S46 controller working charted visual approach procedures was forced to cancel an approach clearance...

Narrative:

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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

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	1057537
Time
Date	201212
Local Time Of Day	1801-2400
Place
Locale Reference	SEA.Airport
State Reference	WA
Aircraft 1
Make Model Name	Dash 8 Series Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Final Approach
Route In Use	Visual Approach
Flight Plan	IFR
Aircraft 2
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Final Approach
Flight Plan	IFR
Person 1
Function	Approach Departure
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types

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.