A DC9-50 FLC FAILS TO INTERCEPT THE ILS LOC COURSE FOR RWY 21L WHEN THEY DIAL IN THE ILS FOR RWY 21R ON NAV RADIO #2 INSTEAD OF THE CRL VOR FOR DME SW OF DTW; MI.

Narrative

THE FLT WAS VECTORED FROM THE CETUS ARR FOR AN APCH TO RWY 21L AT DTW. APPROX 1`5 MI OUTSIDE OF THE FAF; THE FLT WAS CLRED TO INTERCEPT THE RWY 21L LOC. THE RWY 21R LOC FREQ WAS INADVERTENTLY TUNED TO THE PNF VHF NAV AND THE ACFT WAS FLOWN THROUGH THE RWY 21L FINAL APCH COURSE. ATC THEN INSTRUCTED US TO TURN TO A 190 DEG HDG AND MAINTAIN 5000 FT. WE WERE ALSO INSTRUCTED TO INTERCEPT THE RWY 21L LOC AND CLRED FOR THE RWY 21L APCH. THE FLT THEN CONTINUED AND LANDED WITHOUT ANY INCIDENT. THE RWY 21L ILS APCH AT DTW NEEDS TO BE REDESIGNED SO IT DOES NOT REQUIRE DME INFO FROM THE CRL VORTAC TO IDENT THE FIXES ASSOCIATED WITH THE APCH. THIS CREATES A SIT WHERE 1 VHF NAV RADIO MUST BE TUNED TO SOMETHING OTHER THAN THE LOC FREQ REQUIRED FOR THE APCH. THIS IS A VERY DANGEROUS SIT; ESPECIALLY WITH SIMULTANEOUS APCHS TO SIMILAR RWYS OCCURRING IN A HIGH DENSITY AREA SUCH AS DTW. ALSO AN INCORRECT DME READING OCCURS WHEN THE RWY 21L LOC FREQ 111.5 IS TUNED. THIS CAN BE CONFUSING; BECAUSE THE DISTANCE READING IS NOT THE DISTANCE TO DTW. ALL ILS APCHS SHOULD HAVE A DME AS PART OF THE REQUIRED COMPONENTS. SUPPLEMENTAL INFO FROM ACN 476078: CTLR WAS BUSY; AND ACFT WITH SIMILAR CALL SIGNS ON APCH PARALLEL RWY. FATIGUE AND ARPT CONGESTION AND ACFT WITH SIMILAR CALL SIGNS CONTRIBUTED TO INCIDENT.