Narrative:

Air carrier X had reached 5 DME south of the primary and I issued a left turn to 020 degrees and he complied. I then told air carrier Y to climb to 5000 ft. Here is where the incident began. Y and X were now on converging courses with Y climbing to 5000 ft and X level at 5000 ft. Continuing to work traffic, I cleared the C-172 to 3000 ft, the DH-8 was reclred to 15000 ft as the pilot was not sure of his assigned altitude, the PA-28 was finally cleared to a lower altitude, and the be-35 reported continuing his descent below class B. I then issued traffic to X -- 11 O'clock, 3 mi eastbound at 4000 ft, an SW3. I also issued traffic to Y -- 2 O'clock, 3 mi, northbound, a B-737. I had made 1 more transmission turning the C-172 on course when Y requested confirmation of assigned altitude 5000 ft. I then noticed Y was climbing out of 4200 ft. I immediately told Y, 'negative, maintain 4000.' I then told X to climb to 6000 ft. X acknowledged leaving 5600 for 6000 ft. I could hear his TCASII giving X a climb RA. The 2 aircraft were now increasing separation as their targets merged. I continued to work traffic without further incident until I was relieved from the position a couple of mins later. Looking back at the incident 2 things come to mind. First, I was sure that I had assigned Y 4000 ft, not 5000 ft. I was planning a crossover with X. I knew X was at 5000 ft, therefore, any aircraft crossing underneath would have to be at or below 4000 ft. All this time, I was thinking 4000 ft when I had actually issued 5000 ft to Y. I think that part of effective listening is listening to what you are saying. It's not enough to make sure pilots give a correct readback. You have to listen to yourself and make sure you're saying what you want to say. Second, TCASII works! I think it was the TCASII which prompted his confirming the assigned altitude. Certainly, by the time I issued a climb, he was already responding to an RA. I believe TCASII prevented a very bad situation from becoming the worst possible situation.

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Original NASA ASRS Text

Title: ACR Y CLB TO OCCUPIED ALT HAD LTSS FROM ACR X TCASII RA EVASIVE ACTION TAKEN. SYS ERROR.

Narrative: ACR X HAD REACHED 5 DME S OF THE PRIMARY AND I ISSUED A L TURN TO 020 DEGS AND HE COMPLIED. I THEN TOLD ACR Y TO CLB TO 5000 FT. HERE IS WHERE THE INCIDENT BEGAN. Y AND X WERE NOW ON CONVERGING COURSES WITH Y CLBING TO 5000 FT AND X LEVEL AT 5000 FT. CONTINUING TO WORK TFC, I CLRED THE C-172 TO 3000 FT, THE DH-8 WAS RECLRED TO 15000 FT AS THE PLT WAS NOT SURE OF HIS ASSIGNED ALT, THE PA-28 WAS FINALLY CLRED TO A LOWER ALT, AND THE BE-35 RPTED CONTINUING HIS DSCNT BELOW CLASS B. I THEN ISSUED TFC TO X -- 11 O'CLOCK, 3 MI EBOUND AT 4000 FT, AN SW3. I ALSO ISSUED TFC TO Y -- 2 O'CLOCK, 3 MI, NBOUND, A B-737. I HAD MADE 1 MORE XMISSION TURNING THE C-172 ON COURSE WHEN Y REQUESTED CONFIRMATION OF ASSIGNED ALT 5000 FT. I THEN NOTICED Y WAS CLBING OUT OF 4200 FT. I IMMEDIATELY TOLD Y, 'NEGATIVE, MAINTAIN 4000.' I THEN TOLD X TO CLB TO 6000 FT. X ACKNOWLEDGED LEAVING 5600 FOR 6000 FT. I COULD HEAR HIS TCASII GIVING X A CLB RA. THE 2 ACFT WERE NOW INCREASING SEPARATION AS THEIR TARGETS MERGED. I CONTINUED TO WORK TFC WITHOUT FURTHER INCIDENT UNTIL I WAS RELIEVED FROM THE POS A COUPLE OF MINS LATER. LOOKING BACK AT THE INCIDENT 2 THINGS COME TO MIND. FIRST, I WAS SURE THAT I HAD ASSIGNED Y 4000 FT, NOT 5000 FT. I WAS PLANNING A CROSSOVER WITH X. I KNEW X WAS AT 5000 FT, THEREFORE, ANY ACFT XING UNDERNEATH WOULD HAVE TO BE AT OR BELOW 4000 FT. ALL THIS TIME, I WAS THINKING 4000 FT WHEN I HAD ACTUALLY ISSUED 5000 FT TO Y. I THINK THAT PART OF EFFECTIVE LISTENING IS LISTENING TO WHAT YOU ARE SAYING. IT'S NOT ENOUGH TO MAKE SURE PLTS GIVE A CORRECT READBACK. YOU HAVE TO LISTEN TO YOURSELF AND MAKE SURE YOU'RE SAYING WHAT YOU WANT TO SAY. SECOND, TCASII WORKS! I THINK IT WAS THE TCASII WHICH PROMPTED HIS CONFIRMING THE ASSIGNED ALT. CERTAINLY, BY THE TIME I ISSUED A CLB, HE WAS ALREADY RESPONDING TO AN RA. I BELIEVE TCASII PREVENTED A VERY BAD SIT FROM BECOMING THE WORST POSSIBLE SIT.

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.