According to our MEL we are certainly allowed to enter RVSM airspace without a working TCAS. As we are based in germany we have to repair a u/s TCAS unit within 3 days which is an excemption to the european 10 day repair rule.
By the way, Eurocontrol has some very nice info freely available on their webpage, the following excerpts are directly pulled from there:
Flying with an inoperative ACAS II is permitted, including within RVSM airspace, provided it is done in accordance with the applicable Minimum Equipment List (MEL).
The MEL for TCAS II throughout Europe is Class C - 10 days (excluding the day of discovery). Operation under the terms of the EASA-OPS 1 TCAS II MEL has been agreed and accepted by the ECAC Member States. JAA TGL 26 (which is still applicable) states that TCAS II "may be inoperative provided the system is deactivated and secured, and repairs or replacements are carried out within 10 calendar days. Note: Local Authorities may impose a more restrictive rectification interval days.
However, one State in Europe applies a more restrictive requirement: in German airspace the time period during which TCAS II may be inoperative is reduced to 3 days (refer to German AIP GEN 1.5 para. 5). This applies to all aircraft.
In Europe, there is no requirement to notify ATC or to make a remark in the flight plan about TCAS II being inoperative. There are different requirements outside Europe, e.g. India, where ACAS equipage needs to be stated in the flight plan. Please refer to the individual States authorities for more information.
“The procedures to be applied for the provision of air traffic services to aircraft equipped with ACAS shall be identical to those applicable to non-ACAS equipped aircraft” (ICAO PANS-ATM, Doc 4444, para 15.7.3.1).
All quotes taken from
EUROCONTROL - ACAS II Equipage Requirements
However, and that is probably what you mean, TCAS is a mandatory equipment in the ECAC airspace. It can be inoperative though for up to 10 days at a time (if you do not enter german airspace).