A B757-200 CREW WITH AN AIRSPEED MALFUNCTION CONTINUED IN RVSM FLT WHEN A DESCENT OUT OF RVSM WAS REQUIRED.

Narrative

AFTER TKOF WE HAD SEVERAL EICAS MESSAGES RELATING TO A DIFFERENCE IN AIRSPD INDICATIONS. WE ANALYZED THE PROBS AND COMMUNICATED WITH DISPATCH AND MAINT CTL TO SEE HOW TO RESOLVE THEM. IT WAS DETERMINED THAT WE HAD ALREADY DONE EVERYTHING WE COULD BY SELECTING THE ALTERNATE ADC ON THE CAPT'S SIDE. THE AUTOPLT WAS WORKING; BUT AFTER AWHILE AT CRUISE IT TRIPPED OFF AND WOULD NOT RESET DUE TO AN 'ELEV ASYMM' STATUS MESSAGE. INITIALLY I THOUGHT OF TELLING ATC; BUT FIRST COMMUNICATED WITH DISPATCH ABOUT COMMUNICATING AGAIN WITH MAINT CTL ON DIVERTING OR CONTINUING ON TO DEST DEPENDING ON IF THEY COULD FIX THE ACFT AT DEST. THE FO AND I HAD ALREADY DISCUSSED WHETHER TO CONTINUE BY HAND-FLYING OR DIVERT. WE DETERMINED THAT WE WERE WELL RESTED AND COULD HAND-FLY BY SWAPPING OUT EVERY HALF HR. I DID NOT ADVISE ATC OF THE AUTOPLT FAILURE AS WE HAD EVERYTHING UNDER CTL. IN REVIEWING WHAT WE DID IT WAS MY ONLY MISTAKE DUE TO BEING IN RVSM AIRSPACE; AND SHOULD HAVE ADVISED THEM DUE TO THE AUTOPLT FAILURE. WHILE WE WERE ABLE TO MAINTAIN OUR ALT FAIRLY EASILY AND DID NOT HAVE ANY CONFLICTS DUE TO THE TIME OF DAY; THE GOOD WX; AND AIRSPACE WE WERE FLYING IN. HAD WE BEEN CHALLENGED BY ROUGH AIR; WX; OR ANOTHER PROB; MAINTAINING ALT COULD HAVE BEEN A PROB.