ALTDEV ALT EXCURSION ALT XING RESTRICTION NOT MET IN PREMATURE DSCNT IN IAP ILS APCH.

Narrative

INBOUND TO MDW; LNDG RWY 4; WE WERE CLRED DOWN TO 4000 FT APPROX 25 MI FROM FIELD. THE INITIAL SUGGESTED ALT FOR THE APCH IS 5000 FT OVER BANER WHICH IS DEFINED OFF THE CGT 293 DEG RADIAL. THE CAPT SET UP CGT 293 DEG RADIAL AND UPON XING; HAVING BEEN CLRED FOR THE APCH; WE BOTH OVERLOOKED THAT WE WERE ALREADY AT 4000 FT WHICH IS THE NEXT XING RESTRICTION. OR; PROBABLY MORE CORRECT; WE BOTH FORGOT THAT WE WERE XING BANER AND NOT CADON. AS THE CAPT CALLED OUT THAT WE WERE XING THE RADIAL; I BEGAN THE DSCNT TO THE FINAL APCH FIX ALT OF 2600 FT. THE CAPT REALIZED OUR ERROR SHORTLY BUT NOT BEFORE I HAD DSNDED TO APPROX 3500 FT. AT THIS POINT; I LEVELED OFF. I MAINTAINED THAT ALT UNTIL SUCH TIME WE WERE SURE THAT WE WERE PAST CADON. THE APCH PROCEEDED WITHOUT FURTHER INCIDENT TO A SAFE AND NORMAL LNDG. SIMPLY; I MISTOOK XING BANER FOR CADON BY NOT PROPERLY XCHKING WHAT RADIAL THE CAPT HAD DIALED IN. PARTLY; I BELIEVE; THAT WE WERE LURED INTO THIS MISTAKE BY BEING CLRED TO 4000 FT SO FAR OUT. I DO NOT BELIEVE SUCH A MISTAKE COULD BE MADE IF THE MDW ILS RWY 4 HAD DME. SUPPLEMENTAL INFO FROM ACN 206666: I BELIEVE THIS INCIDENT HIGHLIGHTS THE NEED FOR ALL ILS'S TO BE EQUIPPED WITH DME. THE PF THE APCH; CONCENTRATING ON LINE UP RELIES ON THE PNF TO HAVE THE CORRECT RADIAL TO IDENT THE VARIOUS LET DOWN FIXES; FOR HIM TO VERIFY THE PROPER RADIAL REQUIRES A CROSS COCKPIT CHK WHICH AT NIGHT IN THE WX IS NOT THE OPTIMUM SITUATION. IF DME WAS ON ALL ILS THE PF WOULD KNOW HIS POS AT ALL TIMES AND WOULD ALLOW BOTH PLTS TO BE ON THE ILS THROUGHOUT THE APCH. THIS WOULD ENHANCE SAFETY BY ALLOWING BOTH PLTS TO MONITOR THE APCHS.