Narrative:

I was captain flying. Overhead hobby, 210 degree heading at 6000 ft, we got garbled clearance to 3000 ft. My first officer requested altitude confirmation to 3000 ft, which was confirmed. 30 seconds later, heading to 260 degrees, received and complied with. Due to potential for quick visual, speed brakes up, rapid descent. Approaching 3500 ft, speed brakes down to level off when TCASII alert. 10 mi scale on deu showed 2 O'clock position traffic a few hundred ft below us and close. 5 mi range scale indicated inside 1 mi. More aggressive leveloff started when approach queried us about 'climbing to 4000 ft' with traffic only -200 ft below and closing on constant bearing at 2 O'clock position. Then TCASII RA to 'descend' at 1800 FPM. Reversed pull up to a push over to comply, and, with 'new' procedures, maintained heading (with great trepidation). Told first officer to look at 2 O'clock position for visual. Looking like less than half mi, merge plot on the deu, and now co-altitude, I thought to myself, 'this is not right, we are going to hit unless I turn away.' so, without a visual, rolled rapidly left to 30 degree angle of bank and 'pulled' a little more aggressively to generate lateral separation. Just then, the TCASII reversed its command to 'climb' (probably because of seeing my initial leveloff attempts at about 3200 ft). Rolled wings level and now pulled aggressively up to comply. First officer then picked up high wing, single engine (cessna type) aircraft just aft of our right wing, level, at about 1000 ft range. It is my opinion that aggressively turning away, even without the visual (per fom), prevented a very close miss or midair. Recommendation: our 'new' TCASII procedure states 'do not deviate from existing lateral flight path unless visual contact with the conflicting traffic requires other action.' with the target getting ever closer (seemingly almost merge plot on the deu) at 2-3 O'clock position, and now approximately co-altitude, I genuinely feared collision. Old fighter experience and common sense told me to turn away now! Contrary to our written TCASII procedure, I especially felt the need to turn away from the threat precisely because we had not visually acquired the traffic yet, yet I knew where the threat sector was on the display! I rapidly rolled away at 30 degrees left angle of bank, with a little extra 'pull' added for turn rate, as we were descending. Constant bearing, decreasing range on radar/TCASII contacts always mean 'collision' courses. That is exactly what I was seeing on my 5 mi range display, and 'collision' is exactly what I feared was imminent if I did not turn away, especially without a visual on the traffic and especially since our altitude difference was now zero. Turning away will always enhance lateral separation. The odds of traffic being where you are turning to are the same as straight ahead (probably less if on an airway). Take these handcuffs off. Supplement information from acn 587277: level at 6000 ft we were instructed to descend and maintain 3000 ft. The controller was stepped on but I could make out the instructions. Still, I decided to have the controller verify the altitude just in case and he repeated that we are cleared to 3000 ft. As we were leveling off 3000 ft approach called us and asked if we were climbing to 4000 ft. I responded, 'no we are leveling at 3000 ft as instructed.' I was getting worried since we had lost time stuck at 3000 ft going up and down, and I had my head glued to my window trying to get a visual on this aircraft. Shortly after we started a climb, I then spotted the cessna type single engine high wing aircraft off our right wingtip. I called out the traffic to my captain as he made a left break away maneuver. We leveled off at 4000 ft and received vectors for the approach to runway 12R and landed with no additional problems. I requested a contact number and the captain called on the ground. The controller first blamed us for descending to 3000 ft. It was not until we departed hub for mdw, we where instructed to call ATC again upon landing in mdw. At that point, ATC admitted having made a mistake by clearing us to 3000 ft.

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Original NASA ASRS Text

Title: CRITICAL AIRBORNE CONFLICT OCCURS AFTER A B737-700 FLT DSNDS TO AN ALT OCCUPIED BY XING TFC, A CESSNA TYPE ACFT AT 3000 FT 6 MI SW OF HOUSTON, TX.

