Paul,

Is it normal practice for a Mode S transponder to display the aircraft
tail number to ATC?


Mode S transponder have the option of setting a flight ID or the tail
number in a data field designated for that info. This is broadcast with
the Mode S data packet and is independent of the unique transponder hex
code. In airline operations, the flight crew normally sets the flight ID
for each flight, e.g. UA011 for United flight 011. In GA operations, the
aircraft tailnumber is set by the installer.

Controllers with mode S radar equipment have the option of displaying
this data field next to the "blip". I do not know how common such
equipment is in the US, but it is getting more and more widespread in
Europe.

In central Europe, where Mode S recently became mandatory, ATC does not
yet rely on that data field for aircraft identification, however. They
say the percentage of erroneous (sp?) entries in that data field is so
high that it can't be relied upon. Thus, when I do an IFR pickup, I
still get a squawk and are radar identified by that squawk, even though
the controller would be able to see my call sign directly on his screen.
In fact, the controller DOES see it on his screen, which I have verified
by asking them.

AFAIK, the big database correlating call signs and hex codes inside the
radar equipment is a myth.

--
Thomas Borchert (EDDH)