S46 controller described a TCAS RA event between an IFR bfi departure aircraft and an unknown VFR ai...

Narrative:

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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

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.

Attributes
ACN	1005565
Time
Date	201204
Local Time Of Day	1201-1800
Place
Locale Reference	BFI.Airport
State Reference	WA
Aircraft 1
Make Model Name	Citation V/Ultra/Encore (C560)
Operating Under FAR Part	Part 91
Flight Phase	Initial Climb
Flight Plan	IFR
Person 1
Function	Approach Departure
Qualification	Air Traffic Control Developmental
Events
Anomaly	ATC Issue All Types Conflict Airborne Conflict

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.