Narrative:

I was working local control to 1 of 2 parallel runways. I had 5 single engine aircraft in the pattern all doing touch and go's. The first time around in the pattern I pointed everyone out to everyone and issued the pattern sequence. The second time around, I only issued a sequence with landing clearance. The third trip in the pattern I again issued the sequence with landing clearance. After clearing a departure for on course, I glanced out on final and observed 2 smas stacked on top of each other with about 50 ft vertical separation. I know one had been cleared for the touch and go and I wasn't sure of the call sign of the other (I had 3 smas in the pattern). I issued a go around to small aircraft X then made a blanket broadcast that there were 2 smas on final and that I didn't know the call signs but were stacked on top of each other. Both aircraft started to go around. Small aircraft X started a slow climb. Small aircraft Y saw his traffic below him and took evasive action. From my viewpoint there was 10 to 20 ft between aircraft. In hindsight, I should have issued an immediate turn to the 1 aircraft call sign that I knew and an opposite turn to the aircraft call sign which I suspected. Everyone at my facility issues the pattern sequence diligently, but, if there are no changes to the sequence, we expect the pilots to remember who they are following! I plan to include this in our next rainchk program and to talk about it in our controller FBO liaison program.

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Original NASA ASRS Text

Title: SMA X HAD NMAC WITH SMA Y ON FINAL APCH TO ARPT. SEE AND AVOID CONCEPT.

Narrative: I WAS WORKING LCL CTL TO 1 OF 2 PARALLEL RWYS. I HAD 5 SINGLE ENG ACFT IN THE PATTERN ALL DOING TOUCH AND GO'S. THE FIRST TIME AROUND IN THE PATTERN I POINTED EVERYONE OUT TO EVERYONE AND ISSUED THE PATTERN SEQUENCE. THE SECOND TIME AROUND, I ONLY ISSUED A SEQUENCE WITH LNDG CLRNC. THE THIRD TRIP IN THE PATTERN I AGAIN ISSUED THE SEQUENCE WITH LNDG CLRNC. AFTER CLRING A DEP FOR ON COURSE, I GLANCED OUT ON FINAL AND OBSERVED 2 SMAS STACKED ON TOP OF EACH OTHER WITH ABOUT 50 FT VERT SEPARATION. I KNOW ONE HAD BEEN CLRED FOR THE TOUCH AND GO AND I WASN'T SURE OF THE CALL SIGN OF THE OTHER (I HAD 3 SMAS IN THE PATTERN). I ISSUED A GAR TO SMA X THEN MADE A BLANKET BROADCAST THAT THERE WERE 2 SMAS ON FINAL AND THAT I DIDN'T KNOW THE CALL SIGNS BUT WERE STACKED ON TOP OF EACH OTHER. BOTH ACFT STARTED TO GAR. SMA X STARTED A SLOW CLB. SMA Y SAW HIS TFC BELOW HIM AND TOOK EVASIVE ACTION. FROM MY VIEWPOINT THERE WAS 10 TO 20 FT BTWN ACFT. IN HINDSIGHT, I SHOULD HAVE ISSUED AN IMMEDIATE TURN TO THE 1 ACFT CALL SIGN THAT I KNEW AND AN OPPOSITE TURN TO THE ACFT CALL SIGN WHICH I SUSPECTED. EVERYONE AT MY FACILITY ISSUES THE PATTERN SEQUENCE DILIGENTLY, BUT, IF THERE ARE NO CHANGES TO THE SEQUENCE, WE EXPECT THE PLTS TO REMEMBER WHO THEY ARE FOLLOWING! I PLAN TO INCLUDE THIS IN OUR NEXT RAINCHK PROGRAM AND TO TALK ABOUT IT IN OUR CTLR FBO LIAISON PROGRAM.

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.