Narrative:

TCASII RA, near miss. Descending on a downwind for runway 16L we were given a westerly heading for a base leg shortly after we called airport in sight and we were cleared for a visual to runway 16L. Traffic was issued to us as a cessna caravan having us in sight to follow us to runway 16L. We initially had the caravan in sight, then after turning final we were physically unable to keep the caravan in sight and I was not concerned as he was supposed to be following us to the runway. Moments after joining the final for runway 16L we received a TCASII RA. We executed a missed approach and climbed to 4000 ft as directed by ATC. When the TCASII RA was first received the TCASII displayed a target directly below and slightly to the right, altitude readout was 200 ft below. During the missed approach I observed the TCASII altitude readout momentarily indicate 100 ft below. We continued our missed approach to an uneventful 2ND approach and landing. Upon arrival at the gate I was approached by a deadheading (non revenue) same air carrier captain. He advised me that his estimation on separation was less than 200 ft and without adherence to the TCASII RA, a collision would have most likely occurred. Numerous passenger commented on the proximity of the cessna caravan during the missed approach. A PA announcement was made explaining spacing was not adequate and we would be landing shortly. No details of the proximity were given to the passenger. I contacted seattle TRACON and spoke with supervisor. He advised me he was standing next to the controller and watched the incident occur. He confirmed with me the situation and that the caravan was to follow us to the runway. He was unable to confirm the distances between us other than that when the targets converged the caravan's speed was 10 KTS less than us. In retrospect, some consideration needs to be given to the situation and a resolution to ensure it does not reoccur. We did have the traffic in sight but after turning final we were physically unable to see the traffic as he had moved to past our 3 O'clock position. At this point my focus was completing a stable approach, not keeping up with an aircraft that I can no longer see. I am wondering if the caravan ever really had us in sight, or did he have traffic in front of us and believed that was who he was following. Should more spacing have been given prior to allowing him to follow us. These are just few initial questions I have. I am unsure of the actual distances involved and they may be greater than what I have reported. These are the details as I remember them to the best of my knowledge and from the information provided to me from the captain in seat X. I am very thankful for TCASII being installed and operative on this aircraft. I am also extremely thankful for air carrier's policy and training to always follow a TCASII RA regardless of the situation. In this case, if the distances reported are correct, the TCASII advisory is responsible for avoiding a serious collision and accident. Supplemental information from acn 534646: we, captain flying, arrived at sea on the chins 3 arrival and had entered a downwind to runway 16R, 5 mi behind an MD11. Approach control gave us a descent to 4000 ft and a base leg inside of the MD11 for a visual for runway 16L while he was still going to runway 16R. Once established on the base leg with slats extended and flaps at 23 degrees the controller called traffic at our 12 O'clock position to 12:30 O'clock position, a caravan below and at a horizontal distance, I cannot recall, I seem to remember him stating that the caravan was on approach to the parallel runway 16R, as well as the field at our 10-11 O'clock position. We saw this traffic and the field and informed the controller as such. I believe he gave us an intercept heading which we turned to, out of the turn intercepting he called traffic at our 12 O'clock position, an MD80 which would precede us to runway 16L. We called this traffic in sight. He then cleared us for a visual, informed us that the caravan was maintaining visual separation and to contact tower. I called tower as we intercepted the localizer and GS for backup on the visual. Tower cleared us to land behind the MD80. The captain called for the gear. While it was in transit we received a TA apparently from an aircraft at boeing/king field which we were over at the time, (separation indicated in excess of 1000 ft). The TA went out after a second. Once the gear was down we started to finish the mechanical landing checklist when we received an RA for an aircraft 150 ft below us and just off our right wing. The captain had already turned off the autoplt and autothrottles for the visual approach so he smoothly executed the climb indicated by the RA. At one point indicated vertical separation was 100 ft. Once the conflict was resolved, we informed tower we were executing a missed approach due to the RA. Following ATC's direction we made an uneventful approach and landing to runway 16R. Upon landing the captain was informed by an airline captain non-revenuing that had the captain not immediately complied with the RA we would have impacted the conflicting aircraft. The captain notified the system operations control of the incident and also called the TRACON manager, who indicated that the conflicting aircraft was the caravan which had been cleared to follow us to runway 16L. He conjectured that the caravan mistook the previous MD80 for us and picked up too sharp an intercept to final. He also indicated there had been no deviation. We did not see the aircraft which was in conflict.

