DH8 FO DSNDED BELOW HIS CLRNC ALT.

Narrative

DURING RADAR VECTOR FOR A VISUAL APCH TO RWY 18R AT CLT; CLRED HDG 090 DEGS AND DOWN TO 7000 FT. AUTOPLT ENGAGED. WE DSNDED VIA 'FPM' VERT SPD MODE. THIS CAN CAUSE OVERSPD WARNING HORN. THE WARNING HORN WENT OFF. I SLOWED AND THE HORN SILENCED. TO SLOW DOWN; I DISENGAGED THE AUTOPLT THEN ESTABLISHED PROPER AIRSPD AND RE-ENGAGED AUTOPLT. AS ATTN WAS GIVEN TO THIS TASK; 7000 FT ASSIGNED ALT WAS BUSTED THROUGH TO 6580 FT UNTIL DSCNT WAS STOPPED AND 7000 FT WAS REGAINED. THE CAPT HELPED BY ADJUSTING PWR AND ATTITUDE TO MAINTAIN 7000 FT. THE TOTAL TIME OUT OF 7000 FT WAS APPROX 10 SECONDS AND NO RADIO XMISSIONS WERE MADE; ON THE MATTER AT HAND. I AM NEW IN THE ACFT; FIRST MONTH OUT OF TRAINING; AND THIS CONTRIBUTED TO THE INCIDENT. BASICALLY; I FIXATED ON THE AUTOPLT AND READOUT AND DSNDED THROUGH THE ASSIGNED ALT. SUPPLEMENTAL INFO FROM ACN 421170: FO WAS DSNDING TO 7000 FT MSL AND DID NOT HAVE THE FLT DIRECTOR SET PROPERLY AND CONTINUED DSCNT TO 6600 FT BEFORE I NOTICED THE DEV. I TOOK OVER CTLS AND CLBED BACK TO 7000 FT MSL.