Narrative:

We were making an ILS approach to runway 12R at stl. At the OM we were cleared to land. From the OM, we could see the runway but there was a low cloud on the approach which prevented us from maintaining visual contact with the runway. When we entered the cloud, we received a TCASII traffic alert showing a plane directly over us, 700 ft higher. Because there are a lot of TCASII warnings in that area, I confirmed with the first officer that the other aircraft was actually above us and not below as is the usual case in this area. We were still in the cloud when the other aircraft started a rapid descent and the TCASII showed the vertical separation decreasing to 400 ft. At that point, I began an evasive maneuver by increasing my descent rate and flying nearly 2 dots below the glide path. When we broke out of the clouds, the other plane was directly above us and the TCASII showed 300 ft separation. The first officer told the tower that a plane was on top of us. The tower then called the other plane and asked him to confirm that he was making a visual approach to runway 12L. From the sound of his reply it was clear that he thought he was cleared for a visual approach to runway 12R. The tower then told the other plane to make a visual approach to runway 12L and contact the other tower frequency (120.05). I believe that the tower mistakenly cleared the other plane for a visual approach to runway 12R and that we would have collided had I not taken evasive action. The fact that the other plane was on the frequency for runway 12R (118.5) leads me to believe that this near midair collision was caused by the tower. His call sign sounded similar to ours which may have led to the confusion. Callback conversation with reporter revealed the following information: reporter stated the other aircraft was a jetstream on an lda approach to runway 13. Reporter indicated his company received a copy of the radar printout plotting the tracks of both aircraft. Reporter alleges the plot of the jetstream does not show the aircraft going to his right before turning toward runway 13. Reporter stated that when he saw the aircraft, it was on about a 30 degree divergent heading away from his aircraft. Reporter stated they had not been informed of the jetstream and indicated the facility is not treating this as a loss of separation. Reporter stated they did not get a low altitude alert from the tower when the evasive descent from 700 ft to 400 ft while IMC in the cloud. Duration of time in the cloud was about 30 seconds according to reporter.

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Original NASA ASRS Text

Title: NMAC RPTED WHEN 2 ACFT MAKE APCHS TO THE SAME RWY WITH ONE ON AN ILS AND THE OTHER ON A VISUAL APCH. MISCOM OF CLRNCS SEEMS TO BE THE PROB. CASE OF SIMILAR CALL SIGNS. UNKNOWN IF ATC OR FLC FAULT. TCASII EVASIVE ACTION TAKEN. LTSS.

Narrative: WE WERE MAKING AN ILS APCH TO RWY 12R AT STL. AT THE OM WE WERE CLRED TO LAND. FROM THE OM, WE COULD SEE THE RWY BUT THERE WAS A LOW CLOUD ON THE APCH WHICH PREVENTED US FROM MAINTAINING VISUAL CONTACT WITH THE RWY. WHEN WE ENTERED THE CLOUD, WE RECEIVED A TCASII TFC ALERT SHOWING A PLANE DIRECTLY OVER US, 700 FT HIGHER. BECAUSE THERE ARE A LOT OF TCASII WARNINGS IN THAT AREA, I CONFIRMED WITH THE FO THAT THE OTHER ACFT WAS ACTUALLY ABOVE US AND NOT BELOW AS IS THE USUAL CASE IN THIS AREA. WE WERE STILL IN THE CLOUD WHEN THE OTHER ACFT STARTED A RAPID DSCNT AND THE TCASII SHOWED THE VERT SEPARATION DECREASING TO 400 FT. AT THAT POINT, I BEGAN AN EVASIVE MANEUVER BY INCREASING MY DSCNT RATE AND FLYING NEARLY 2 DOTS BELOW THE GLIDE PATH. WHEN WE BROKE OUT OF THE CLOUDS, THE OTHER PLANE WAS DIRECTLY ABOVE US AND THE TCASII SHOWED 300 FT SEPARATION. THE FO TOLD THE TWR THAT A PLANE WAS ON TOP OF US. THE TWR THEN CALLED THE OTHER PLANE AND ASKED HIM TO CONFIRM THAT HE WAS MAKING A VISUAL APCH TO RWY 12L. FROM THE SOUND OF HIS REPLY IT WAS CLR THAT HE THOUGHT HE WAS CLRED FOR A VISUAL APCH TO RWY 12R. THE TWR THEN TOLD THE OTHER PLANE TO MAKE A VISUAL APCH TO RWY 12L AND CONTACT THE OTHER TWR FREQ (120.05). I BELIEVE THAT THE TWR MISTAKENLY CLRED THE OTHER PLANE FOR A VISUAL APCH TO RWY 12R AND THAT WE WOULD HAVE COLLIDED HAD I NOT TAKEN EVASIVE ACTION. THE FACT THAT THE OTHER PLANE WAS ON THE FREQ FOR RWY 12R (118.5) LEADS ME TO BELIEVE THAT THIS NEAR MIDAIR COLLISION WAS CAUSED BY THE TWR. HIS CALL SIGN SOUNDED SIMILAR TO OURS WHICH MAY HAVE LED TO THE CONFUSION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THE OTHER ACFT WAS A JETSTREAM ON AN LDA APCH TO RWY 13. RPTR INDICATED HIS COMPANY RECEIVED A COPY OF THE RADAR PRINTOUT PLOTTING THE TRACKS OF BOTH ACFT. RPTR ALLEGES THE PLOT OF THE JETSTREAM DOES NOT SHOW THE ACFT GOING TO HIS R BEFORE TURNING TOWARD RWY 13. RPTR STATED THAT WHEN HE SAW THE ACFT, IT WAS ON ABOUT A 30 DEG DIVERGENT HEADING AWAY FROM HIS ACFT. RPTR STATED THEY HAD NOT BEEN INFORMED OF THE JETSTREAM AND INDICATED THE FACILITY IS NOT TREATING THIS AS A LOSS OF SEPARATION. RPTR STATED THEY DID NOT GET A LOW ALT ALERT FROM THE TWR WHEN THE EVASIVE DSCNT FROM 700 FT TO 400 FT WHILE IMC IN THE CLOUD. DURATION OF TIME IN THE CLOUD WAS ABOUT 30 SECONDS ACCORDING TO RPTR.

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.