Dash 8 Series Undifferentiated or Other Model
Air Traffic Control Fully Certified
Situational Awareness
ATC Issue All Types
Air Traffic Control Issued New Clearance
Procedure; Weather
I had just taken the Final position. The briefing stated that Dash 8 was cleared for a Bay visual approach Runway 16C to maintain 3200 until established on the localizer from the west side and that the B737 was established on a 250 heading to 4;000 from the east. The briefing also included that visuals were working. It appeared that the Dash 8 was starting his turn for the bay visual approach and the ground speed indicated 220 KTS. I turned the B737 to a dogleg heading of 190 and cleared him for the ILS approach to Runway 16C with a speed reduction to 170 KTS for spacing. I then noticed that the Dash 8 was still northeast bound over the north shore of Elliot Bay; I instructed the Dash 8 to start his turn to SEATAC. I then called traffic to the B737 to get the Dash 8 in sight. The B737 was leaving 4;300 FT and instructed me that he was in the haze and unable to see the Dash 8. I instructed the B737 to cancel his approach clearance; maintain 4;000; and continue inbound on the localizer. The B737 then acquired the Dash 8 and was issued a visual approach clearance to follow the Dash 8 to Runway 16C. Assumptions made by myself were that the Dash 8 would turn through the middle of Elliot Bay as detailed in the published charted visual approach procedure and that visuals were not a problem so the B737 would be able to get the Dash 8 in sight. If you need to assign a heading on a charted visual approach procedure that is higher than the altitudes on the plate you should consider controlled vectors to a straight in final. I believe that that altitude restriction made the pilot feel that he could proceed north of the charted routing to provide himself a stabilized approach. Briefings that suggest visuals are working should contain detail as to the plane to plane visibility conditions.
S46 Controller working charted visual approach procedures was forced to cancel an approach clearance when traffic failed to see preceding traffic; the reporter indicating visibility changes were taking place.