ALTDEV ALT EXCURSION.

Narrative

WHILE DSNDING TO 6000 FT WE WERE GIVEN A TURN TO JOIN A VISUAL FINAL. WE WERE GIVEN THE TURN VERY LATE AND WERE BADLY OVERSHOOTING THE FINAL; THE CAPT WAS TRYING TO TELL APCH WE WERE UNFAMILIAR WITH THE ARPT; WHILE APCH WAS ASKING US TO TIGHTEN OUR TURN. HAVING JUST COME FROM THE MD-80 TO THE DC-10 AND KNOWING THE SIMILARITIES BTWN THE 2 FLT GUIDANCE SYS I ATTEMPTED TO TIGHTEN THE TURN BY EXTENDING THE SLATS AND ENGAGING THE ILS. IN DOING SO I HAD INADVERTENTLY DISENGAGED THE ALT ARM; AND THE ACFT PROCEEDED TO DSND THROUGH 6000 FT. I NOTICED THE DEV AT APPROX 5900 FT; DISENGAGED THE AUTOPLT AND INITIATED A RECOVERY AT ABOUT 5800 FT. BY THE TIME THE ATTITUDE CHANGE TOOK EFFECT WE WERE AT 5600 FT; AT WHICH POINT APCH ISSUED AN ALT ALERT AND AN IMMEDIATE CLB. WE PROCEEDED TO CLB BACK UP TO 6000 FT AND CONTINUED THE REST OF THE APCH HAND FLOWN AND VISUAL TO AN UNEVENTFUL LNDG. SUPPLEMENTAL INFO FROM ACN 295355: I THINK THE FO ALSO ARMED THE ILS CAPTURE FEATURE. THE ACFT DID CAPTURE THE ALT. THE FIELD ELEVATION WAS 4200 FT. GIVEN OUR RANGE FROM THE FIELD; WE WERE AT GS CAPTURE ALT. I BELIEVE THE ILS ARMED FEATURE CAPTURED THE GS AND STARTED TO DSND WITHOUT US BEING AWARE OF IT. I NOTICED THE ALTDEV AT 5400 FT MSL AND STARTED TO INFORM THE FO WHEN APCH CTL CALLED US ABOUT AN LOW ALT ALERT AND GAVE US AN IMMEDIATE CLBING TURN. WE WERE NOT CLRED FOR THE ILS APCH. SAME OLD THING. SOMEBODY MUST FLY THE ACFT. SINCE WE WERE IN A 30 DEG ANGLE OF BANK R TURN I SHOULD HAVE TAKEN THE ACFT FROM THE FO AND FLOWN ON INSTS WHILE THE FO LOOKED FOR THE ACFT.