AN A300 IN THE PROFILE DSCNT MODE INDICATED PROPER FMA ALT CAPTURE ANNUNCIATIONS; BUT FAILED TO CAPTURE THE PROGRAMMED ALT.

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ON ARR TO ZZZ; ON XXXXX ARR. FO WAS FLYING. CLRED TO DSND TO FL240; ACFT WAS IN PROFILE DSCNT MODE. CAPT WAS REVIEWING ARR CHART WHEN ALT DEV WARNING WENT OFF. I (CAPT) LOOKED AWAY FROM CHART TO SEE ACFT WAS NOW AT FL236. FO; WHO WAS FLYING; WAS LOOKING DOWN AT HIS CHART. I IMMEDIATELY TOOK CTL OF ACFT; CLICKED AUTOPLT OFF; AND CLBED BACK UP TO FL240. APCH DIDN'T SAY ANYTHING AND EVENT WAS OVER WITHIN 5 TO 10 SECONDS. ACFT WILL ALWAYS NOT PERFORM PROPERLY WHILE IN PROFILE MODE. COMPLACENCY ESPECIALLY IN PROFILE MODE SHOULD BE AVOIDED; ESPECIALLY PRIOR TO ACFT LEVELOFF. ADDITIONAL; CREW WAS AWAKE FOR APPROX 24 HRS AT THE TIME OF EVENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT IN THIS EVENT BOTH THE MCP ALT SET AND THE FMC CONTAINED FL240 AS THE CONSTRAINING ALT. THE ACFT WAS SLIGHTLY PAST THE CONSTRAINT POINT AND WAS ATTEMPTING TO MEET THE FIX REQUIREMENT BUT DROPPED THE ALT FOR SOME UNKNOWN REASON. THE AUTOPLT DID NOT OVERSPEED TRYING TO MEET THE CONSTRAINT.