Narrative:

We got vectored pretty far north of the final approach course for 16R on the west side after descending via the HAWKS6 landing south. ATC turned us on a right base to intercept just south of pae. Slowed to 170 pretty early by ATC and joined the localizer inbound for 16R. As we were approaching downtown we were told to maintain 4000 ft 'til established; cleared for the visual 16R and maintain speed 170. Shortly thereafter we noticed to the right of our position on the mfd that there was TCAS traffic +1000-+1500 ft within a couple miles of us being vectored by the same controller to pass over the top of us and execute the visual approach for 16L. We could not find them visually; but kept our scans going. As we continued inbound I mentioned to the captain that the traffic was descending toward us; approaching 1000 ft above. I recall shortly thereafter watching the altitude difference decrease below 1000 ft and verbalized 'I think this is going to be an RA'. Sure enough; we got the 'traffic; traffic' from TCAS and we immediately started looking; followed by the RA calling for 'descend; descend now' at approximately 2000 ft per minute. I disengaged the autopilot and over rode the throttles; and immediately complied with the TCAS RA. We descended several hundred feet and the TCAS advised us to shallow our descent; which I complied with. Hoping the event was over; we took a sigh and immediately got another 'descend; descend now' right back down toward 2000 ft/min. I immediately complied with the new RA again and this time the captain started getting aggressive with approach control about what was going on. Approach seemed somewhat uninterested; which made me think he wasn't getting the collision alert on his end. Aircraft Y piped up on the frequency and said 'I don't know what your problem is man; we have had you in sight the whole time'. Which leads us to believe they didn't get an RA and we did. If two transport category aircraft have TA/RA both should be getting RA's; but we were the only ones reporting responding to an RA. Oddly enough; after the TCAS advised we were 'clear of conflict' the target reappeared as blue at our 2 O'clock position and about -200 ft relative to us and remained that way all the way down to the runway. Discussing with the captain we believed that the TCAS traffic was the aircraft Y aircraft at our 10-11 o'clock which was approximately 200 ft lower than us; and was not on our TCAS at all. This led us to believe after a thorough scan of the 2 o clock position that there was no traffic. We both decided since we were on short final that we selected TA only in case the TCAS was malfunctioning and giving us false RA's. The only reason we did this was because we reasonably thought that the TCAS was displaying incorrect information since the aircraft Y aircraft was not on the screen but a 'bogey' target was. We landed without any further events; cleared the runway and after parking at the gate; proactively obtained the TRACON's phone number. We need to determine why we were the only aircraft that got an RA. Either the actual traffic conflict was GA and not being controlled; or aircraft Y had an RA and didn't respond to it; or either TCAS system wasn't functioning properly. We relied on our training and thankfully had a safe outcome. However; an investigation is needed to get to the root of this. Both the captain and I were almost certain we were about to experience a midair collision based off of the information we had.

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Original NASA ASRS Text

Title: Air Carrier First Officer reported responding to several RAs due to a potential TCAS system anomaly during an approach.

Narrative: We got vectored pretty far north of the final approach course for 16R on the west side after descending via the HAWKS6 landing south. ATC turned us on a right base to intercept just south of PAE. Slowed to 170 pretty early by ATC and joined the localizer inbound for 16R. As we were approaching downtown we were told to maintain 4000 ft 'til established; cleared for the visual 16R and maintain speed 170. Shortly thereafter we noticed to the right of our position on the MFD that there was TCAS traffic +1000-+1500 ft within a couple miles of us being vectored by the same controller to pass over the top of us and execute the visual approach for 16L. We could not find them visually; but kept our scans going. As we continued inbound I mentioned to the captain that the traffic was descending toward us; approaching 1000 ft above. I recall shortly thereafter watching the altitude difference decrease below 1000 ft and verbalized 'I think this is going to be an RA'. Sure enough; we got the 'TRAFFIC; TRAFFIC' from TCAS and we immediately started looking; followed by the RA calling for 'DESCEND; DESCEND NOW' at approximately 2000 ft per minute. I disengaged the autopilot and over rode the throttles; and immediately complied with the TCAS RA. We descended several hundred feet and the TCAS advised us to shallow our descent; which I complied with. Hoping the event was over; we took a sigh and immediately got another 'DESCEND; DESCEND NOW' right back down toward 2000 ft/min. I immediately complied with the new RA again and this time the Captain started getting aggressive with approach control about what was going on. Approach seemed somewhat uninterested; which made me think he wasn't getting the collision alert on his end. Aircraft Y piped up on the frequency and said 'I don't know what your problem is man; we have had you in sight the whole time'. Which leads us to believe they didn't get an RA and we did. If two transport category aircraft have TA/RA both should be getting RA's; but we were the only ones reporting responding to an RA. Oddly enough; after the TCAS advised we were 'Clear of Conflict' the target reappeared as blue at our 2 O'clock position and about -200 ft relative to us and remained that way all the way down to the runway. Discussing with the captain we believed that the TCAS traffic was the Aircraft Y aircraft at our 10-11 o'clock which was approximately 200 ft lower than us; and was not on our TCAS at all. This led us to believe after a thorough scan of the 2 o clock position that there was no traffic. We both decided since we were on short final that we selected TA only in case the TCAS was malfunctioning and giving us false RA's. The only reason we did this was because we reasonably thought that the TCAS was displaying incorrect information since the Aircraft Y aircraft was not on the screen but a 'bogey' target was. We landed without any further events; cleared the runway and after parking at the gate; proactively obtained the TRACON's phone number. We need to determine why we were the only aircraft that got an RA. Either the actual traffic conflict was GA and not being controlled; or Aircraft Y had an RA and didn't respond to it; or either TCAS system wasn't functioning properly. We relied on our training and thankfully had a safe outcome. However; an investigation is needed to get to the root of this. Both the Captain and I were almost certain we were about to experience a midair collision based off of the information we had.

Data retrieved from NASA's ASRS site 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.