
up a new altimeter setting to you, or just tell you that he sees 9300, as a
hint that he is watching, or that you need to get it looked at. If your
altimeter setting is not set to "local", his computer radar will
misinterpret the altitude ModeC information from your aircraft.

All controllers verify altitude readouts on the radar display on initial
contact, whether you call in, "out of 8500 for 9000" or, "level 9000", he
compares to what he sees on his data. If you do not offer up an altitude, or
in the case of an "IFR pickup" in the air, he will ask altitude leaving or
verify alt so he can compare his data.

BT

"Magnus" wrote in message
...
I thought it was +/- 200 feet before you've violated if you are flying
IFR, but someone just told me it's a 100 feet, which seems a little
steep considering they use 1000 feet separations.