FLT CREW OF B747-400 IS CONCERNED THAT AUTOFLT VERT NAV MALFUNCTION IN PACIFIC RVSM AIRSPACE RESULTED IN A MORE SERIOUS SEPARATION HAZARD THAN PRIOR TO RVSM PROCS.

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APPROX 3-5 MINS PRIOR TO THIS INCIDENT; THE ACFT GOT 1 SINGLE JOLT OF LIGHT TO MODERATE TURB. THERE WAS NO TURB PRIOR TO OR AFTER THIS INCIDENT FOR QUITE AWHILE. THIS JOLT SUCCEEDED IN WAKING UP THE OFF WATCH CAPT WHO THEN WENT BACK FOR A RESTROOM VISIT. FOLLOWING NORMAL SOP PROCS; I GOT OUT OF THE L SEAT TO LET HIM BACK INTO THE COCKPIT. WHEN I WAS GETTING BACK INTO THE L SEAT; THE OTHER RELIEF PLT CALLED OUT THAT WE WERE IN A VERT SPD DSCNT OF 100 FPM! WE IMMEDIATELY PROCEEDED TO STOP THE DSCNT. I FIRST HIT 'FLT LEVEL CHANGE' SINCE FL350 WAS STILL IN THE ALT WINDOW; BUT THE AUTOPLT REMAINED IN VERT SPD. SO I JUST ROLLED UP THE VERT SPD TO +200 TO CLB BACK UP. WE BOTH READ OFF THE ALT AT FL347.3. PRIOR TO THIS INCIDENT; WE HAD REQUESTED FL360 FROM NADI. ONCE ESTABLISHED BACK AT FL350 AND IN VNAV; WE RECEIVED OUR CLB CLRNC TO FL360. THE PNF SELECTED FL360 IN THE ALT WINDOW AND MUCH TO OUR AMAZEMENT; THE VERT SPD WINDOW OPENED ON ITS OWN SHOWING +100 FPM! AFTER CLBING TO FL360 WE LATER TRIED DUPLICATING THE PROB. UNABLE TO DUPLICATE. DID VNAV REVERT BACK TO VERT SPD ON ITS OWN; DID THE TURB DO IT; OR DID I INADVERTENTLY HIT THE VERT SPD SELECT BUTTON WHILE GETTING OUT OF MY SEAT? WE TESTED THE LAST THEORY; NOT AN EASY THING TO DO. BUT THIS WAS OUR ONLY RATIONALE -- UNTIL WE DSNDED INTO SYDNEY. NOW THE CAPT WAS BACK IN THE L SEAT; HAND FLYING; AUTOPLT OFF; IN VMC. WE RECEIVED CLRNC FROM 9000 FT TO 8000 FT; BUT WHEN THE PNF PUSHED FLT LEVEL CHANGE INSTEAD OF GETTING FLT LEVEL CHANGE; THE VERT SPD WINDOWS OPENED UP AGAIN. THIS HAPPENED SEVERAL TIMES AND WE WROTE IT UP. THERE WAS SOMETHING WRONG WITH THE MCP. DID IT CONTRIBUTE TO OUR ALTDEV; OR WAS IT JUST A COINCIDENCE? THIS INCIDENT PROVES THAT EVEN WITH DILIGENT PLTS; MINOR ALTDEVS CAN HAPPEN. SHOULD WE BE MOTORING DOWN THE MIDDLE OF AIRWAYS WITH HEAD-ON TFC IN RVSM AIRSPACE? OR WOULD IT BE BETTER TO HAVE A BUILT-IN OFFSET TO PRECLUDE THE OBVIOUS? THE NECESSARY DOOR SECURITY PROCS ARE DEFINITELY A DISTR. IT IS STILL PRUDENT TO ALLOW THE AUTOPLT TO DEVIATE FROM THE ALT WINDOW BY MORE THAN 200 FT (MAX ALLOWED IN RVSM) WITHOUT A WARNING? I SHUDDER AT THE CONSEQUENCES OF OUR NOT RECOGNIZING THE ALTDEV AS QUICKLY AS WAS DONE IF THERE HAD BEEN INTERVENING TFC BELOW US IN RVSM AIRSPACE. THE SOP TO CALL OUT CHANGES WHEN THE OTHER PLT RETURNS TO HIS SEAT IS A GOOD ONE; AND MAY HAVE SAVED US; BUT IT WAS STILL TOO LITTLE TOO LATE.