A B747-200 FLT CREW RPTED THAT FATIGUE AND LACK OF AN ALT ALERTER CAUSED THEM TO DEVIATE FROM THEIR ASSIGNED ALT BY 1500 FT IN VHHH'S AIRSPACE.

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ASSIGNED ALT WAS 13000 FT AT MELON INTXN. DSCNT BELOW ALT TO 11500 FT WITH IMMEDIATE RETURN TO 13000 FT. CAPT RPTED LEVEL AT 13000 FT AND APCH INSTRUCTED US TO CONTINUE TO 8000 FT. FACTORS: EXTREME FATIGUE. THE NEARLY 11-HR FLT WAS PRECEDED BY A 5-HR MAINT DELAY. THE CREW RETURNED TO THE HOTEL FOR 5 HRS AFTER THE MAINT DELAY (AND BEFORE DEP). NONE OF THE CREW WERE ABLE TO SLEEP MORE THAN ABOUT 1 HR. 'BACKSIDE OF THE CLOCK' FLYING. IN THE DAYS PRECEDING THIS TRIP; THE PF FLEW ANC-LAX-HNL-LAX-HNL-LAX-JFK-ANC; ALL AT VARYING TIMES; MOSTLY AFTER MIDNIGHT. THE FLT IN QUESTION WAS SCHEDULED TO DEPART IN THE EVENING BUT; BECAUSE OF THE MAINT DELAY/DUTY BREAK; DEPARTED AROUND 13 HRS LATER; THE NEXT DAY. DURING THE APPROX 32 HRS BEFORE THE END OF THE FLT; THE PF HAD SLEPT APPROX 4 HRS. MECHANICAL DEFECT: THE ALT SELECT WINDOW WAS INOP. IN FACT; THIS WAS THE CAUSE OF THE MAINT DELAY THE PREVIOUS NIGHT. THIS ITEM WAS DEFERRED BY MAINT; AS THERE WAS NO REPLACEMENT PART AVAILABLE. THIS DEFECTIVE ITEM RENDERED THE ALT ALERT SYS INOP. THE CREW WROTE EACH ASSIGNED ALT ON A PIECE OF NOTE PAPER AND PLACED THE NOTE PAPER FORWARD OF THE THROTTLES. CONFUSION AS TO THE ASSIGNED ALT; MAINLY DUE TO FATIGUE. THE ASSIGNED ALT WAS 13000 FT. THE STAR INCLUDES A NOTE DIRECTING ACFT AT MELON TO MAINTAIN 250 KTS BELOW 11000 FT. ALSO; THE PF ASKED THE CAPT WHAT THE TRANSITION ALT WAS. THE CAPT REPLIED; '11000 FT.' DESPITE THE FACT THAT THE CORRECT ASSIGNED ALT WAS WRITTEN ON A PIECE OF PAPER; THESE 2 SUGGESTIONS OF '11000 FT;' COMBINED WITH ATC'S INSTRUCTION TO 'MAINTAIN HIGH SPD' IN THE DSCNT; EXTREME FATIGUE; AND AN INOP ALT ALERTER; LED TO AN UNINTENTIONAL DSCNT TO 11000 FT. AT APPROX 12300 FT; THE CAPT ADVISED THE PF OF THE CORRECT ASSIGNED ALT. AN IMMEDIATE RECOVERY WAS INITIATED; BUT 11500 FT WAS REACHED PRIOR TO ESTABLISHING A CLB. SUPPLEMENTAL INFO FROM ACN 630170: ON DSCNT INTO HONG KONG (VHHH); WE DEVIATED FROM AN ASSIGNED ALT. WE WERE CLRED TO DSND TO AND MAINTAIN FL130. WE WERE ON THE ELATO 1B ARR; BTWN POINT ELATO AND MAGOG. WE DSNDED TO APPROX FL115 BEFORE CORRECTIVE ACTION WAS TAKEN. WE RETURNED TO FL130 AND NO OTHER ACTION WAS TAKEN. WE WERE BELOW FL130 FOR APPROX 45 SECONDS. NO MENTION OF THE ALT EXCURSION WAS MADE BY ATC. CONTRIBUTING FACTORS: FATIGUE. WE HAD BEEN ON DUTY FOR 17 HRS. DISTR. WE WERE DISTR WITH DEVIATING AROUND CUMULONIMBUS IN THE AREA ON DSCNT. DEFERRED MAINT ITEM. THE ALT ALERTING SYS; AND THE ALT SELECT SYS OF THE AUTOPLT WERE DEFERRED AND INOP.