Narrative:

As pilot of trinidad TB21, I was cleared for a downwind departure from bfi, runway 13R. After turning downwind following takeoff, I encountered what I assumed to be wake turbulence from a twin engine heavy on final to sea-tac, which I could see to my port, at or below my altitude approximately 1/2 mi to the south. My altitude at the time of the wake turbulence encounter was approximately 1000 ft. My aircraft rolled to starboard, and experienced a lifting action which brought my altitude to 1100-1300 ft. I am unclr of the precise altitude inasmuch as my main concern was righting my aircraft and maintaining level flight. I corrected for the roll, and saw to my starboard a freighter jet on final to sea tac with another jet behind him a couple of mi. At the time I saw the freighter jet, it appeared to me that he and I were close to the same altitude. I spoke to bfi tower chief, and she informed me that they had heard from the freighter pilot who wanted to know if downwind departures from bfi when incoming heavies to sea tac are overflying bfi. Informed that this was normal, the freighter pilot indicated that he thought he was about 1400 ft, and that I was 1100-1200 ft. He took evasive action with a slight turn to port. I did not take any evasive action, in that in my judgement, none was required. In any case, my flight path was such that I was diverging from his as fast as was possible anyway. I am quite sure that I broke the 1100 ft class B airspace that overlies the south end of bfi during the wake turbulence encounter. On reflection, the following procedural changes should be considered by ATC at bfi. When visual approachs to sea tac from the north are active, downwind departures from runway 13R at bfi should not be authorized. I realize that it is always the PIC who must take responsibility for whether a particular departure is appropriate, but to pilots unfamiliar with how the traffic flow from an adjacent class B airspace affect operations from the airport they are landing or departing from, help from ATC may be vital for fully informed decisions. Since heavies inbound to sea tac are passing within 300 ft or so of downwind runway 13R traffic, wake turbulence encounters are unavoidable. I taxied from FBO at bfi, and was cleared for a mid-field departure from runway 13R. Such mid-field departures result in GA aircraft turning downwind at the place where overflying heavies inbound to sea tac are the closest to the tpa at bfi. While mid-field departures from runway 13R are the most convenient when you start out at FBO these coupled with downwind departures are dangerous. In the future, I will refuse such clrncs, and opt for the runway 13L downwind departure, which puts GA aircraft on a safe departure which avoids heavies inbound to sea tac. I received no wake turbulence warnings on my takeoff clearance from bfi. Such warnings should be SOP, as they may cause pilots unfamiliar with the dynamics of the interaction between the 2 airports (like me) to modify their departure request.

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Original NASA ASRS Text

Title: POTENTIAL CONFLICT IN PROX OF APCH RTE. INFLT WAKE TURB ENCOUNTER.

Narrative: AS PLT OF TRINIDAD TB21, I WAS CLRED FOR A DOWNWIND DEP FROM BFI, RWY 13R. AFTER TURNING DOWNWIND FOLLOWING TKOF, I ENCOUNTERED WHAT I ASSUMED TO BE WAKE TURB FROM A TWIN ENG HVY ON FINAL TO SEA-TAC, WHICH I COULD SEE TO MY PORT, AT OR BELOW MY ALT APPROX 1/2 MI TO THE S. MY ALT AT THE TIME OF THE WAKE TURB ENCOUNTER WAS APPROX 1000 FT. MY ACFT ROLLED TO STARBOARD, AND EXPERIENCED A LIFTING ACTION WHICH BROUGHT MY ALT TO 1100-1300 FT. I AM UNCLR OF THE PRECISE ALT INASMUCH AS MY MAIN CONCERN WAS RIGHTING MY ACFT AND MAINTAINING LEVEL FLT. I CORRECTED FOR THE ROLL, AND SAW TO MY STARBOARD A FREIGHTER JET ON FINAL TO SEA TAC WITH ANOTHER JET BEHIND HIM A COUPLE OF MI. AT THE TIME I SAW THE FREIGHTER JET, IT APPEARED TO ME THAT HE AND I WERE CLOSE TO THE SAME ALT. I SPOKE TO BFI TWR CHIEF, AND SHE INFORMED ME THAT THEY HAD HEARD FROM THE FREIGHTER PLT WHO WANTED TO KNOW IF DOWNWIND DEPS FROM BFI WHEN INCOMING HEAVIES TO SEA TAC ARE OVERFLYING BFI. INFORMED THAT THIS WAS NORMAL, THE FREIGHTER PLT INDICATED THAT HE THOUGHT HE WAS ABOUT 1400 FT, AND THAT I WAS 1100-1200 FT. HE TOOK EVASIVE ACTION WITH A SLIGHT TURN TO PORT. I DID NOT TAKE ANY EVASIVE ACTION, IN THAT IN MY JUDGEMENT, NONE WAS REQUIRED. IN ANY CASE, MY FLT PATH WAS SUCH THAT I WAS DIVERGING FROM HIS AS FAST AS WAS POSSIBLE ANYWAY. I AM QUITE SURE THAT I BROKE THE 1100 FT CLASS B AIRSPACE THAT OVERLIES THE S END OF BFI DURING THE WAKE TURB ENCOUNTER. ON REFLECTION, THE FOLLOWING PROCEDURAL CHANGES SHOULD BE CONSIDERED BY ATC AT BFI. WHEN VISUAL APCHS TO SEA TAC FROM THE N ARE ACTIVE, DOWNWIND DEPS FROM RWY 13R AT BFI SHOULD NOT BE AUTHORIZED. I REALIZE THAT IT IS ALWAYS THE PIC WHO MUST TAKE RESPONSIBILITY FOR WHETHER A PARTICULAR DEP IS APPROPRIATE, BUT TO PLTS UNFAMILIAR WITH HOW THE TFC FLOW FROM AN ADJACENT CLASS B AIRSPACE AFFECT OPS FROM THE ARPT THEY ARE LNDG OR DEPARTING FROM, HELP FROM ATC MAY BE VITAL FOR FULLY INFORMED DECISIONS. SINCE HEAVIES INBOUND TO SEA TAC ARE PASSING WITHIN 300 FT OR SO OF DOWNWIND RWY 13R TFC, WAKE TURB ENCOUNTERS ARE UNAVOIDABLE. I TAXIED FROM FBO AT BFI, AND WAS CLRED FOR A MID-FIELD DEP FROM RWY 13R. SUCH MID-FIELD DEPS RESULT IN GA ACFT TURNING DOWNWIND AT THE PLACE WHERE OVERFLYING HEAVIES INBOUND TO SEA TAC ARE THE CLOSEST TO THE TPA AT BFI. WHILE MID-FIELD DEPS FROM RWY 13R ARE THE MOST CONVENIENT WHEN YOU START OUT AT FBO THESE COUPLED WITH DOWNWIND DEPS ARE DANGEROUS. IN THE FUTURE, I WILL REFUSE SUCH CLRNCS, AND OPT FOR THE RWY 13L DOWNWIND DEP, WHICH PUTS GA ACFT ON A SAFE DEP WHICH AVOIDS HEAVIES INBOUND TO SEA TAC. I RECEIVED NO WAKE TURB WARNINGS ON MY TKOF CLRNC FROM BFI. SUCH WARNINGS SHOULD BE SOP, AS THEY MAY CAUSE PLTS UNFAMILIAR WITH THE DYNAMICS OF THE INTERACTION BTWN THE 2 ARPTS (LIKE ME) TO MODIFY THEIR DEP REQUEST.

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.