AN MD11 IN CRUISE AT FL370 WITH AUTOPLT IN ALT HOLD MODE BEGAN A SMOOTH GENTLE CLB TO FL378 WITH NO ALT ALERT WARNING. ALT SELECT STILL INDICATED FL370.

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AT FL370 WHILE IN CLR; DAYLIGHT; SMOOTH CONDITIONS; THE ACFT BEGAN A SMOOTH CLB. THE CAPT NOTICED IT FIRST; CALLED IT TO MY ATTN (I WAS LOOKING AT ANOTHER MD11 BELOW US). I DISCONNECTED THE AUTOPLT AND RETURNED TO FL370. WE HAD BEEN AT FL370 FOR ABOUT 6 HRS (FLT WAS LAX-NRT) AND THE ONLY FMS INPUT WAS FOR A DSCNT TO CROSS A FIX INBOUND TO NRT. THE ONLY GLARESHIELD (AUTOFLT) INPUT WAS A HDG CHANGE. THE CAPT SAW FL370 STILL IN ALT WINDOW DURING THIS EXCURSION. WE WERE DSNDING BACK TO FL370 WHEN TOKYO ASKED IF WE WERE AVOIDING WX. WE SAID NO; WE WERE RETURNING TO FL370. NO CONFLICT AROSE AND NOTHING FURTHER WAS SAID. REMAINDER OF FLT UNEVENTFUL. THE MECH IN NRT RAN A DIAGNOSTIC ON THE AUTOFLT/FMS/GLARESHIELD AND FOUND ONLY A SPURIOUS SPD INPUT TO FMS DSCNT. HE FELT IT HAD NOTHING TO DO WITH THIS EVENT. I HAVE HEARD OF ONLY 1 OTHER SUCH TYPE EXCURSION AND HAVE BEEN ON THIS ACFT FOR 6 YRS. I AM UPSET THAT I DO NOT KNOW HOW OR WHY IT OCCURRED OR HOW TO PREVENT IT FROM RECURRING. I WILL VISIT WITH OUR MD11 TYPE MGR NEXT WEEK TO RESEARCH THIS MATTER. I WOULD BE MOST APPRECIATIVE IF YOU HAVE ANY INFO ON WHAT MIGHT HAVE CAUSED THIS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT BEGAN A SLOW SMOOTH GENTLE CLB FROM FL370 TO FL378 WHEN THE DEV WAS DISCOVERED. THE RPTR SAID AT NO TIME DID THE ALT ALERT SOUND AND THE ALT SELECT STILL READ FL370. THE RPTR STATED THE RPT WAS WRITTEN UP AND TOKYO MAINT DID A COMPREHENSIVE TEST OF THE AUTOFLT; FMS AND GLARESHIELD COMPONENTS AND FOUND NOTHING THAT WOULD CAUSE THE ALTDEV. THE RPTR SAID THE OUTBOUND CAPT WHO HAD EXTENSIVE MD11 EXPERIENCE WAS ADVISED OF THE EVENT AND COMMENTED IT HAD HAPPENED ONCE BEFORE. THE RPTR HAS 6 YRS FLYING THE MD11 WITH NO PREVIOUS EXPERIENCE OF ALTDEV FROM THE FLT CTL COMPUTER. THE RPTR STATED A VISIT WAS MADE TO ACR MD11 FLT MGR TO REQUEST INFO ON PREVIOUS AUTOPLT ALT HOLD RPTS ON THE MD11 BUT NONE WERE IN THE MAINT HISTORY.