B737 CREW DSNDED BASED ON ILS DME. THE ILS DME WAS UNRELIABLE.

Narrative

ON THE MORNING OF NOV/XA/98; APCHING SAN; WE HAD ATIS WINDS 290 DEGS AT 5 KTS; VISIBILITY 4 MI BROKEN; CEILING 1000 FT BROKEN. LOC RWY 27 WAS IN USE. I WAS FLYING AND THE CAPT WAS THE PNF. CAPT TOLD ME HE IDENTED LOC. WE WERE VECTORED FOR THE APCH AND DSNDED ON PROFILE. DSNDING THROUGH ABOUT 1500 FT THE CTLR ADVISED LOW ALT ALERT THAT WE HAD DSNDED BELOW 2000 FT PRIOR TO REEBO; FAF. WE THEN CLBED UP TO 1500 FT MSL AND SOON HAD VASI IN SIGHT AND DSNDED TO THE ARPT. WE HAD GND CONTACT AND 4 NM FORWARD VISIBILITY THROUGHOUT THE APCH SO SAFETY WAS NOT COMPROMISED. HOWEVER; IT WAS A VALUABLE LESSON FOR THE FUTURE. CONTRIBUTING FACTORS INCLUDED: 1) UNEXPECTED FAILURE OF THE GND BASED DME SYS. 2) APCH CTLR NOT NOTIFYING PLTS OF FAILURE. 3) NOTIFICATION OF FAILURE NOT ON ATIS. 4) FAILURE OF DME NOT ON NOTAMS. 5) FAILURE OF PNF TO IDENT DME IDENT. 6) FAILURE OF PF TO USE TIMING AND VOR XRADIALS FROM POGGI AS STEPDOWN FIXES. THE NEED TO CONSTANTLY XCHK AND NOT ASSUME ANY NAVAIDS ARE ACCURATE IS ESSENTIAL. COMPLACENCY CAN LEAD TO ACCIDENTS.