S46 controller described a loss of separation event discovered several days after the event via an a...

Narrative:

I was working final: south flow; independent/parallel ILS approaches; weather deviations and complications; with our weeknight rush of small; larges; and multiple wake turbulence aircraft. No 'plan alpha' and many bfi arrivals. Boeing final (D) and approach monitor (arm) were open. A caravan was on final at seatac airport. The aircraft was descended to 3;200 and was established on the localizer for runway 16R. As per the facility SOP; we are not to clear the seattle arrival traffic with an aircraft inbound to boeing field (bfi) until that aircraft is descended past 1800 on the localizer. A bizjet was being worked by the boeing final controller (D sector) I saw the bizjet nine miles out; inbound and decided that it was no factor. I cleared my caravan on the ILS to 16R just past bugne. I kept an eye on the bizjet descending and saw no conflict. The approach monitor and the boeing final controller mentioned nothing regarding the traffic and the caravan was shipped at the FAF as per the rules of the facility. I was informed two weeks later that the two aircraft had lost separation and observed the radar data. It appears that the bizjet had seen/had traffic called for a target at approximately 800 ft MSL in front of them. The bizjet climbed from 1;100 to 1;200-1;300 and then descended again. The caravan was at 2;100-2;200 on the final approximately 2.9 miles away with unknown divergence. No traffic was called for the caravan. Both landed. Recommendation; in a perfect control environment there would be no waiver procedures that try to make up separation where there is none. S46/bfi/sea' 'plan alpha' is an attempt at such separation. This was an example of how two airports less than 3 miles apart with crossing arrivals can have problems. In the future to prevent this type of separation loss I should have followed the facilities rules of not clearing the caravan until the bizjet was out of 1;800 ft; descending. While that is not why I lost separation it would have caused me to be too high above the glide-slope with the caravan; and as a result; I would have had to cancel the approach clearance and climb the caravan; therefore taking the aircraft out of the problem.

Original NASA ASRS Text

Title: S46 Controller described a loss of separation event discovered several days after the event via an automated replay; the reporter acknowledging that had facility procedures been followed no error would have occurred.

Narrative: I was working final: South Flow; independent/parallel ILS approaches; weather deviations and complications; with our weeknight rush of small; larges; and multiple wake turbulence aircraft. No 'Plan Alpha' and many BFI arrivals. Boeing Final (D) and Approach Monitor (ARM) were open. A Caravan was on final at SeaTac airport. The aircraft was descended to 3;200 and was established on the localizer for Runway 16R. As per the facility SOP; we are not to clear the Seattle arrival traffic with an aircraft inbound to Boeing Field (BFI) until that aircraft is descended past 1800 on the localizer. A bizjet was being worked by the Boeing Final Controller (D Sector) I saw the bizjet nine miles out; inbound and decided that it was no factor. I cleared my Caravan on the ILS to 16R just past BUGNE. I kept an eye on the bizjet descending and saw no conflict. The Approach Monitor and the Boeing Final Controller mentioned nothing regarding the traffic and the Caravan was shipped at the FAF as per the rules of the facility. I was informed two weeks later that the two aircraft had lost separation and observed the RADAR data. It appears that the bizjet had seen/had traffic called for a target at approximately 800 FT MSL in front of them. The bizjet climbed from 1;100 to 1;200-1;300 and then descended again. The Caravan was at 2;100-2;200 on the final approximately 2.9 miles away with unknown divergence. No traffic was called for the Caravan. Both landed. Recommendation; in a perfect control environment there would be no waiver procedures that try to make up separation where there is none. S46/BFI/SEA' 'Plan Alpha' is an attempt at such separation. This was an example of how two airports less than 3 miles apart with crossing arrivals can have problems. In the future to prevent this type of separation loss I should have followed the facilities rules of not clearing the Caravan until the bizjet was out of 1;800 FT; descending. While that is not why I lost separation it would have caused me to be too high above the glide-slope with the Caravan; and as a result; I would have had to cancel the approach clearance and climb the Caravan; therefore taking the aircraft out of the problem.

Data retrieved from NASA's ASRS site as of April 2012 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	944775
Time
Date	201104
Local Time Of Day	0001-0600
Place
Locale Reference	S46.TRACON
State Reference	WA
Aircraft 1
Make Model Name	Caravan 208B
Flight Phase	Final Approach
Flight Plan	IFR
Aircraft 2
Make Model Name	Small Transport Low Wing 2 Turbojet Eng
Flight Phase	Landing
Flight Plan	IFR
Person 1
Function	Approach
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Conflict Airborne Conflict Deviation - Procedural Published Material / Policy

S46 controller described a loss of separation event discovered several days after the event via an automated replay; the reporter acknowledging that had facility procedures been followed no error would have occurred.