MISUSE OF AN ALT ALERTER CONTRIBUTES TO AN ALTDEV ON DEP CLB IN AN ATR42 NEAR SJU; PR.

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WE WERE CLBING TO OUR FINAL ALT OF 8000 FT. I WAS FLYING BY HAND (AUTOPLT OFF) BUT FOLLOWING THE FLT DIRECTOR BARS. USING THE FLT DIRECTOR BARS HELPS MAINTAIN TIGHTER AIRWORK TOLERANCES; BUT HAND FLYING INCREASES RELIANCE ON YOUR COPLT TO MANAGE THE FLT DIRECTOR. PASSING 6500 FT IN THE CLB; WE WERE GIVEN 'PLT'S DISCRETION TO 6000 FT.' I ASKED THE CAPT IF WE WERE STILL CLRED TO 8000 FT. HE SAID YES AND DIRECTED ME TO CONTINUE THE CLB. HE SET 6000 FT IN THE FLT DIRECTOR AND I CONFIRMED IT AS OUR NEXT ALT. UNFORTUNATELY; NEITHER OF US IMMEDIATELY REALIZED THAT WE HAD A CLB WITHOUT A TARGET ALT. THE FLT DIRECTOR WOULD NOT CAPTURE ALT (8000 FT) SINCE WE RESET IT TO AN ALT WE HAD ALREADY PASSED. THE CAPT; AS PNF; DIRECTED HIS ATTN TO SOME ADMINISTRATIVE COMPANY COMS; WHILE I CONCENTRATED ON THE FLT DIRECTOR BARS. AT 8500 FT; WE BOTH REALIZED THE ERROR AND I BEGAN A DSCNT BACK TO 8000 FT (WHICH I OVERSHOT TO 7900 FT). RECOMMENDATION: 1) FOR PNF; FOCUS ON MONITORING TASKS DURING TRANSITIONS. COMPANY CALLS CAN WAIT. 2) FOR PF; DON'T BE A SLAVE TO THE FLT DIRECTOR. ONE MUST DRIVE THE BARS JUST AS WHEN USING THE AUTOPLT TO ENSURE THEY ARE COMMANDING WHAT YOU WANT. MATCHING THE 'PIPPER' TO THE BARS IS SECONDARY. SMALL DEVS FROM PERFECT AIRWORK ARE PREFERABLE TO FLYING THE WRONG ALT; HDG OR AIRSPD PERFECTLY.