B767 ACR DISPATCHER WAS CONCERNED ABOUT ENTERING ATLANTIC RVSM AIRSPACE WITH A TCASII MEL'ED INOP.

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I RECEIVED A CALL FROM THE COCKPIT OF FLT XXX; CAPT XXX. AT ABOUT XX30Z; THE FLT WAS COMING UP ON 20 MI W IN THE NORTH ATLANTIC TRACK SYS. THE FLT HAD BEEN RELEASED WITH TCASII SYS DEFERRAL. THE MEL RTE WAS REQUIRING THE DISPATCHER TO PLAN THE FLT TO AVOID RVSM AIRSPACE WHICH HAD BEEN DONE BY FILING THE OCEANIC PORTION AT FL310 BENEATH THE FL330 FLOOR OF RVSM. CAPT XXX CALLED TO ADVISE ME OF HIS SIT. JUST S OF IRELAND AND STILL IN RADAR CTL PRIOR TO OCEANIC ENTRY HE REQUESTED AND RECEIVED CLRNC TO FL340 IN ORDER TO TOP SOME CONVECTIVE WX IN HIS PATH. INADVERTENTLY THIS SEEMS TO HAVE BOXED THE FLT INTO THAT ALT. HAVING CLRED THE THREATENING WX; SHANWICK WAS UNABLE TO APPROVE THE FLT BACK DOWN TO FL310. THE FLT MADE CONTINUED REQUESTS FOR FL310 EVEN ADVISING THE SHANWICK CTLR THAT THEIR TCASII WAS INOP WHICH RESTRICTED THEM FROM RVSM. THE CTLR RPTEDLY REPLIED THAT TCASII WAS NOT REQUIRED FOR RVSM AIRSPACE. CAPT XXX FELT THAT HIS OPTIONS WERE EITHER TO PROCEED 30 MI N OR TO STAY AT FL340 IN VMC ON THE PRESENTLY CLRED ROUTING. I COULD OFFER NO FURTHER SUGGESTIONS AND CONCURRED. I ADVISED THE CAPT OF THE FOLLOWING POINTS. FIRST; THAT WE HAD COMPLIED WITH THE MEL BY PLANNING THE FLT BENEATH RVSM AIRSPACE. I HAVE BEEN AWARE OF THE DEFERRAL FROM MY TURNOVER BRIEFING LOG. NEXT; WITHIN RADAR CTL; HIGHER ALT WAS REQUESTED -- IN THE BEST JUDGEMENT OF THE CAPT; THE MOST PRUDENT COURSE OF ACTION FOR WX AVOIDANCE. FINALLY; I CALLED THE CAPT'S ATTN TO B767 OPS MANUAL. PINK BULLETIN REFING RVSM. IT DELINEATES EQUIP WHICH MUST BE OPERATIONAL TO ENTER RVSM AIRSPACE. TCASII IS NOT MENTIONED. HE SAID HE WAS ALREADY ON THE PAGE. HENCE; THE SEQUENCE OF EVENTS. FLT WAS PROPERLY PLANNED. WX AVOIDANCE CAUSED UNUSUAL EVENT. UNABLE TO COMPLY WITH ORIGINAL PLAN; ACCOUNT ATC CONSTRAINTS. STILL IN COMPLIANCE WITH OPERATING MANUAL. WE AGREED THAT GOING 1 DEG N OF THE TRACK AND DSNDING WAS TANTAMOUNT TO DECLARING AN EMER; WHICH WE DID NOT INTEND. OUR BEST COURSE WAS TO REMAIN AT FL340 AS CLRED BY ATC; WHICH WAS ALSO IN VMC.