AN ACR SF34 FLC DSNDED BELOW THE MINIMUM XING ALT OF AN APCH FIX WHILE ON A GLIDESLOPE OUT ILS APCH.

Narrative

WE HAD BEEN CLRED FOR THE ILS RWY 12R 'GLIDESLOPE UNUSABLE' AT THE MSP ARPT. WE WERE CLRED FOR THE APCH AT 20 DME FROM THE MSP VOR. THE 12R RWY IS UNDER CONSTRUCTION WITH THE THRESHOLD DISPLACED 4000 FT; THUS THE UNUSABLE GLIDESLOPE. THERE WAS SOME CONFUSION AND DISCUSSION IN THE COCKPIT ABOUT THE CLRNC. I WAS PREOCCUPIED WITH EXPLAINING THE CLRNC AS THE FO PROCEEDED TO DSND TO THE MINIMUM ALT TO CROSS THE FINAL APCH FIX (2800 FT). WE DSNDED FROM 5000 FT TO 2800 FT. UNFORTUNATELY; I FAILED TO NOTICE WE WERE STILL OUTSIDE THE 13.9 DME FIX WHICH HAS A MINIMUM XING ALT OF 5000 FT. THE MISTAKE WAS BROUGHT TO OUR ATTN BY THE CTLR; AT WHICH TIME I BECAME AWARE OF OUR ERROR. I TOLD THE CTLR WE WERE CORRECTING OUR ALT. BY THIS TIME WE WERE INSIDE THE 13.9 DME FIX. THE CTLR TOLD US TO STAY AT 2800 FT; THAT HE WANTED US TO TAKE NOTE OF THE XING RESTRICTION. THERE WAS NO CONFLICT TO ANY DEGREE AS A RESULT OF OUR MISTAKE. MY ONLY EXPLANATION AS TO WHY THIS HAPPENED IS HUMAN ERROR. WE WERE AWARE OF THE MINIMUM ALT; BUT FAILED TO NOTICE OUR POS WAS STILL OUTSIDE THE REQUIRED FIX.