FLT CREW OF B733 UTILIZED WRONG DME TO DETERMINE STEPDOWN FIXES WHEN CLRED FOR RWY 21L ILS TO DTW. DSNDED FROM CLRED ALT PREMATURELY AS A RESULT.

Narrative

FO'S LEG TO DTW. UNEVENTFUL FLT WITH FOLLOWING EXCEPTIONS: 1) MULTIPLE NON-STANDARD SPD ASSIGNMENTS ENRTE FOR FLOW INTO DTW RESULTED IN LESS THAN 5000 LBS OF FUEL AT DTW (LANDED WITH 4200 LBS). 2) GIVEN EXTENDED VECTORS FOR RWY 21L. 3) RWY 21L HAS ILS WITHOUT DME CAPABILITY. DME FOR APCH IS PROVIDED BY OFF-ARPT NAVAID (CARLETON; APPROX 12 NM FROM DTW). 4) CLRNC WAS: 'MAINTAIN 5000 FT TO ROBBI; HEADING APPROX 240 DEGS; INTERCEPT THE LOC; CLRED ILS APCH TO RWY 21L' (AS NEAR AS I CAN REMEMBER). FOCUS AND CONCERN OVER FUEL STATUS HAD US USING DXO TO PROVIDE 'TO THE FIELD' DISTANCES FOR DSCNT PLANNING AND THAT; COUPLED WITH HIGH TFC VOLUME LED TO THE ALTDEV. THE FO (ME) SWITCHED TO THE ILS AND THE CAPT REMAINED ON DXO VERSUS CHANGING TO CRL. CAPT MADE ALT CALLS BASED ON DXO VERSUS CRL. LOWEST ALT WAS APPROX 3800 FT MSL (APPROX 1200 FT LOW). AT THAT POINT WE RECOGNIZED OUR MISTAKE AND CLBED TO 4000 FT. AT 3800 FT; WE WERE ON GLIDE PATCH; BUT NOT YET AT ROBBI. CTLR QUERIED OUR ALT; WE FESSED UP. REMAINDER OF APCH AND LNDG UNEVENTFUL. CTLR DID NOT MAKE CONFRONTATIONAL COMMENTS OR INDICATE HE WAS OR WOULD TAKE ANY ACTION. I THINK THIS ASAP RPT IS PRECAUTIONARY. THIS COULD HAPPEN TO ANYONE. TERPS SPECIALISTS SHOULD CONSIDER BASING APCHS TO DTW OFF OF DXO DME OR DTW SHOULD INSTALL ILS/DME EQUIP FOR RWY 21L.