AFTER BEING CLRED TO LOWER ALT; PF INTENDED TO SET A DSCNT OF 1000 FPM IN THE MCU (MODE CTRL PANEL); BUT APPARENTLY; INSTEAD OF SETTING IN A DSCNT; SET IN A CLB. NEITHER OF THE FLC NOTICED UNTIL ATC INTERVENED WHEN THEIR ALT WAS 1200 FT ABOVE THE ORIGINAL ALT.

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WE WERE AT FL370 CLRED TO FL240. I WAS PNF AND SET FL240 IN FCU AND BOTH PF AND I AGREED. PF THOUGHT IT WAS OUR DISCRETION AND I SAID IT WAS NOT. HE THEN SAID HE'D USE V/S AT 1000/MIN AS WE WERE NOT AT OPTIMAL DSCNT POINT. I WAS LOOKING AT BOIDS ARRIVAL PAGE WHEN PF SAID SOMETHING. I LOOKED UP AND SAW WE WERE AT FL380. COPLT WENT TO OPEN DSCNT. AT FL382 ATC ASKED OUR ALT. HE THOUGHT HIS ENCODER WAS WRONG. WE SAID WHAT HAPPENED; AND HE SAID TO EXPEDITE THROUGH FL310 OUR DISCRETION AFTER. WE DO NOT KNOW IF WE CONFLICTED WITH ANYONE ELSE. THIS INCIDENT RE-EMPHASIZES THE EXTRA CARE WITH AN AUTOMATIC AIRPLANE ONE MUST EXERCISE TO MAKE SURE IT DOES WHAT YOU WANT. INTERESTINGLY; NO ALT ALERT WAS SOUNDED AND I DON'T RECALL IF THE ALT WINDOW WAS FLASHING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: BECAUSE THE CAPT'S RPT DIDN'T SPECIFY WHY THE ACFT CLBED WHEN THE FLC HAD BEEN CLRED TO DSND; THE RPTR WAS CONTACTED. HE SAID THAT; THOUGH HE WASN'T SURE WHY IT OCCURRED; HE ASSUMED THAT THE FO HAD SET A VERT CLB OF 1000 FPM INSTEAD OF THE DSCNT. SINCE THAT ACTION WOULD GENERALLY BE CONSIDERED SOMETHING AN INEXPERIENCED PLT ON THE ACFT MIGHT DO; THE CAPT WAS ASKED ABOUT THE FO EXPERIENCE LEVEL. HE HAD HIS RECORDS OF FLTS AVAILABLE SO CLARIFIED THAT THE FO WAS VERY EXPERIENCED ON THE ACFT; AND MUST HAVE BEEN DISTRACTED BY SOME OTHER TASK WHEN HE INITIATED WHAT HE THOUGHT WAS A DSCNT. RPTR SAID THAT HIS COMPANY HAS INCLUDED IN THE ANNUAL PROFICIENCY CHK MORE ADVANCED TRAINING ON THE A320 COMPUTER SYS; WHICH HE BELIEVES IS VERY POSITIVE AND NECESSARY.