Narrative: I WAS CAPT FLYING. OVERHEAD HOBBY, 210 DEG HDG AT 6000 FT, WE GOT GARBLED CLRNC TO 3000 FT. MY FO REQUESTED ALT CONFIRMATION TO 3000 FT, WHICH WAS CONFIRMED. 30 SECONDS LATER, HDG TO 260 DEGS, RECEIVED AND COMPLIED WITH. DUE TO POTENTIAL FOR QUICK VISUAL, SPD BRAKES UP, RAPID DSCNT. APCHING 3500 FT, SPD BRAKES DOWN TO LEVEL OFF WHEN TCASII ALERT. 10 MI SCALE ON DEU SHOWED 2 O'CLOCK POS TFC A FEW HUNDRED FT BELOW US AND CLOSE. 5 MI RANGE SCALE INDICATED INSIDE 1 MI. MORE AGGRESSIVE LEVELOFF STARTED WHEN APCH QUERIED US ABOUT 'CLBING TO 4000 FT' WITH TFC ONLY -200 FT BELOW AND CLOSING ON CONSTANT BEARING AT 2 O'CLOCK POS. THEN TCASII RA TO 'DSND' AT 1800 FPM. REVERSED PULL UP TO A PUSH OVER TO COMPLY, AND, WITH 'NEW' PROCS, MAINTAINED HDG (WITH GREAT TREPIDATION). TOLD FO TO LOOK AT 2 O'CLOCK POS FOR VISUAL. LOOKING LIKE LESS THAN HALF MI, MERGE PLOT ON THE DEU, AND NOW CO-ALT, I THOUGHT TO MYSELF, 'THIS IS NOT RIGHT, WE ARE GOING TO HIT UNLESS I TURN AWAY.' SO, WITHOUT A VISUAL, ROLLED RAPIDLY L TO 30 DEG ANGLE OF BANK AND 'PULLED' A LITTLE MORE AGGRESSIVELY TO GENERATE LATERAL SEPARATION. JUST THEN, THE TCASII REVERSED ITS COMMAND TO 'CLB' (PROBABLY BECAUSE OF SEEING MY INITIAL LEVELOFF ATTEMPTS AT ABOUT 3200 FT). ROLLED WINGS LEVEL AND NOW PULLED AGGRESSIVELY UP TO COMPLY. FO THEN PICKED UP HIGH WING, SINGLE ENG (CESSNA TYPE) ACFT JUST AFT OF OUR R WING, LEVEL, AT ABOUT 1000 FT RANGE. IT IS MY OPINION THAT AGGRESSIVELY TURNING AWAY, EVEN WITHOUT THE VISUAL (PER FOM), PREVENTED A VERY CLOSE MISS OR MIDAIR. RECOMMENDATION: OUR 'NEW' TCASII PROC STATES 'DO NOT DEVIATE FROM EXISTING LATERAL FLT PATH UNLESS VISUAL CONTACT WITH THE CONFLICTING TFC REQUIRES OTHER ACTION.' WITH THE TARGET GETTING EVER CLOSER (SEEMINGLY ALMOST MERGE PLOT ON THE DEU) AT 2-3 O'CLOCK POS, AND NOW APPROX CO-ALT, I GENUINELY FEARED COLLISION. OLD FIGHTER EXPERIENCE AND COMMON SENSE TOLD ME TO TURN AWAY NOW! CONTRARY TO OUR WRITTEN TCASII PROC, I ESPECIALLY FELT THE NEED TO TURN AWAY FROM THE THREAT PRECISELY BECAUSE WE HAD NOT VISUALLY ACQUIRED THE TFC YET, YET I KNEW WHERE THE THREAT SECTOR WAS ON THE DISPLAY! I RAPIDLY ROLLED AWAY AT 30 DEGS L ANGLE OF BANK, WITH A LITTLE EXTRA 'PULL' ADDED FOR TURN RATE, AS WE WERE DSNDING. CONSTANT BEARING, DECREASING RANGE ON RADAR/TCASII CONTACTS ALWAYS MEAN 'COLLISION' COURSES. THAT IS EXACTLY WHAT I WAS SEEING ON MY 5 MI RANGE DISPLAY, AND 'COLLISION' IS EXACTLY WHAT I FEARED WAS IMMINENT IF I DID NOT TURN AWAY, ESPECIALLY WITHOUT A VISUAL ON THE TFC AND ESPECIALLY SINCE OUR ALT DIFFERENCE WAS NOW ZERO. TURNING AWAY WILL ALWAYS ENHANCE LATERAL SEPARATION. THE ODDS OF TFC BEING WHERE YOU ARE TURNING TO ARE THE SAME AS STRAIGHT AHEAD (PROBABLY LESS IF ON AN AIRWAY). TAKE THESE HANDCUFFS OFF. SUPPLEMENT INFO FROM ACN 587277: LEVEL AT 6000 FT WE WERE INSTRUCTED TO DSND AND MAINTAIN 3000 FT. THE CTLR WAS STEPPED ON BUT I COULD MAKE OUT THE INSTRUCTIONS. STILL, I DECIDED TO HAVE THE CTLR VERIFY THE ALT JUST IN CASE AND HE REPEATED THAT WE ARE CLRED TO 3000 FT. AS WE WERE LEVELING OFF 3000 FT APCH CALLED US AND ASKED IF WE WERE CLBING TO 4000 FT. I RESPONDED, 'NO WE ARE LEVELING AT 3000 FT AS INSTRUCTED.' I WAS GETTING WORRIED SINCE WE HAD LOST TIME STUCK AT 3000 FT GOING UP AND DOWN, AND I HAD MY HEAD GLUED TO MY WINDOW TRYING TO GET A VISUAL ON THIS ACFT. SHORTLY AFTER WE STARTED A CLB, I THEN SPOTTED THE CESSNA TYPE SINGLE ENG HIGH WING ACFT OFF OUR R WINGTIP. I CALLED OUT THE TFC TO MY CAPT AS HE MADE A L BREAK AWAY MANEUVER. WE LEVELED OFF AT 4000 FT AND RECEIVED VECTORS FOR THE APCH TO RWY 12R AND LANDED WITH NO ADDITIONAL PROBS. I REQUESTED A CONTACT NUMBER AND THE CAPT CALLED ON THE GND. THE CTLR FIRST BLAMED US FOR DSNDING TO 3000 FT. IT WAS NOT UNTIL WE DEPARTED HUB FOR MDW, WE WHERE INSTRUCTED TO CALL ATC AGAIN UPON LNDG IN MDW. AT THAT POINT, ATC ADMITTED HAVING MADE A MISTAKE BY CLRING US TO 3000 FT.

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.