Narrative:

I was approaching from vashon island for a VFR landing. Bfi tower 120.6 instructed me to follow a high winged aircraft ahead and report the west seattle reservoir for left traffic 31L. The high winged aircraft ahead had been given the same instructions but drifted south in sea airspace. I followed the instructions and reported over the reservoir. I told tower I was over the reservoir and that the other high winged aircraft was way off to my right. Bfi tower 120.6 changed my clearance to cross over mid-field and join right downwind for 31R. Bfi tower 120.6 did not switch me to bfi tower 118.3 nor did he warn me that there was another airplane in the 31R downwind. I crossed over mid-field and turned downwind and it was a good thing I did because the second aircraft missed me by 300 ft off my nose as I did that. I am convinced that if I had not turned at that point; we would have collided. I never saw the second aircraft before it flashed by from left to right. After the collision had been avoided; bfi tower 120.6 called to say 'traffic ahead and to your right; follow him on the downwind'. The second was on my left but also the traffic call was 20 seconds late to avoid an accident. It was only luck that I chose to turn downwind when I did that avoided a mid-air collision. I believe the cause of this near midair collision was the bfi tower 120.6 controller becoming distracted dealing with the other high winged aircraft in sea's airspace and forgetting that he had cleared me across the airport into the path of another airplane. I had no heard the other traffic on 31R downwind because they were correctly on 118.3. I had glanced across the airport for traffic in the downwind but had not looked far enough left to see the second aircraft.

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Original NASA ASRS Text

Title: A pilot on a right downwind to BFI Runway 31R reported a near miss after the Tower Controller apparently lost track of a third aircraft in the pattern for 31R and so failed to point out that aircraft.

Narrative: I was approaching from Vashon Island for a VFR landing. BFI Tower 120.6 instructed me to follow a high winged aircraft ahead and report the West Seattle Reservoir for left traffic 31L. The high winged aircraft ahead had been given the same instructions but drifted south in SEA airspace. I followed the instructions and reported over the reservoir. I told Tower I was over the reservoir and that the other high winged aircraft was way off to my right. BFI Tower 120.6 changed my clearance to cross over mid-field and join right downwind for 31R. BFI Tower 120.6 did not switch me to BFI Tower 118.3 nor did he warn me that there was another airplane in the 31R downwind. I crossed over mid-field and turned downwind and it was a good thing I did because the second aircraft missed me by 300 FT off my nose as I did that. I am convinced that if I had not turned at that point; we would have collided. I never saw the second aircraft before it flashed by from left to right. After the collision had been avoided; BFI Tower 120.6 called to say 'traffic ahead and to your right; follow him on the downwind'. The second was on my left but also the traffic call was 20 seconds late to avoid an accident. It was only luck that I chose to turn downwind when I did that avoided a mid-air collision. I believe the cause of this NMAC was the BFI Tower 120.6 Controller becoming distracted dealing with the other high winged aircraft in SEA's airspace and forgetting that he had cleared me across the airport into the path of another airplane. I had no heard the other traffic on 31R downwind because they were correctly on 118.3. I had glanced across the airport for traffic in the downwind but had not looked far enough left to see the second aircraft.

Data retrieved from NASA's ASRS site as of July 2013 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.