Narrative:

On takeoff roll, tower requested an immediate hard right turn and best rate of climb to avoid conflict with another aircraft also turning right, having just departed on the parallel runway 31L. I was departing on runway 31R, behind the other aircraft. After lift-off, with the other departing aircraft in sight, I became unsure as to when to make my right turn, since a sign at the taxiway entrance to runway 31R specified no turns before reaching 600 ft, due to the high hill on the northeast side of the airport. I made best rate of climb to 600 ft and made an immediate right turn. However, shortly before I reached 600 ft the bfi tower told the other aircraft that I had him in sight, I would be turning immediately after departure and that he could start his right turn. The other aircraft's turn 'early' combined with my turn only after reaching 600 ft caused us to converge rapidly. I was overtaking and converging and chose to fly behind and to the left of the other aircraft. As we converged, I stayed below the other aircraft (a high wing cessna) so that we could see one another. I thought we were going to clear by a wide margin as I overtook and as the other aircraft continued his right turn. But the other aircraft stopped turning right, may have turned slightly left as I began to pass on his left. I altered course further left to widen our margin of clearance, but my correction was inadequate to widen our separation adequately (in light of what I believe was his continuing left turn, probably brought on by not knowing my exact position below and behind him). As we came parallel to one another, we both executed turns in opposite directions, him right, me left to keep clear. Prior to executing my decision on climb out to stay left (behind) and below the other departing aircraft, I broadcast on the tower frequency that I would turn north (and left) of the other departing aircraft as he continued his right turn. This would keep me behind the other aircraft. I received no response from the tower or the other aircraft. At no time was the other aircraft out of my sight. We kept the other aircraft in sight until well clear ahead before climbing above 1200 ft MSL. In retrospect, I should have turned prior to reaching 600 ft, or at least asked the tower frequency for clarification of its instructions in that regard. In addition, I probably should have tightened my turn to the right instead of choosing to proceed behind the other aircraft as I overtook it, this would have kept us clrer of one another. However, turning into what appeared to be the path of the other r-turning aircraft was repugnant to me at the time. In addition, by proceeding further right, I feared at the time, I might find myself in conflict with the renton airport control zone in short order.

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Original NASA ASRS Text

Title: NMAC IN GA TFC PATTERN.

Narrative: ON TKOF ROLL, TWR REQUESTED AN IMMEDIATE HARD R TURN AND BEST RATE OF CLB TO AVOID CONFLICT WITH ANOTHER ACFT ALSO TURNING R, HAVING JUST DEPARTED ON THE PARALLEL RWY 31L. I WAS DEPARTING ON RWY 31R, BEHIND THE OTHER ACFT. AFTER LIFT-OFF, WITH THE OTHER DEPARTING ACFT IN SIGHT, I BECAME UNSURE AS TO WHEN TO MAKE MY R TURN, SINCE A SIGN AT THE TXWY ENTRANCE TO RWY 31R SPECIFIED NO TURNS BEFORE REACHING 600 FT, DUE TO THE HIGH HILL ON THE NE SIDE OF THE ARPT. I MADE BEST RATE OF CLB TO 600 FT AND MADE AN IMMEDIATE R TURN. HOWEVER, SHORTLY BEFORE I REACHED 600 FT THE BFI TWR TOLD THE OTHER ACFT THAT I HAD HIM IN SIGHT, I WOULD BE TURNING IMMEDIATELY AFTER DEP AND THAT HE COULD START HIS R TURN. THE OTHER ACFT'S TURN 'EARLY' COMBINED WITH MY TURN ONLY AFTER REACHING 600 FT CAUSED US TO CONVERGE RAPIDLY. I WAS OVERTAKING AND CONVERGING AND CHOSE TO FLY BEHIND AND TO THE L OF THE OTHER ACFT. AS WE CONVERGED, I STAYED BELOW THE OTHER ACFT (A HIGH WING CESSNA) SO THAT WE COULD SEE ONE ANOTHER. I THOUGHT WE WERE GOING TO CLR BY A WIDE MARGIN AS I OVERTOOK AND AS THE OTHER ACFT CONTINUED HIS R TURN. BUT THE OTHER ACFT STOPPED TURNING R, MAY HAVE TURNED SLIGHTLY L AS I BEGAN TO PASS ON HIS L. I ALTERED COURSE FURTHER L TO WIDEN OUR MARGIN OF CLRNC, BUT MY CORRECTION WAS INADEQUATE TO WIDEN OUR SEPARATION ADEQUATELY (IN LIGHT OF WHAT I BELIEVE WAS HIS CONTINUING L TURN, PROBABLY BROUGHT ON BY NOT KNOWING MY EXACT POS BELOW AND BEHIND HIM). AS WE CAME PARALLEL TO ONE ANOTHER, WE BOTH EXECUTED TURNS IN OPPOSITE DIRECTIONS, HIM R, ME L TO KEEP CLR. PRIOR TO EXECUTING MY DECISION ON CLB OUT TO STAY L (BEHIND) AND BELOW THE OTHER DEPARTING ACFT, I BROADCAST ON THE TWR FREQ THAT I WOULD TURN N (AND L) OF THE OTHER DEPARTING ACFT AS HE CONTINUED HIS R TURN. THIS WOULD KEEP ME BEHIND THE OTHER ACFT. I RECEIVED NO RESPONSE FROM THE TWR OR THE OTHER ACFT. AT NO TIME WAS THE OTHER ACFT OUT OF MY SIGHT. WE KEPT THE OTHER ACFT IN SIGHT UNTIL WELL CLR AHEAD BEFORE CLBING ABOVE 1200 FT MSL. IN RETROSPECT, I SHOULD HAVE TURNED PRIOR TO REACHING 600 FT, OR AT LEAST ASKED THE TWR FREQ FOR CLARIFICATION OF ITS INSTRUCTIONS IN THAT REGARD. IN ADDITION, I PROBABLY SHOULD HAVE TIGHTENED MY TURN TO THE R INSTEAD OF CHOOSING TO PROCEED BEHIND THE OTHER ACFT AS I OVERTOOK IT, THIS WOULD HAVE KEPT US CLRER OF ONE ANOTHER. HOWEVER, TURNING INTO WHAT APPEARED TO BE THE PATH OF THE OTHER R-TURNING ACFT WAS REPUGNANT TO ME AT THE TIME. IN ADDITION, BY PROCEEDING FURTHER R, I FEARED AT THE TIME, I MIGHT FIND MYSELF IN CONFLICT WITH THE RENTON ARPT CTL ZONE IN SHORT ORDER.

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.