Narrative:

Air carrier X was level at 4000 ft on assigned heading 310 degrees for a right base leg for ILS 04R approach at bos. We were being given radar vectors by bos approach control. Flight conditions were solid IMC. Our first indication of a potential problem was when an aircraft Y symbol appeared on our TCASII screen at approximately 12 to 1 O'clock and 6 mi at 1000 ft above our altitude (500 ft MSL). The target was moving towards our aircraft as indicated by the TCASII display. Shortly after becoming visible on our TCASII display, aircraft Y began indicating a descent from 5000 ft. When the target was approximately 4 mi away, we received a 'traffic' warning. Shortly thereafter, the TCASII unit issued a RA, 'monitor vertical speed,' followed by 'descend.' as we started to descend from 4000 ft to avoid the traffic, which was now indicated at 12 to 1 O'clock, 1 mi 600 ft above our aircraft and still descending, approach control apparently noticed the impending conflict and issued a left turn to 270 degrees to our aircraft. We descended to 3500 ft and rolled out on a heading of 270 degrees. We notified bos approach that we had vacated our assigned altitude in response to a TCASII RA. The other aircraft Y involved reported that they had initiated a climb in response to a RA from their TCASII system. The controller then issued us a right turn to an intercept heading for the ILS localizer at 4000 ft and we were subsequently cleared for the ILS 04R approach without further incident. Lesson learned: TCASII works. The occasional nuisance warnings on parallel visual approachs are a small price to pay for a real-life save in this instance.

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Original NASA ASRS Text

Title: ACR X TCASII TA RA DSND HAD LTSS FROM ACFT Y TCASII RA CLB. EVASIVE ACTION TAKEN. DSCNT TO OCCUPIED ALT. SYS ERROR.

Narrative: ACR X WAS LEVEL AT 4000 FT ON ASSIGNED HDG 310 DEGS FOR A R BASE LEG FOR ILS 04R APCH AT BOS. WE WERE BEING GIVEN RADAR VECTORS BY BOS APCH CTL. FLT CONDITIONS WERE SOLID IMC. OUR FIRST INDICATION OF A POTENTIAL PROBLEM WAS WHEN AN ACFT Y SYMBOL APPEARED ON OUR TCASII SCREEN AT APPROX 12 TO 1 O'CLOCK AND 6 MI AT 1000 FT ABOVE OUR ALT (500 FT MSL). THE TARGET WAS MOVING TOWARDS OUR ACFT AS INDICATED BY THE TCASII DISPLAY. SHORTLY AFTER BECOMING VISIBLE ON OUR TCASII DISPLAY, ACFT Y BEGAN INDICATING A DSCNT FROM 5000 FT. WHEN THE TARGET WAS APPROX 4 MI AWAY, WE RECEIVED A 'TFC' WARNING. SHORTLY THEREAFTER, THE TCASII UNIT ISSUED A RA, 'MONITOR VERT SPD,' FOLLOWED BY 'DSND.' AS WE STARTED TO DSND FROM 4000 FT TO AVOID THE TFC, WHICH WAS NOW INDICATED AT 12 TO 1 O'CLOCK, 1 MI 600 FT ABOVE OUR ACFT AND STILL DSNDING, APCH CTL APPARENTLY NOTICED THE IMPENDING CONFLICT AND ISSUED A L TURN TO 270 DEGS TO OUR ACFT. WE DSNDED TO 3500 FT AND ROLLED OUT ON A HDG OF 270 DEGS. WE NOTIFIED BOS APCH THAT WE HAD VACATED OUR ASSIGNED ALT IN RESPONSE TO A TCASII RA. THE OTHER ACFT Y INVOLVED RPTED THAT THEY HAD INITIATED A CLB IN RESPONSE TO A RA FROM THEIR TCASII SYS. THE CTLR THEN ISSUED US A R TURN TO AN INTERCEPT HDG FOR THE ILS LOC AT 4000 FT AND WE WERE SUBSEQUENTLY CLRED FOR THE ILS 04R APCH WITHOUT FURTHER INCIDENT. LESSON LEARNED: TCASII WORKS. THE OCCASIONAL NUISANCE WARNINGS ON PARALLEL VISUAL APCHS ARE A SMALL PRICE TO PAY FOR A REAL-LIFE SAVE IN THIS INSTANCE.

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.