Narrative:

Initial clearance: seattle 3, maintain 9000 ft. We were #3 in line on taxiway B behind 2 aircraft. While the other 2 aircraft were holding short of runway 16L, we were told to taxi across runway 16L and cleared for takeoff on runway 16R. We were told that we could expect an early right turn and that the other traffic on runway 16L would be straight out. We conducted a normal rolling takeoff on runway 16R. At approximately 1000 ft MSL, we received a TCASII RA of 'descend, descend, descend.' I pushed the nose over leveling the aircraft while looking for the conflicting traffic. The first and only time I saw the other aircraft was at this point. He was about 400 ft to our left and 75 ft below us (8 O'clock low position). Concentrating on flying the jet, I lost sight of aircraft (behind and below us). At approximately 3 mi on departure, we were switched over to departure control, where we were asked if we had visual contact with aircraft Y. We replied 'negative.' when aircraft Y checked in with departure, they were asked the same question. They also replied 'negative.' observation of the aircraft altitude reporting showed aircraft Y directly above us with 400 ft of separation. Departure control issued us a right turn, while aircraft Y continued straight ahead.

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Original NASA ASRS Text

Title: A B737 FLT CREW RECEIVE A TCASII RA FOR AN NMAC JUST AFTER DEPARTING SEA WITH IFR TFC DEPARTING OFF PARALLEL RWY.

Narrative: INITIAL CLRNC: SEATTLE 3, MAINTAIN 9000 FT. WE WERE #3 IN LINE ON TXWY B BEHIND 2 ACFT. WHILE THE OTHER 2 ACFT WERE HOLDING SHORT OF RWY 16L, WE WERE TOLD TO TAXI ACROSS RWY 16L AND CLRED FOR TKOF ON RWY 16R. WE WERE TOLD THAT WE COULD EXPECT AN EARLY R TURN AND THAT THE OTHER TFC ON RWY 16L WOULD BE STRAIGHT OUT. WE CONDUCTED A NORMAL ROLLING TKOF ON RWY 16R. AT APPROX 1000 FT MSL, WE RECEIVED A TCASII RA OF 'DSND, DSND, DSND.' I PUSHED THE NOSE OVER LEVELING THE ACFT WHILE LOOKING FOR THE CONFLICTING TFC. THE FIRST AND ONLY TIME I SAW THE OTHER ACFT WAS AT THIS POINT. HE WAS ABOUT 400 FT TO OUR L AND 75 FT BELOW US (8 O'CLOCK LOW POS). CONCENTRATING ON FLYING THE JET, I LOST SIGHT OF ACFT (BEHIND AND BELOW US). AT APPROX 3 MI ON DEP, WE WERE SWITCHED OVER TO DEP CTL, WHERE WE WERE ASKED IF WE HAD VISUAL CONTACT WITH ACFT Y. WE REPLIED 'NEGATIVE.' WHEN ACFT Y CHKED IN WITH DEP, THEY WERE ASKED THE SAME QUESTION. THEY ALSO REPLIED 'NEGATIVE.' OBSERVATION OF THE ACFT ALT RPTING SHOWED ACFT Y DIRECTLY ABOVE US WITH 400 FT OF SEPARATION. DEP CTL ISSUED US A R TURN, WHILE ACFT Y CONTINUED STRAIGHT AHEAD.

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.