Mode S U/S, what now?
 

Yesterday, the ATCO suddenly told us that he sees no data on the screen; i.e only mode A and C active but not mode S. However, there was no fault indication in our equipment. If the problem persists, are we allowed to continue flights in that condition until next MX (aircraft is C525 so I guess flights in RVSM airspace are ruled out or all IFR flights in general?),

Cheers

Cecco

Well, it depends on the airspace. While in EPWW FIR I'm pretty sure you're not required to be equipped with mode S at all. Situation can be different in other countries.

But AFAIR without mode S your TCAS is quite downgraded, so this might be a more important issue.

Correct, without Mode-S TCAS is not fully functional and that is a 3 day repair period item if you fly within german or french airspace. In other european countries it is 10 days.

However, several european countries require mandatory mode s carriage and for a jet (IFR traffic with more than 5,7t MTOW, 250kts or more cruise speed) that would be the enhanced surveillance version. Namely germany, france and the UK require mode s equipment, netherlands and belgium above FL245. If you want to fly through the airspace of any of those countries you need an exemption from the relevant CAA for each country. Best to get in touch with your governing CAA and talk with them about it, get it repaired asap in any case.

Were you squawking 1000?