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Original NASA ASRS Text

Title: MD80 CREW HAD AN NMAC WITH A CESSNA CARAVAN IN S46 CLASS B.

Narrative: TCASII RA, NEAR MISS. DSNDING ON A DOWNWIND FOR RWY 16L WE WERE GIVEN A WESTERLY HEADING FOR A BASE LEG SHORTLY AFTER WE CALLED ARPT IN SIGHT AND WE WERE CLRED FOR A VISUAL TO RWY 16L. TFC WAS ISSUED TO US AS A CESSNA CARAVAN HAVING US IN SIGHT TO FOLLOW US TO RWY 16L. WE INITIALLY HAD THE CARAVAN IN SIGHT, THEN AFTER TURNING FINAL WE WERE PHYSICALLY UNABLE TO KEEP THE CARAVAN IN SIGHT AND I WAS NOT CONCERNED AS HE WAS SUPPOSED TO BE FOLLOWING US TO THE RWY. MOMENTS AFTER JOINING THE FINAL FOR RWY 16L WE RECEIVED A TCASII RA. WE EXECUTED A MISSED APCH AND CLBED TO 4000 FT AS DIRECTED BY ATC. WHEN THE TCASII RA WAS FIRST RECEIVED THE TCASII DISPLAYED A TARGET DIRECTLY BELOW AND SLIGHTLY TO THE R, ALT READOUT WAS 200 FT BELOW. DURING THE MISSED APCH I OBSERVED THE TCASII ALT READOUT MOMENTARILY INDICATE 100 FT BELOW. WE CONTINUED OUR MISSED APCH TO AN UNEVENTFUL 2ND APCH AND LNDG. UPON ARR AT THE GATE I WAS APCHED BY A DEADHEADING (NON REVENUE) SAME ACR CAPT. HE ADVISED ME THAT HIS ESTIMATION ON SEPARATION WAS LESS THAN 200 FT AND WITHOUT ADHERENCE TO THE TCASII RA, A COLLISION WOULD HAVE MOST LIKELY OCCURRED. NUMEROUS PAX COMMENTED ON THE PROX OF THE CESSNA CARAVAN DURING THE MISSED APCH. A PA ANNOUNCEMENT WAS MADE EXPLAINING SPACING WAS NOT ADEQUATE AND WE WOULD BE LNDG SHORTLY. NO DETAILS OF THE PROX WERE GIVEN TO THE PAX. I CONTACTED SEATTLE TRACON AND SPOKE WITH SUPVR. HE ADVISED ME HE WAS STANDING NEXT TO THE CTLR AND WATCHED THE INCIDENT OCCUR. HE CONFIRMED WITH ME THE SIT AND THAT THE CARAVAN WAS TO FOLLOW US TO THE RWY. HE WAS UNABLE TO CONFIRM THE DISTANCES BTWN US OTHER THAN THAT WHEN THE TARGETS CONVERGED THE CARAVAN'S SPD WAS 10 KTS LESS THAN US. IN RETROSPECT, SOME CONSIDERATION NEEDS TO BE GIVEN TO THE SIT AND A RESOLUTION TO ENSURE IT DOES NOT REOCCUR. WE DID HAVE THE TFC IN SIGHT BUT AFTER TURNING FINAL WE WERE PHYSICALLY UNABLE TO SEE THE TFC AS HE HAD MOVED TO PAST OUR 3 O'CLOCK POS. AT THIS POINT MY FOCUS WAS COMPLETING A STABLE APCH, NOT KEEPING UP WITH AN ACFT THAT I CAN NO LONGER SEE. I AM WONDERING IF THE CARAVAN EVER REALLY HAD US IN SIGHT, OR DID HE HAVE TFC IN FRONT OF US AND BELIEVED THAT WAS WHO HE WAS FOLLOWING. SHOULD MORE SPACING HAVE BEEN GIVEN PRIOR TO ALLOWING HIM TO FOLLOW US. THESE ARE JUST FEW INITIAL QUESTIONS I HAVE. I AM UNSURE OF THE ACTUAL DISTANCES INVOLVED AND THEY MAY BE GREATER THAN WHAT I HAVE RPTED. THESE ARE THE DETAILS AS I REMEMBER THEM TO THE BEST OF MY KNOWLEDGE AND FROM THE INFO PROVIDED TO ME FROM THE CAPT IN SEAT X. I AM VERY THANKFUL FOR TCASII BEING