A320 CAPT INADVERTENTLY MISC - PROGRAMMED AUTOPLT TO CLB INSTEAD OF DSND AS INTENDED CAUSING AN ALT EXCURSION.

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THIS WAS MY FIRST FLT AFTER COMPLETING IOE ON THE AIRBUS 320. MY PREVIOUS ACFT WAS THE B767/757 ON WHICH I HAD 2 YRS EXPERIENCE. THE CAPT HAD APPROX 80-100 HRS ON THE AIRBUS 320 ON THIS DATE AND ALSO HAD RECENTLY COME FROM THE B767/757 FLEET. WE WERE ABOUT 4 HRS INTO THE 5 HR FLT; LEVEL AT A CRUISE ALT OF FL330; APPROX 100 NM SW OF THE FLAT ROCK VOR (FAK) HEADING DIRECT. THE ATC CTR WE WERE ON WITH WAS ZDC. ZDC ISSUED A CLRNC TO 'CROSS 60 NM SW OF FAK; AND MAINTAIN FL250.' I READ BACK THE CLRNC; SET THE ALT IN FLT CTL UNIT ALT WINDOW AND CONFIRMED IT WITH THE CAPT. AT THAT POINT I PROCEEDED TO TYPE THE CLRNC INTO OUR FMS COMPUTERS. WHILE I WAS DOING THAT; THE CAPT DECIDED TO START A DSCNT (SLOW) TO FL250 USING THE VERT SPD MODE ON THE AUTOPLT AT A DSCNT RATE OF -500 FPM. UNFORTUNATELY; INSTEAD OF SETTING 500 FPM DOWN THE CAPT INADVERTENTLY SET 500 FPM UP IN THE AUTOPLT WINDOW. THE CLB WAS SO GRADUAL THAT NEITHER OF US NOTICED A PITCH CHANGE AND THE ENGS NEVER SPOOLED UP TO A HIGHER THRUST SETTING THAT WE COULD HEAR. AFTER I FINISHED PROGRAMMING THE FMS TO THE CLRNC I LOOKED UP AT THE ALTIMETER AND NOTICED OUR ALT WAS APPROX FL335 AND CLBING. I ANNOUNCED THIS TO THE CAPT AT WHICH TIME WE BOTH FIGURED OUT WHAT HAD CAUSED US TO CLB AND THE CAPT IMMEDIATELY TOOK CORRECTIVE ACTION AND BEGAN THE DSCNT BACK TO FL330. AS WE WERE CORRECTING; ZDC QUERIED OUR ALT THEN ADVISED US TO EXPEDITE OUR DSCNT BACK TO FL330 AND THEN STILL CROSS 60 NM SW OF FAK AT FL250. WE COMPLIED AND MET THE RESTR. AFTER THAT; NOTHING WAS SAID AGAIN ABOUT THE DEV BY US OR ATC AND WE WERE EVENTUALLY HANDED OFF TO DULLES APCH AND LANDED SAFELY.