B737 Undifferentiated or Other Model
Air Traffic Control Fully Certified
Other / Unknown; Situational Awareness
ATC Issue All Types; Conflict Airborne Conflict
General None Reported / Taken
Human Factors; Procedure
B737 was on the 16R final for Seattle south flow. King Air was on departure north flow Runway 31L from BFI. We were using 'Split Flow' procedures; but no separation was ensured and best I could tell IFR separation was not maintained. This is a procedural issue. The Seattle final is descending to cross the marker (FINKA) at 1;900.  The BFI departure is climbing unrestricted to 2;100. The aircraft are on converging courses. Again no serparation is ensured in this procedure. The aircraft did receive a collision alert. I believe the aircraft lost 1;000 FT separation prior to diverging. A minimum of 45 degrees is required for 'divergence' and this procedure only has 30 degrees. The LOA says that BFI Tower will provide visual separation between the 2 aircraft until they diverge; but they are not talking to either aircraft; so that doesn't seem appropriate. Another prior example was Air Carrier Y on short final for Runway 31L at BFI and B737 on final for Runway 16R at SEA. If Air Carrier Y were to go-around; he would climb to 6;000 on a 285 heading; in other words he would climb directly through the SEA final. This is extremely unsafe. The LOA also says that SEA Tower will provide visual between the 2 aircraft; but since they are not talking to Air Carrier Y they can not immediately react to a go-around. This is also unsafe. A new go-around procedure for Runway 31 at BFI needs to be developed; or we need to build a hole.  Split flow is not a safe procedure and we do not have the protocol in place to ensure the safety of the fyling public. 'Split Flow' needs an immediate and thourough review. Recommendation; A departure or procedure needs to be developed. Releasing aircraft north off BFI (climbing to 2;100) while Seattle traffic is descending to cross the marker (which is right over BFI) at 1;900 is not in line with the 7110.65. A departure procedure needs to be developed off BFI which ensures IFR separation and/or divergence OR we need to build holes in a 'split flow' scenario.
S46 Controller described a conflict between a BFI departure and a SEA arrival aircraft. The reporter provided additional examples of potential conflicts when 'Split Flow' procedures are utilized; and recommended some procedural changes to BFI departures.