Narrative:

We were cleared for the ILS runway 3, GS OTS. I had the ILS frequency in my navigation radio and the first officer initially had spa VOR frequency in his navigation radio. Greer IAF frequency 287 was set in the ADF. Raw data on the localizer was monitored and confirmed for the localizer intercept. The map display appeared to be accurate as I went back to map on my navigation display. The first officer monitored raw data (ILS frequency runway 3) on his navigation display. The map display on my navigation display showed that we were passed the IAF and the ADF needle bounced a few times. I (PF) decided to descend due to the fact that we had the ground in sight and the forward visibility was such (estimated 4-5 mi) that the airfield environment (no trees) appeared to be in sight. (Approach lights were OTS.) the first officer was unsure that we had passed the ADF but I thought we had based upon the fact that the IAF on the map display was now well behind us and I had seen the ADF needle moving. We were descending at 1500 FPM when we got a call from the tower that the tower had a low altitude alert. We confirmed we had the ground in sight. Realizing we were not where I thought we were. I stopped the descent and drove in toward the field at approximately 2000 ft MSL. Although we would not have descended in IMC without a more positive identify on the IAF (ie, radar fix), with the ADF needle locking and unlocking, the fact is that this was confusing due to the lack of DME and a map display that appeared to be correct but was obviously not.

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Original NASA ASRS Text

Title: CAPT OF AN MD88 DSNDED BELOW PUBLISHED APCH ALT DURING AN ILS APCH RESULTING IN TWR CTLR ISSUING A LOW ALT ALERT.

Narrative: WE WERE CLRED FOR THE ILS RWY 3, GS OTS. I HAD THE ILS FREQ IN MY NAV RADIO AND THE FO INITIALLY HAD SPA VOR FREQ IN HIS NAV RADIO. GREER IAF FREQ 287 WAS SET IN THE ADF. RAW DATA ON THE LOC WAS MONITORED AND CONFIRMED FOR THE LOC INTERCEPT. THE MAP DISPLAY APPEARED TO BE ACCURATE AS I WENT BACK TO MAP ON MY NAV DISPLAY. THE FO MONITORED RAW DATA (ILS FREQ RWY 3) ON HIS NAV DISPLAY. THE MAP DISPLAY ON MY NAV DISPLAY SHOWED THAT WE WERE PASSED THE IAF AND THE ADF NEEDLE BOUNCED A FEW TIMES. I (PF) DECIDED TO DSND DUE TO THE FACT THAT WE HAD THE GND IN SIGHT AND THE FORWARD VISIBILITY WAS SUCH (ESTIMATED 4-5 MI) THAT THE AIRFIELD ENVIRONMENT (NO TREES) APPEARED TO BE IN SIGHT. (APCH LIGHTS WERE OTS.) THE FO WAS UNSURE THAT WE HAD PASSED THE ADF BUT I THOUGHT WE HAD BASED UPON THE FACT THAT THE IAF ON THE MAP DISPLAY WAS NOW WELL BEHIND US AND I HAD SEEN THE ADF NEEDLE MOVING. WE WERE DSNDING AT 1500 FPM WHEN WE GOT A CALL FROM THE TWR THAT THE TWR HAD A LOW ALT ALERT. WE CONFIRMED WE HAD THE GND IN SIGHT. REALIZING WE WERE NOT WHERE I THOUGHT WE WERE. I STOPPED THE DSCNT AND DROVE IN TOWARD THE FIELD AT APPROX 2000 FT MSL. ALTHOUGH WE WOULD NOT HAVE DSNDED IN IMC WITHOUT A MORE POSITIVE IDENT ON THE IAF (IE, RADAR FIX), WITH THE ADF NEEDLE LOCKING AND UNLOCKING, THE FACT IS THAT THIS WAS CONFUSING DUE TO THE LACK OF DME AND A MAP DISPLAY THAT APPEARED TO BE CORRECT BUT WAS OBVIOUSLY NOT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.