INSTALLED AND OPERATIVE ON THIS ACFT. I AM ALSO EXTREMELY THANKFUL FOR ACR'S POLICY AND TRAINING TO ALWAYS FOLLOW A TCASII RA REGARDLESS OF THE SIT. IN THIS CASE, IF THE DISTANCES RPTED ARE CORRECT, THE TCASII ADVISORY IS RESPONSIBLE FOR AVOIDING A SERIOUS COLLISION AND ACCIDENT. SUPPLEMENTAL INFO FROM ACN 534646: WE, CAPT FLYING, ARRIVED AT SEA ON THE CHINS 3 ARR AND HAD ENTERED A DOWNWIND TO RWY 16R, 5 MI BEHIND AN MD11. APCH CTL GAVE US A DSCNT TO 4000 FT AND A BASE LEG INSIDE OF THE MD11 FOR A VISUAL FOR RWY 16L WHILE HE WAS STILL GOING TO RWY 16R. ONCE ESTABLISHED ON THE BASE LEG WITH SLATS EXTENDED AND FLAPS AT 23 DEGS THE CTLR CALLED TFC AT OUR 12 O'CLOCK POS TO 12:30 O'CLOCK POS, A CARAVAN BELOW AND AT A HORIZ DISTANCE, I CANNOT RECALL, I SEEM TO REMEMBER HIM STATING THAT THE CARAVAN WAS ON APCH TO THE PARALLEL RWY 16R, AS WELL AS THE FIELD AT OUR 10-11 O'CLOCK POS. WE SAW THIS TFC AND THE FIELD AND INFORMED THE CTLR AS SUCH. I BELIEVE HE GAVE US AN INTERCEPT HEADING WHICH WE TURNED TO, OUT OF THE TURN INTERCEPTING HE CALLED TFC AT OUR 12 O'CLOCK POS, AN MD80 WHICH WOULD PRECEDE US TO RWY 16L. WE CALLED THIS TFC IN SIGHT. HE THEN CLRED US FOR A VISUAL, INFORMED US THAT THE CARAVAN WAS MAINTAINING VISUAL SEPARATION AND TO CONTACT TWR. I CALLED TWR AS WE INTERCEPTED THE LOC AND GS FOR BACKUP ON THE VISUAL. TWR CLRED US TO LAND BEHIND THE MD80. THE CAPT CALLED FOR THE GEAR. WHILE IT WAS IN TRANSIT WE RECEIVED A TA APPARENTLY FROM AN ACFT AT BOEING/KING FIELD WHICH WE WERE OVER AT THE TIME, (SEPARATION INDICATED IN EXCESS OF 1000 FT). THE TA WENT OUT AFTER A SECOND. ONCE THE GEAR WAS DOWN WE STARTED TO FINISH THE MECHANICAL LNDG CHKLIST WHEN WE RECEIVED AN RA FOR AN ACFT 150 FT BELOW US AND JUST OFF OUR R WING. THE CAPT HAD ALREADY TURNED OFF THE AUTOPLT AND AUTOTHROTTLES FOR THE VISUAL APCH SO HE SMOOTHLY EXECUTED THE CLB INDICATED BY THE RA. AT ONE POINT INDICATED VERT SEPARATION WAS 100 FT. ONCE THE CONFLICT WAS RESOLVED, WE INFORMED TWR WE WERE EXECUTING A MISSED APCH DUE TO THE RA. FOLLOWING ATC'S DIRECTION WE MADE AN UNEVENTFUL APCH AND LNDG TO RWY 16R. UPON LNDG THE CAPT WAS INFORMED BY AN AIRLINE CAPT NON-REVENUING THAT HAD THE CAPT NOT IMMEDIATELY COMPLIED WITH THE RA WE WOULD HAVE IMPACTED THE CONFLICTING ACFT. THE CAPT NOTIFIED THE SYS OPS CTL OF THE INCIDENT AND ALSO CALLED THE TRACON MGR, WHO INDICATED THAT THE CONFLICTING ACFT WAS THE CARAVAN WHICH HAD BEEN CLRED TO FOLLOW US TO RWY 16L. HE CONJECTURED THAT THE CARAVAN MISTOOK THE PREVIOUS MD80 FOR US AND PICKED UP TOO SHARP AN INTERCEPT TO FINAL. HE ALSO INDICATED THERE HAD BEEN NO DEV. WE DID NOT SEE THE ACFT WHICH WAS IN CONFLICT.

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.