Narrative:

Air carrier X 6000 ft MSL 18 mi from bur talking to bur approach on 120.4. A closure rate developed, observed first visually, with a small aircraft at the same altitude. Just as it became obvious that an escape maneuver was necessary, TCASII commanded an immediate descent. A descent was initiated while notifying approach control.approach control did not show a conflict until after maneuver was begun. Approach cleared us to 5000. We had descended to 4800 MSL and climbed back up to 5000 ft. We were within 1/2 mi of the small aircraft and closing fast as maneuver was begun. Our closest point of approach was approximately 400 ft. The small aircraft did not see us in time to avoid a midair collision. Approximately 2 mins later, a second alert was issued to climb just as approach turned us to a heading of 160 degrees to avoid traffic. TCASII commanded a climb while in turn and we once again told approach control of our deviation. This traffic was never seen visually. Aircraft gained 250 ft during this maneuver before 'clear' was given.

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Original NASA ASRS Text

Title: ACR X TCASII RA DSND HAD CONFLICT WITH SMA. UNAUTHORIZED DSCNT FROM AND THROUGH ASSIGNED ALT. PLTDEV. SEE AND AVOID CONCEPT.

Narrative: ACR X 6000 FT MSL 18 MI FROM BUR TALKING TO BUR APCH ON 120.4. A CLOSURE RATE DEVELOPED, OBSERVED FIRST VISUALLY, WITH A SMA AT THE SAME ALT. JUST AS IT BECAME OBVIOUS THAT AN ESCAPE MANEUVER WAS NECESSARY, TCASII COMMANDED AN IMMEDIATE DSCNT. A DSCNT WAS INITIATED WHILE NOTIFYING APCH CTL.APCH CTL DID NOT SHOW A CONFLICT UNTIL AFTER MANEUVER WAS BEGUN. APCH CLRED US TO 5000. WE HAD DSNDED TO 4800 MSL AND CLBED BACK UP TO 5000 FT. WE WERE WITHIN 1/2 MI OF THE SMA AND CLOSING FAST AS MANEUVER WAS BEGUN. OUR CLOSEST POINT OF APCH WAS APPROX 400 FT. THE SMA DID NOT SEE US IN TIME TO AVOID A MIDAIR COLLISION. APPROX 2 MINS LATER, A SECOND ALERT WAS ISSUED TO CLB JUST AS APCH TURNED US TO A HDG OF 160 DEGS TO AVOID TFC. TCASII COMMANDED A CLB WHILE IN TURN AND WE ONCE AGAIN TOLD APCH CTL OF OUR DEV. THIS TFC WAS NEVER SEEN VISUALLY. ACFT GAINED 250 FT DURING THIS MANEUVER BEFORE 'CLR' WAS GIVEN.

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.