Europe ops Transponder "Call sign" vs. Radio Call sign

[ksjc]

We operate a US registered and based bizjet. Transponder always set to transmit our N reg number to ATC and we always operate under a different company radio alpha-numeric callsign. Never had an issue in US nor for many years of ops in Europe until last week when we had 2 comments from Euro ATC that Transponder should return same as radio call sign. Geneva-Copenhagen-Hamburg legs. Is this a regulation? Did something change?

Anyone with Euro ATC knowledge out there who can enlighten me?

Of course I am able to adjust the transponder to send flight number code but reluctant to until I understand this better.

[FLEXJET]

See Honeywell document: I am afraid you'll have to change your ATC call-sign in your RMU each time. Maybe it has to do with your type of transponder.

http://www51.honeywell.com/aero/common/documents/White_Paper_Mode_S_and_European_Mandates.pdf


"Important Note about ID: The MST 67A has a feature that allows it to automatically determine the aircraft Registration Number (Tail Number) of US registered (N numbered) aircraft. If the aircraft is US Registered and operated where the tail number is used as the call sign (Flight ID),
then the operator does not have to manually enter the Flight ID – therefore a new Flight ID control head is not required. The transponder automatically reads the Mode S address assigned to the aircraft.
If it falls within the range of addresses assigned to the US by ICAO, it will automatically convert that Mode S address to the tail number of the aircraft and place it in the Flight ID register for compliance with Elementary surveillance. If the aircraft is operated where the Flight ID is not the tail number or the aircraft is not US registered, then a Flight ID control head is required to allow for manual entry of Flight ID."

[eglnyt]

In the UK you are required to enter the Aircraft Id you put in Item 7 of the flight plan. UK AIC 37/2007 refers which also suggests that the same requirement is in ICAO Doc 4444 and ICAO Doc 8168 although I haven't checked either of them.

[ksjc]

Thanks for the informative replies.