Air Traffic Control Developmental
Situational Awareness
ATC Issue All Types; Conflict Airborne Conflict
Flight Crew Took Evasive Action
Human Factors; Procedure
A C560 was departing BFI; checked on the frequency climbing to assigned altitude of 2;100. I advised them to expect a 'step up climb' due to a heavy B767 departing SEA 4 miles in trail. Soon after RADAR identification; the C560 advised that they were responding to a TCAS RA and descending 'back down' to 2;100. When I observed the C560's Mode C; it indicated 2;300 and I issued a traffic call referencing an unidentified VFR target indicating 1;600 at 1 o'clock and 1 mile. On initial contact; I did not observe the VFR traffic as a potential conflict for the C560. The traffic was no longer a factor and the C560 continued on their route and climb without further incident. I am not certain that BFI Tower issued a traffic advisory to the C560 prior to frequency change while both aircraft were still within the lateral limits of BFI's Class D airspace. Better communication between me and the BFI Tower Controller or between BFI Tower and the C560 might have prevented the TCAS RA.
S46 Controller described a TCAS RA event between an IFR BFI departure aircraft and an unknown VFR aircraft operating nearby; the reporter listing less than complete communication between ATC personnel and the VFR aircraft.