Ce-560XL flight crew reported climbing in response to a RA on approach to bfi.

Attributes
ACN	1547251
Time
Date	201806
Local Time Of Day	1201-1800
Place
Locale Reference	BFI.Airport
State Reference	WA
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Citation Excel (C560XL)
Operating Under FAR Part	Part 91
Flight Phase	Initial Approach
Route In Use	Vectors
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	Climb
Person 1
Function	Pilot Flying Captain
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Person 2
Function	First Officer Pilot Not Flying
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Conflict Airborne Conflict Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

Narrative:

In VMC while on an IFR approach; level at 2200 feet assigned; we were being vectored on base leg preparing to join localizer course for bfi runway 13R; when we received a TCAS traffic alert. Target aircraft was straight ahead on our course temporarily level 500ft below. As we closed; the target aircraft began to climb; causing an RA commanded climb for us. Simultaneously the approach controller announced the target aircraft climbing and assigned us a heading to deconflict. As required; I had already initiated the commanded RA climb; and my pilot monitoring bugged my new heading and informed the controller we had an RA. The pilot monitoring announced the RA twice. When clear of conflict; I slowly descended back down to our assigned altitude. ATC then gave us a new heading back to intercept the localizer. The remaining approach and landing were uneventful.

Original NASA ASRS Text

Title: CE-560XL flight crew reported climbing in response to a RA on approach to BFI.

Narrative: In VMC while on an IFR approach; level at 2200 feet assigned; we were being vectored on base leg preparing to join localizer course for BFI Runway 13R; when we received a TCAS traffic alert. Target aircraft was straight ahead on our course temporarily level 500ft below. As we closed; the target aircraft began to climb; causing an RA commanded climb for us. Simultaneously the Approach Controller announced the target aircraft climbing and assigned us a heading to deconflict. As required; I had already initiated the commanded RA climb; and my pilot monitoring bugged my new heading and informed the controller we had an RA. The pilot monitoring announced the RA twice. When clear of conflict; I slowly descended back down to our assigned altitude. ATC then gave us a new heading back to intercept the localizer. The remaining approach and landing were uneventful.

Data retrieved from NASA's ASRS site 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.