A CL65 FLC DSNDS BELOW THE FAF XING ALT WHEN THE APCH MODE WAS INADVERTENTLY DISENGAGED 2 MI OUTSIDE OF THE LOM FOR RDU; NC.

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ON VECTORS TO LAND ON RWY 23R AT RDU ARPT; WE HAD BEEN GIVEN A DSCNT CLRNC TO 2200 FT. DURING THE DSCNT; I CALLED THE ARPT IN SIGHT; THE CAPT NOTIFIED APCH AND WE WERE CLRED FOR THE APCH. I UNDERSTOOD THE CLRNC 'CLRED FOR THE VISUAL TO RWY 23R.' AS WE CONTINUED THE DSCNT TO 2200 FT; WITH THE AUTOPLT ON; I RESET THE ALT SELECTOR TO 2200 FT WHICH WAS THE MINIMUM ALT TO INTERCEPT THE GS AT THE FAF (BODLY). WHEN I LOOKED AT THE PFD I REALIZED THAT THE AUTOPLT WOULD NOT BE ABLE TO INTERCEPT THE LOC WITH THE INTERCEPT ANGLE WE WERE ON; SO I SWITCHED TO HDG MODE TO BRING US CLOSER TO THE LOC COURSE. WHEN I DID THIS THE APCH MODE WAS DISENGAGED. AS I DID THIS THE ACFT CONTINUED TO DSND. AS THIS IS MY FIRST EXPERIENCE WITH FMS; IT TOOK A MOMENT TO REALIZE WHAT THE ACFT WAS DOING. BY THE TIME I REALIZED WHAT HAD HAPPENED; WE HAD DSNDED TO 1600 FT MSL. I IMMEDIATELY DISENGAGED THE AUTOPLT AND BEGAN TO CLB TO GET BACK ON PROFILE. AS I DID THIS; APCH CTL ANNOUNCED 'FLT XY; 2200 FT OR ABOVE TO BODLY.' DURING THIS EVENT; THE CAPT WAS ALSO TRYING TO FIGURE OUT WHAT HAD HAPPENED TO ALLOW THE PLANE TO DSND. AFTER REGAINING ALT JUST AS WE REACHED BODLY WE WERE BACK ON PROFILE AND LANDED WITHOUT INCIDENT. A POSTFLT DISCUSSION BTWN MYSELF AND THE CAPT REVEALED SEVERAL THINGS: 1) MY LACK OF EXPERIENCE WITH THE FMS HAD SOMEHOW ALLOWED ME TO GO TO ROLL MODE AND DSND BELOW THE SELECTED ALT. 2) THERE WAS CONFUSION ABOUT OUR APCH CLRNC; I WAS UNDER THE IMPRESSION WE HAD BEEN CLRED FOR THE VISUAL; BUT WE HAD BEEN CLRED FOR THE ILS. 3) IN THE BRIEF PERIOD OF TIME FROM THE CLRNC TO LAND; A TURNING DSCNT ALONG WITH ATC COM HAD DIVERTED THE CAPT'S ATTN. BY THE TIME HE HAD RECOGNIZED WHAT THE ACFT WAS DOING AND COMMANDED CORRECTIVE ACTION; I HAD SIMULTANEOUSLY DISCONNECTED THE AUTOPLT AND WAS BEGINNING THE RECOVERY. I FEEL THE MAIN REASON FOR THIS TRANSGRESSION WAS A LACK OF EXPERIENCE ON FMS ACFT AND THE MISINTERP OF WHICH MODE THE ACFT AUTOPLT WAS IN. ALSO; THE TIMING OF THE EVENTS WHICH REQUIRED THE CAPT TO COMMUNICATE WITH ATC.