ACR FLT CREW IN CRUISE AT FL390 HAS TAT PROBE FAILURE; AND DETERMINES AFTER SOME DISCUSSION THAT THEY ARE NO LONGER RVSM CAPABLE. A DESCENT IS REQUESTED AND INITIATED TO FL290.

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I AM NOT SURE THAT I HAVE DEVIATED FROM AN RVSM PROC; BUT IT WOULD HAVE BEEN HELPFUL TO HAVE THE INFO EASIER TO FIND. ONCE THE DISPATCHER WAS NOTIFIED; MAINT AND HIMSELF RESEARCHED THE PROB ALSO AND ACARS US BACK ABOUT THE ACFT BEING NO LONGER RVSM AS WELL AS AUTOLAND DOWNGRADED. IT WAS VFR WX IN ATL SO IT WAS NO PROB. IT WOULD HAVE REDUCED OUR WORKLOAD TO HAVE HAD THE INFO SENT TO US INSTEAD OF DIGGING FOR IT. I KNEW WHERE TO LOOK BUT IT WAS CUMBERSOME. WE LOST OUR TAT PROBE INFO AT FL390. OUR AUTOPLT REMAINED OPERATIVE IN THE LNAV MODE AND PITCH WAS IN CWP. WE RELAYED THE MESSAGE TO OUR DISPATCH WHAT HAD OCCURRED AND THE REPLY WAS THAT MAINT SAID TO JUST PERFORM OUR CHKLIST ITEMS AND WE WILL MEET YOU IN ATL. THERE IS NO CHKLIST ITEMS FOR THIS LOSS. WE THEN BEGAN TO TRY AND TROUBLESHOOT THE PROB TO DETERMINE IF WE COULD GET THE AUTOPLT TO FUNCTION WITH JUST THE MCP BUT COULD NOT. I THEN ASKED THE COPLT TO GET OUT THE MEL BOOK TO FIND OUT IF WE WERE STILL RVSM CAPABLE. IT WAS NOT EASY TO FIND; IN FACT; AFTER HE COULD NOT FIND IT WE XFERRED THE CTLS AND I LOOKED AT THE MEL. WE HAD ALREADY ASKED FOR A DSCNT TO LOWER ALT TO GET OUT OF RVSM AIRSPACE. OUR MISTAKE WAS NOT INFORMING ATC OF WHY WE WANTED TO DSND BUT HE DIDN'T ASK EITHER. UPON FURTHER DSCNT I FINALLY FOUND IN THE MEL -- THAT WITHOUT ALT HOLD FUNCTION WE WERE NOT RVSM CAPABLE. WE WERE ALREADY CLRED TO FL290 BY THAT TIME AND I INFORMED ATC THAT WE WERE NOT RVSM CAPABLE AND INQUIRED ABOUT THE ALT AND WAS TOLD THAT IT WAS NOT A PROB. WE CONTINUED TO ATL IN LNAV AND CWP MODES ON THE AUTOPLT AND THERE WERE NO DEVS FROM ASSIGNED CLRNCS.