B747 FLC ENTERED RVSM AIRSPACE WITH MEL'ED INOP ALT ALERTER.

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ON FEB/TUE/00 AS ACR FLT ABCD WE ENTERED THE NEW RVSM AIRSPACE OVER THE PACIFIC OCEAN WITH A REQUIRED PIECE OF EQUIP (ALT ALERTER) TECHNICALLY INOP. HOWEVER; IT ACTUALLY DID FUNCTION PROPERLY INFLT. SEVERAL THINGS LED TO OUR MISTAKE. IT WAS THE LAST DAY OF OUR 10 DAY TRIP AND A 'LATE NIGHT' FLT (THROUGH THE NIGHT). SO THE ANTICIPATION OF GETTING HOME AND THE HURRY TO GET THE SHOW ON THE ROAD CONTRIBUTED TO OUR OVERSIGHT. ALSO WE WERE PROBABLY TOO COMPLACENT AND TRUSTING IN OUR DISPATCH AND MAINT DEPTS; SINCE THEY ALWAYS DO SUCH A GREAT JOB. BUT THE BACKGROUND MAY HELP. IN THE DAYS LEADING UP THE CHANGE-OVER TO RVSM; IT WAS KNOWN THAT MOST; BUT NOT ALL OF OUR AIRPLANES WOULD BE COMPLIANT. OF COURSE; THERE WAS ANXIETY ABOUT WHETHER ONE WOULD GET STUCK WITH AN AIRPLANE THAT WAS NOT COMPLIANT; SINCE THAT WOULD MEAN FLYING A LOT LOWER THAN ONE WOULD LIKE. WHILE CHKING ON FOR THE FLT I HEARD THAT OUR AIRPLANE WAS RVSM COMPLIANT; SINCE IT HAD THE NEW ALTIMETERS NECESSARY FOR IT. THE FLT RELEASE LISTED THE ALT ALERTER AS AN MEL ITEM; BUT ALSO HAD A 'W' IN THE TYPE ACFT CODE (INDICATING THE ACFT WAS COMPLIANT). IT WAS THE THIRD RELEASE; WHICH SEEMED TO INDICATE DISPATCH HAD PUT SOME EXTRA WORK INTO THE FLT. THE ALT ALERTER ITSELF WAS NOT PLACARDED IN THE ACFT (AS IT SHOULD HAVE BEEN). THE WRITE-UP IN THE LOGBOOK WAS THAT THE ALT ALERTER HAD SOUNDED ON LNDG. WE DIDN'T THINK IT MEANT TO MAKE THE ENTIRE SYS INOP (ALTHOUGH; AS WE FOUND OUT LATER; TECHNICALLY IT DID). WE TALKED ABOUT IT; AND IT SEEMED FAIRLY HARMLESS; SINCE THE SYS HAD OTHERWISE FUNCTIONED NORMALLY. AND IN ANY CASE WE HAD ALL FLOWN AIRPLANES BEFORE (NON RVSM) WITHOUT ALT ALERTERS WITHOUT ANY PROBS. IT DIDN'T SEEM LIKE A BIG DEAL; SO WE LEFT. THIS WAS A NEW REQUIREMENT (RVSM); WITH NEW PROCS THAT NONE OF US IN FLT OPS; DISPATCH OR MAINT SEEMED TO BE COMPLETELY FAMILIAR WITH. THE COMPANY HAD PUT OUT A MEMO ABOUT RVSM A FEW DAYS BEFORE WHICH SPECIFICALLY MENTIONED THE ALT ALERTER AS REQUIRED EQUIP. BUT NO ONE REMEMBERED THAT. WE WERE FOCUSED ON THE NEW ALTIMETERS; WHICH WERE SUPPOSED TO ALLOW US TO GO RVSM. THE MISTAKE WAS NOT DISCOVERED UNTIL LNDG IN HNL WHEN MAINT ASKED US ABOUT THE ALT ALERTER AND HOW IT HAD WORKED. WE HAD FORGOTTEN ALL ABOUT IT; SINCE IT HAD WORKED FINE FOR US. 'NO RVSM' PLACARDS IN THE ACFT WHEN IT IS NOT COMPLIANT (LIKE 'NO CAT II' PLACARDS) MIGHT BE A GOOD IDEA IN THE FUTURE. OR AT LEAST A STICKER IN THE LOGBOOK. THAT MAY NOT HAVE HELPED IN THIS CASE; THOUGH; SINCE EVEN THE BASIC PLACARDING WAS NOT DONE. HOWEVER; WE SHOULD HAVE ACTUALLY CHKED THE MEL IN THE BOOK; EVEN THOUGH IT SEEMED SO HARMLESS; AND THIS WHOLE PROB WOULD HAVE BEEN AVOIDED.