A DO328 CREW MAKES AN EARLY DSCNT WHEN ATTEMPTING AN ILS; THEN; WITH NO GS VISIBLE; A LOC ONLY APCH TO RWY 3R AT DTW; MI.

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WE WERE CLRED FOR THE ILS RWY 3R AT DTW. A CHAIN OF EVENTS 'SUCKERED' ME INTO DOING SOMETHING I SHOULDN'T; AND NORMALLY WOULDN'T DO. THE ILS FREQ HAD BEEN IDENTED AND WE WERE ON THE LOC; CLRED FOR THE APCH. THE CAPT NOTICED WE EITHER DIDN'T HAVE OR LOST THE GS. BOTH OF US WERE RELATIVELY UNFAMILIAR WITH THE ARPT BUT WE SHOULD'VE ADVISED ATC RIGHT THEN THAT WE DIDN'T HAVE IT. HOWEVER; DUE TO THE GOOD WX; WE WERE IN AND OUT OF THE BASES; I ASKED THE CAPT IF HE WANTED TO DO THE LOC ONLY PROC BECAUSE WE CERTAINLY WOULD BREAK OUT. AS PER COMPANY PROC; I SET THE STEPDOWN ALT AND BRIEFED THE CAPT THAT HE COULD STEP DOWN AT 2.9 DME OFF CRL. ATC WAS CONSTANTLY SLOWING US DOWN ON THIS APCH SO WE CONFIGURED AND I RAN THE BEFORE LNDG CHKS. AS I WAS RUNNING THE CHKS; I NOTICED THE CAPT DSNDING FROM 3000 FT TO 2700 FT. I FINISHED THE CHKLIST AND DIDN'T FEEL RIGHT. I EXAMINED THE SIT AND REALIZED THE CAPT HAD DSNDED AT 2.9 'OUTSIDE' CRL WHERE THE STEPDOWN IS 2.9 'INSIDE' CRL. I NOTICED THIS ALMOST IMMEDIATELY; NOTIFIED THE CAPT; BUT HE OPTED TO FLY THE REMAINING 5 OR SO MI TO THE FIX 300 FT LOW. AS PREDICTED; WE DID BREAK OUT. REST OF THE APCH WAS NORMAL; NO COMMENT FROM ATC. CONTRIBUTING FACTORS AND COMMENTS: WE WERE ONLY 300 FT LOW; WAY OUT ON THE APCH. WE WERE 300 FT LOW. THIS IS VERY SIGNIFICANT. FURTHER IN THE APCH; THIS MIGHT HAVE CREATED A GND OBSTACLE HAZARD. OUR LACK OF FAMILIARITY CAUSED US TO PANIC OR RUSH WHEN WE DIDN'T HAVE THE GS. WE THOUGHT WE WERE A LOT CLOSER THAN WE WERE. I BRIEFED ORIGINALLY THE ILS TO THE CAPT POINTING OUT THE POSSIBLE DME CONFUSION WITH THE CAPT BY SAYING 'FAF IS HURON; DME 4.0 ON THE OTHER SIDE OF CRL.' EVEN BEING AWARE OF POSSIBLE CONFUSION; THE CAPT DIDN'T CATCH IT. A POSSIBLE FACTOR IS CLUTTER IN THE PROFILE VIEW OF THE APCH CHART. THE VOR (CRL) IS NOTED BUT IS TOUGH TO PICK OUT AT A GLANCE; WHICH; WHILE RUSHING; IS ALL WE DID. AS I SUGGESTED THE LOC ONLY PROC I THOUGHT TO MYSELF 'WE SHOULDN'T DO THIS WITHOUT BRIEFING IT FIRST.' BUT THEN I SAID 'NAH; IT'S BETTER THAN VFR WX; WE'LL BREAK OUT.' HORRIBLE IDEA; ESPECIALLY SINCE I KNEW AND EVEN THOUGHT ABOUT IT! CONSIDERATIONS INCLUDE MDA; MISSED APCH TIMING; ETC. I WOULD NEVER DO THAT NORMALLY; SO WHY DID I THINK IT WAS OK THIS TIME? AFTER NOTIFYING THE CAPT; HE OPTED NOT TO CORRECT THE SIT. THIS IS BAD. WE COULD'VE AVOIDED A POTENTIAL CONFLICT HAD THE PROC BEEN DIFFERENT. HIS RESPONSE WAS 'WE'RE FINE.' THE PROC; I BELIEVE; IS DESIGNED TO KEEP YOU WITHIN CLASS B AIRSPACE. THE EARLY DSCNT PUT US OUTSIDE (BELOW) CLASS B AIRSPACE IN VIOLATION OF PART 91.131 A(2). THE CAPT MADE THE STEPDOWN SILENTLY; IE; DID NOT VERBALIZE EITHER DSCNT OR FIX PASSAGE. WITH MY HEAD IN THE CHKLIST; THIS MAY HAVE ALERTED ME AND PREVENTED SUCH A GROSS DEV. THIS OCCURRENCE WAS A GENUINE MISTAKE. I ACTED IN GOOD FAITH AS A PROFESSIONAL CREW MEMBER. HOWEVER; AS I THINK ABOUT IT; I KEEP COMING BACK TO THE FACT THAT I KNEW WE SHOULDN'T DO IT WITHOUT A BRIEFING AND I LET MYSELF DO IT. THANK GOODNESS I DECIDED TO TAKE THIS ACTION IN GOOD WX IN A FORGIVING ENVIRONMENT WITH ONLY POTENTIAL LEGAL REPERCUSSIONS. IMAGINE IF IT'S NIGHT; LOW IMC. BELIEVE ME; LESSON LEARNED. HOPEFULLY THIS IS THE END OF THIS.