Originally Posted by RFSchaller
This thread prompted me to look at FAR 91.227 out of curiosity. It looks like when you don?t file a flight plan or request ATC services you can Omit your call sign and ICAO code but if you file or request ATC services you have to broadcast your ICAO code. See 91.227(d) (8) and (11).
Yes. Exactly right. That is specifically why the anonymous switch only works when squawking VFR code (1200 in USA). That's when the ICAO code is scrambled and randomized. Any other discrete code entered into the transponder disables anonymous mode and transmits the true ICAO code regardless of the switch position. And there are certain circumstances where ATC requires non-anonymous mode even whilst squawking VFR which is why the FAA doesn't allow automatic anonymous mode (i.e. permanently grounding pin 5 on the GDL 82 to airframe ground point) but rather requires a pilot selectable switch (grounds pin 5 through the switch for enabling anonymous mode). For example, I believe flying on a VFR flight plan and squawking 1200 then anonymous mode must be disabled.