Narrative:

Flying to ads via dumpy 2 arrival; visibility was more than 10 mi with thunderstorms northwest of the field. ATIS at ads indicated runway 15 in use and we had the airport in sight. We could see that rain was nearing the airport. Just as we should have been given a vector to turn towards a downwind leg for runway 15; there were multiple lightning strikes north of the field. Approach cleared us for the ILS to runway 33. We accepted the clearance and I (as PNF) changed the approach loaded in the GPS to ILS runway 33. I then got out the approach plate for ILS runway 33 as the autoplt intercepted the localizer about 10 mi from the runway still using the GPS for guidance. As we continued on this path we were descending towards 3000 ft which was the altitude ATC had given in the approach clearance. The rain began to obscure our view of the airport and we switched autoplt guidance to the VOR. When we did this; the rate of descent increased and the pilot asked me to read the altitudes on the approach plate. I told him the altitude over the FAF (portr) was 2000 ft and gave him the decision ht for the approach. As we descended through 2000 ft; the pilot remarked that we were not yet at the FAF and asked if my GS indicator was alive. I answered that it was and that we were right on the approach. At 1700 ft the aircraft started to bank to the left and the rate of turn and rate of descent were increasing rapidly. Both of us (pilots) were trying to assess the situation and the pilot said the autoplt had malfunctioned; and declared that he was abandoning the approach. He started to reconfigure the aircraft for a go around. At this time; we received a call from the tower that we were low on the approach and immediately also got a taws warning. Seconds later; we made visual contact with the runway and continued to an uneventful landing at the airport. Neither of us were able to immediately understand the events we had encountered. The pilot said that the indications were consistent only with having been tuned to a back course so the next morning I went to the airplane to check to see if I had dialed an incorrect frequency into the VOR. As I was examining the approach plate; I noticed that the ILS for runway 15 and for runway 33 shared the same frequency. I believe that the lightning distraction the personnel in the tower and that they forgot to change the ILS transmitter. The GPS had centered the airplane on the localizer perfectly so that we were able to continue for approximately 1 1/2 mins without the autoplt trying to correct the course. The GS we were following was a reflected signal from the opposite runway and would have ended 1/2 mi short of the runway at a steep angle. I will never again fly an ILS approach without carefully checking the morse code identify. I also think that this situation could happen again as long as the 2 ILS approachs to this airport share a common frequency.

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Original NASA ASRS Text

Title: BE20 FLT CREW FLIES AN APCH TO RWY 33 AT ADS WHEN TWR HAD ILS SET FOR RWY 15; CAUSING CONFUSION.

Narrative: FLYING TO ADS VIA DUMPY 2 ARR; VISIBILITY WAS MORE THAN 10 MI WITH TSTMS NW OF THE FIELD. ATIS AT ADS INDICATED RWY 15 IN USE AND WE HAD THE ARPT IN SIGHT. WE COULD SEE THAT RAIN WAS NEARING THE ARPT. JUST AS WE SHOULD HAVE BEEN GIVEN A VECTOR TO TURN TOWARDS A DOWNWIND LEG FOR RWY 15; THERE WERE MULTIPLE LIGHTNING STRIKES N OF THE FIELD. APCH CLRED US FOR THE ILS TO RWY 33. WE ACCEPTED THE CLRNC AND I (AS PNF) CHANGED THE APCH LOADED IN THE GPS TO ILS RWY 33. I THEN GOT OUT THE APCH PLATE FOR ILS RWY 33 AS THE AUTOPLT INTERCEPTED THE LOC ABOUT 10 MI FROM THE RWY STILL USING THE GPS FOR GUIDANCE. AS WE CONTINUED ON THIS PATH WE WERE DSNDING TOWARDS 3000 FT WHICH WAS THE ALT ATC HAD GIVEN IN THE APCH CLRNC. THE RAIN BEGAN TO OBSCURE OUR VIEW OF THE ARPT AND WE SWITCHED AUTOPLT GUIDANCE TO THE VOR. WHEN WE DID THIS; THE RATE OF DSCNT INCREASED AND THE PLT ASKED ME TO READ THE ALTS ON THE APCH PLATE. I TOLD HIM THE ALT OVER THE FAF (PORTR) WAS 2000 FT AND GAVE HIM THE DECISION HT FOR THE APCH. AS WE DSNDED THROUGH 2000 FT; THE PLT REMARKED THAT WE WERE NOT YET AT THE FAF AND ASKED IF MY GS INDICATOR WAS ALIVE. I ANSWERED THAT IT WAS AND THAT WE WERE RIGHT ON THE APCH. AT 1700 FT THE ACFT STARTED TO BANK TO THE L AND THE RATE OF TURN AND RATE OF DSCNT WERE INCREASING RAPIDLY. BOTH OF US (PLTS) WERE TRYING TO ASSESS THE SIT AND THE PLT SAID THE AUTOPLT HAD MALFUNCTIONED; AND DECLARED THAT HE WAS ABANDONING THE APCH. HE STARTED TO RECONFIGURE THE ACFT FOR A GAR. AT THIS TIME; WE RECEIVED A CALL FROM THE TWR THAT WE WERE LOW ON THE APCH AND IMMEDIATELY ALSO GOT A TAWS WARNING. SECONDS LATER; WE MADE VISUAL CONTACT WITH THE RWY AND CONTINUED TO AN UNEVENTFUL LNDG AT THE ARPT. NEITHER OF US WERE ABLE TO IMMEDIATELY UNDERSTAND THE EVENTS WE HAD ENCOUNTERED. THE PLT SAID THAT THE INDICATIONS WERE CONSISTENT ONLY WITH HAVING BEEN TUNED TO A BACK COURSE SO THE NEXT MORNING I WENT TO THE AIRPLANE TO CHK TO SEE IF I HAD DIALED AN INCORRECT FREQ INTO THE VOR. AS I WAS EXAMINING THE APCH PLATE; I NOTICED THAT THE ILS FOR RWY 15 AND FOR RWY 33 SHARED THE SAME FREQ. I BELIEVE THAT THE LIGHTNING DISTR THE PERSONNEL IN THE TWR AND THAT THEY FORGOT TO CHANGE THE ILS XMITTER. THE GPS HAD CTRED THE AIRPLANE ON THE LOC PERFECTLY SO THAT WE WERE ABLE TO CONTINUE FOR APPROX 1 1/2 MINS WITHOUT THE AUTOPLT TRYING TO CORRECT THE COURSE. THE GS WE WERE FOLLOWING WAS A REFLECTED SIGNAL FROM THE OPPOSITE RWY AND WOULD HAVE ENDED 1/2 MI SHORT OF THE RWY AT A STEEP ANGLE. I WILL NEVER AGAIN FLY AN ILS APCH WITHOUT CAREFULLY CHKING THE MORSE CODE IDENT. I ALSO THINK THAT THIS SIT COULD HAPPEN AGAIN AS LONG AS THE 2 ILS APCHS TO THIS ARPT SHARE A COMMON FREQ.

Data retrieved from NASA's ASRS site as of January 2009 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.