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.