Narrative:

Air carrier X ILS runway 16R approach, encountered a beech 99 at 10:30 O'clock position, slightly low, left to right, approximately 500-600 ft laterally, that proceeded to fly directly across our flight path at approximately 900 ft on our altitude and 3-4 DME from the field, and exited to the right. We initiated a visual, mild evasive maneuver up and to the left, then continued to a normal landing, after tower initially started to issued a go around then reversed itself. The intruder in question was Y training operation in the bfi VFR traffic pattern. As per supervisor at sea-tac tower, proper paperwork was initiated. Of concern is the fact that although TCASII was set at 5 NM below TA only, no TA was given until visual maneuver was already initiated and intruder was directly in front of our aircraft.

Related reports:

Google
 

Original NASA ASRS Text

Title: ACR X NMAC LTSS FROM Y. SYS ERROR.

Narrative: ACR X ILS RWY 16R APCH, ENCOUNTERED A BEECH 99 AT 10:30 O'CLOCK POS, SLIGHTLY LOW, L TO R, APPROX 500-600 FT LATERALLY, THAT PROCEEDED TO FLY DIRECTLY ACROSS OUR FLT PATH AT APPROX 900 FT ON OUR ALT AND 3-4 DME FROM THE FIELD, AND EXITED TO THE R. WE INITIATED A VISUAL, MILD EVASIVE MANEUVER UP AND TO THE L, THEN CONTINUED TO A NORMAL LNDG, AFTER TWR INITIALLY STARTED TO ISSUED A GAR THEN REVERSED ITSELF. THE INTRUDER IN QUESTION WAS Y TRAINING OP IN THE BFI VFR TFC PATTERN. AS PER SUPVR AT SEA-TAC TWR, PROPER PAPERWORK WAS INITIATED. OF CONCERN IS THE FACT THAT ALTHOUGH TCASII WAS SET AT 5 NM BELOW TA ONLY, NO TA WAS GIVEN UNTIL VISUAL MANEUVER WAS ALREADY INITIATED AND INTRUDER WAS DIRECTLY IN FRONT OF OUR ACFT.

Data retrieved from NASA's ASRS site as of July 2007 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.