Narrative:

TCASII RA. While on the ILS approach over bfi in clear conditions, traffic alert popped up less than 1 mi in front of us at 100 ft. An RA, descend crossing descend, which we responded to followed by another RA of increase descent, which was followed. The RA had us descending at 2500 FPM at 1700 ft gl. No aircraft was seen, however, at that point an aircraft was taking off at bfi under us. Sea tower was informed and they did not see the traffic. After responding at 2500 FPM descent we abandoned the response after it became apparent the RA had to be erroneous information. TCASII was written up in the maintenance logbook, however suspect an erroneous altitude from the transponder of the small VFR departure aircraft taking off at bfi as the traffic symbol followed the departing/diverging aircraft after we passed bfi.

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Original NASA ASRS Text

Title: MD80 CREW AND A LIGHT ACFT OVER BFI HAD A TCASII RA IN SEA CLASS B.

Narrative: TCASII RA. WHILE ON THE ILS APCH OVER BFI IN CLR CONDITIONS, TFC ALERT POPPED UP LESS THAN 1 MI IN FRONT OF US AT 100 FT. AN RA, DSND XING DSND, WHICH WE RESPONDED TO FOLLOWED BY ANOTHER RA OF INCREASE DSCNT, WHICH WAS FOLLOWED. THE RA HAD US DSNDING AT 2500 FPM AT 1700 FT GL. NO ACFT WAS SEEN, HOWEVER, AT THAT POINT AN ACFT WAS TAKING OFF AT BFI UNDER US. SEA TWR WAS INFORMED AND THEY DID NOT SEE THE TFC. AFTER RESPONDING AT 2500 FPM DSCNT WE ABANDONED THE RESPONSE AFTER IT BECAME APPARENT THE RA HAD TO BE ERRONEOUS INFO. TCASII WAS WRITTEN UP IN THE MAINT LOGBOOK, HOWEVER SUSPECT AN ERRONEOUS ALT FROM THE XPONDER OF THE SMALL VFR DEP ACFT TAKING OFF AT BFI AS THE TFC SYMBOL FOLLOWED THE DEPARTING/DIVERGING ACFT AFTER WE PASSED BFI.

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.