ALT OVERSHOOT ON DSCNT.

Narrative

WE WERE GIVEN A HDG TO INTERCEPT THE LOC BY DULLES APCH AND CLRED FROM 9000 FT TO 8000 FT AND GIVEN A FREQ CHANGE. WE BEGAN DSCNT TO 8000 FT WITH AUTOPLT ENGAGED; BEGAN TURN TOWARD NEW ASSIGNED HDG. ONCE BANK WAS ESTABLISHED; REALIZING THE 1/2 BANK MODE WAS STILL ON; I LOOKED DOWN TO DISENGAGE IT AND BECAME DISTRACTED WITH THE STEEPER BANK ANGLE; CHANGING FREQ AND SETTING UP FOR LOC. DURING THIS TIME THE ALT/SEL FUNCTION BECAME DISENGAGED; UNNOTICED BY ME. THE CAPT WAS BUSY CALLING COMPANY AND BRIEFING PAX. HE NOTICED THE ALTDEV AND INSTANTLY RECOVERED. AFTER HIS ACTION; I REALIZED WHAT HAD HAPPENED. THE CTLR ASKED US IF WE WERE ON FREQ YET AND THE CAPT RESPONDED. THROUGH THIS I HAVE ADOPTED A MORE DISCIPLINED XCHK AND SCAN OF ALT AWARENESS; AUTOPLT INDICATIONS AT ALL TIMES; ESPECIALLY DURING ALT AND HDG TRANSITIONS USING THE COMPANY PROC OF 1000 FT; 500 FT CALLS IN ADDITION ADDING ARMED AFTER 500 FT CALL AND SCANNING FOR 300 FT; 200 FT; 100 FT; ALT/CAPTURE; ALT/ON INDICATIONS AND RECHK AND SCAN PROC NOT ALLOWING MYSELF TO BECOME COMPLACENT AND DISTRACTED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR FLIES THE JETSTREAM 4100 FOR A REGIONAL ACR. THE FO IS RELATIVELY LOW TIME IN THE ACFT. THE FO ASSUMES FULL RESPONSIBILITY FOR THE ALTDEV. HE MAY HAVE CHANGED DSCNT MODES WHICH WOULD HAVE KNOCKED OFF THE ALT CAPTURE FEATURE OF THE AUTOPLT. DULLES WAS GIVING A STEP-DOWN DSCNT WITH MANY 1000 FT CHANGES. THE ALT ALERTER WAS GOING OFF CONTINUOUSLY. THE CAPT WAS 'HEAVILY INVOLVED' WITH COMPANY RADIO; ATIS; ETC AND WAS OUT OF THE LOOP. RPTR NOW IS MORE AWARE OF THE TRAPS IN THE AUTOPLT.