Bfi tower controller reported a near midair collision between a C172 departing bfi downwind and a B7...

Attributes
ACN	1374520
Time
Date	201607
Local Time Of Day	0001-0600
Place
Locale Reference	BFI.Tower
State Reference	WA
Aircraft 1
Make Model Name	Skyhawk 172/Cutlass 172
Flight Phase	Initial Climb
Route In Use	None
Flight Plan	VFR
Aircraft 2
Make Model Name	B737 Undifferentiated or Other Model
Flight Phase	Final Approach
Route In Use	Other Instrument Approach
Flight Plan	IFR
Person 1
Function	Local
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 2
Events
Anomaly	ATC Issue All Types Conflict NMAC Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy

Narrative:

Working combined locals. Moderate traffic. 3 in the pattern with steady arrivals and departures. I cleared a C172 for take off with explicate instructions on how to safely fly the down wind departure. The pilot read back the instructions and sounded competent. I had a student pilot that was having difficulty controlling his plane in the pattern. I then focused on ensuring he remained separated from the rest of my pattern traffic. This is an ongoing systemic pattern caused by the proximity of sea and bfi. As stated; this is a systemic problem due to the proximity of sea and bfi. Even with 'proper' departure procedures in place this problem is going to continue due to pilot error. This problem has occurred with the most experienced controllers at bfi. A B737 pilot landing sea declared near midair collision TCAS RA/evasive action VFR to IFR altitude course/routing at 1;400 feet.

Original NASA ASRS Text

Title: BFI Tower Controller reported a NMAC between a C172 departing BFI downwind and a B737 arriving SEA. The C172 pilot apparently did not follow the Tower issued departure instructions.

Narrative: Working combined locals. Moderate traffic. 3 in the pattern with steady arrivals and departures. I cleared a C172 for take off with explicate instructions on how to safely fly the down wind departure. The pilot read back the instructions and sounded competent. I had a student pilot that was having difficulty controlling his plane in the pattern. I then focused on ensuring he remained separated from the rest of my pattern traffic. This is an ongoing systemic pattern caused by the proximity of SEA and BFI. As stated; this is a systemic problem due to the proximity of SEA and BFI. Even with 'proper' departure procedures in place this problem is going to continue due to pilot error. This problem has occurred with the most experienced controllers at BFI. A B737 pilot landing SEA declared NMAC TCAS RA/Evasive Action VFR To IFR Altitude Course/Routing at 1;400 feet.

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.

Bfi tower controller reported a near midair collision between a C172 departing bfi downwind and a B737 arriving sea. The C172 pilot apparently did not follow the tower issued departure instructions.