Narrative:

We requested and were told to taxi to runway 26. We requested runway 21 (surface winds were suitable for either runway), and ground instructed us to taxi to runway 21. During taxi out, ground amended our initial altitude from 10000 ft to 7000 ft. Tower cleared us to take off, instructing us to maintain runway heading. After takeoff (first officer flying), we followed normal clean-up procedures. Immediately after completing flap retraction, tower called out traffic to our right at 7000 ft. I looked out the first officer's side window (R2) and picked up the traffic. I first directed the first officer to level off, then quickly directed him to start a descent, which he did. Very quickly after starting the evasive maneuver, the TCASII generated an RA ('descend'), which we had essentially already started, and we complied with the RA guidance. As we started the descent, I lost sight of the traffic above and behind our aircraft, then regained sight out to the left side of the aircraft. I do not know if the other aircraft took any evasive action -- no radios calls were made by the other aircraft to that effect. Immediately after this, tower directed us to contact departure. When I checked in with departure, I asked departure to confirm that the twin was at 7000 ft (with the implied question of 'why?'). Departure said something indicating there had been a mix-up between tower and departure ('we thought you were taking off on runway 26'). Point of minimum separation occurred above and behind our aircraft and out of my sight. I don't believe we were ever within 500 ft of each other, so I wouldn't classify this as a near midair collision. However, with the caveat that my perceptions are probably affected by the heat of the moment, I would estimate the minimum altitude separation at about 500 ft and minimum horizontal separation at 1000 ft, with about a 30 degree heading crossing angle. In addition, our minimum separation would likely have been less without the evasive action taken. Because of the TCASII's 1200 ft climbing inhibit point for RA's, the TCASII RA was probably delayed in this situation. As a result, I'm not sure how effective the RA would have been in this situation if we had not already started the evasive maneuver. While this situation is obviously somewhat unique, we may want to re-evaluate the way the TCASII is programmed to handle these kinds of sits.

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Original NASA ASRS Text

Title: CAPT OF A B737 DIRECTED THE PF TO LEVEL OFF TO AVOID A TWIN ENG ACFT HE HAD SIGHTED OUT THE PF FO'S WINDOW. A TCASII RA THEN WARNED TO DSND CAUSING ACFT TO PASS WITHIN 500-1000 FT OF EACH OTHER.

Narrative: WE REQUESTED AND WERE TOLD TO TAXI TO RWY 26. WE REQUESTED RWY 21 (SURFACE WINDS WERE SUITABLE FOR EITHER RWY), AND GND INSTRUCTED US TO TAXI TO RWY 21. DURING TAXI OUT, GND AMENDED OUR INITIAL ALT FROM 10000 FT TO 7000 FT. TWR CLRED US TO TAKE OFF, INSTRUCTING US TO MAINTAIN RWY HDG. AFTER TKOF (FO FLYING), WE FOLLOWED NORMAL CLEAN-UP PROCS. IMMEDIATELY AFTER COMPLETING FLAP RETRACTION, TWR CALLED OUT TFC TO OUR R AT 7000 FT. I LOOKED OUT THE FO'S SIDE WINDOW (R2) AND PICKED UP THE TFC. I FIRST DIRECTED THE FO TO LEVEL OFF, THEN QUICKLY DIRECTED HIM TO START A DSCNT, WHICH HE DID. VERY QUICKLY AFTER STARTING THE EVASIVE MANEUVER, THE TCASII GENERATED AN RA ('DSND'), WHICH WE HAD ESSENTIALLY ALREADY STARTED, AND WE COMPLIED WITH THE RA GUIDANCE. AS WE STARTED THE DSCNT, I LOST SIGHT OF THE TFC ABOVE AND BEHIND OUR ACFT, THEN REGAINED SIGHT OUT TO THE L SIDE OF THE ACFT. I DO NOT KNOW IF THE OTHER ACFT TOOK ANY EVASIVE ACTION -- NO RADIOS CALLS WERE MADE BY THE OTHER ACFT TO THAT EFFECT. IMMEDIATELY AFTER THIS, TWR DIRECTED US TO CONTACT DEP. WHEN I CHKED IN WITH DEP, I ASKED DEP TO CONFIRM THAT THE TWIN WAS AT 7000 FT (WITH THE IMPLIED QUESTION OF 'WHY?'). DEP SAID SOMETHING INDICATING THERE HAD BEEN A MIX-UP BTWN TWR AND DEP ('WE THOUGHT YOU WERE TAKING OFF ON RWY 26'). POINT OF MINIMUM SEPARATION OCCURRED ABOVE AND BEHIND OUR ACFT AND OUT OF MY SIGHT. I DON'T BELIEVE WE WERE EVER WITHIN 500 FT OF EACH OTHER, SO I WOULDN'T CLASSIFY THIS AS A NMAC. HOWEVER, WITH THE CAVEAT THAT MY PERCEPTIONS ARE PROBABLY AFFECTED BY THE HEAT OF THE MOMENT, I WOULD ESTIMATE THE MINIMUM ALT SEPARATION AT ABOUT 500 FT AND MINIMUM HORIZ SEPARATION AT 1000 FT, WITH ABOUT A 30 DEG HDG XING ANGLE. IN ADDITION, OUR MINIMUM SEPARATION WOULD LIKELY HAVE BEEN LESS WITHOUT THE EVASIVE ACTION TAKEN. BECAUSE OF THE TCASII'S 1200 FT CLBING INHIBIT POINT FOR RA'S, THE TCASII RA WAS PROBABLY DELAYED IN THIS SIT. AS A RESULT, I'M NOT SURE HOW EFFECTIVE THE RA WOULD HAVE BEEN IN THIS SIT IF WE HAD NOT ALREADY STARTED THE EVASIVE MANEUVER. WHILE THIS SIT IS OBVIOUSLY SOMEWHAT UNIQUE, WE MAY WANT TO RE-EVALUATE THE WAY THE TCASII IS PROGRAMMED TO HANDLE THESE KINDS OF SITS.

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.