AN MD83 HAS A CLOSE CALL WITH A DH8 THAT HAD OVERTAKEN THEM DURING A VISUAL APCH TO PARALLEL RWYS 34L&R AT SEA; WA.

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SUBJECT: VISUAL SEPARATION; VIRTUALLY NO SEPARATION! FLT; AN MD83. WE WERE BEING VECTORED ON DOWNWIND FOR A TURN TO BASE TO ACCEPT A VISUAL APCH TO RWY 34R AT SEA. PRIOR TO TURNING FINAL; WE WERE ASKED TO IDENT A HVY DC8 ON FINAL TO RWY 35R AND ALSO A B737 ON PARALLEL APCH TO RWY 34L. WE IDENTED BOTH AND WERE CLRED TO FOLLOW THE HVY DC8 FOR A VISUAL APCH TO RWY 34R. (THIS WAS R TFC TO RWY 34R.) AS THE PIC AND THE PF; I WAS UNABLE TO SEE THE FIELD AT THIS TIME. I TURNED BASE AND ROLLED WINGS LEVEL MOMENTARILY TO FIND THE FIELD; THEN TURNED TO AN INTERCEPT HDG FOR THE RWY 34R LOC. WE WERE TOLD TO RPT THE 'MALL;' WHICH WE DID. WE WERE SWITCHED TO LCL AND RECEIVED LNDG CLRNC FOR RWY 34R AND ADVISED THAT AN ACFT WOULD 'IN POS.' SHORTLY THEREAFTER; WE GOT AN RA (NO TA FIRST). THE COMMAND WAS 'DSND; DSND; DSND NOW -- MONITOR VERT SPD' IN RAPID SUCCESSION (THE DSND COMMAND WAS IN THE AREA OF 1500-2000 FPM). AT THE FIRST RA; I ASKED THE FO TO QUERY LCL ON TFC. BECAUSE OF THE VOLUME OF VOICE TFC HE WAS UNABLE TO GET THROUGH. THE INTRUDER (ON TCASII) LOOKED AS THOUGH HE WERE RIGHT ON TOP OF US DSNDING. I WAS UNABLE TO LOCATE THE INTRUDER VISUALLY -- HE WAS IN OUR 8 O'CLOCK HIGH POS AS OBSERVED ON TCASII. ADDING TO THE PROB IS THE POOR QUALITY REAR SIDE WINDOW THAT DOUGLAS USES. I WAS ABLE TO ACQUIRE THE INTRUDER VISUALLY AT OUR 9 O'CLOCK LEVEL POS (APPROX 150-200 FT LATERALLY). OUR SPD WAS APPROX BUG 15-20 KTS DECELERATING TO BUG 5 KTS. TO THE BEST I CAN REMEMBER; WE WERE BTWN THE FINAL FIX AND DECISION ALT WHEN I ACQUIRED THE INTRUDER VISUALLY. UNBELIEVABLY HE PASSED US AND CONTINUED TO RWY 34L. WE LANDED RWY 34R RELATIVELY UNEVENTFULLY; AND BY THAT I MEAN OUR PAX WERE NONE THE WISER FOR WHAT I FEEL WAS AN EXTREMELY HAZARDOUS POS PUT ON US BY THE CARELESS AND RECKLESS ACTIONS OF THE COMMUTER DASH 8 PLT. WE RESEARCHED THE DEFINITION OF 'VISUAL SEPARATION' IN THE AIM AND APPROPRIATE FAR REGS; UNBELIEVABLY THERE DOESN'T APPEAR TO BE A STANDARD. FURTHER COMPLICATING THIS INCIDENT IS THE CLOSENESS OF THE PARALLEL RWYS AT SEA. I ESTIMATED 600 FT. MY FO ESTIMATED 800 FT. I RECOMMEND THAT THE FLT RECORDERS BE PULLED ON BOTH ACFT AND THAT THE ATC RADAR TAPE BE ANALYZED. MY INTERNAL DEBATE ON WHAT ESCAPE MANEUVERS WERE AVAILABLE HAPPENED IN A MICROSECOND. I DECIDED NOT TO FOLLOW THE RA COMMAND ALL THE WAY BECAUSE OF OUR PROX TO THE GND. A PULL UP MAY HAVE CAUSED A COLLISION WITH THE INTRUDER OR KNOCKED HIM OUT OF THE AIR WITH OUR WAKE. A TURN L MAY HAVE CAUSED A COLLISION ALSO. A R TURN WOULD HAVE PUT ME IN A HAZARDOUS POS TO THE DOWNWIND. I CONTINUED THE APCH WITH A SLIGHTLY HIGHER RATE OF DSCNT. AFTER ACQUIRING THE INTRUDER VISUALLY WE WERE ABLE TO COMPLETE THE APCH WITHIN PARAMETERS. THIS HAD A HAPPY ENDING. I CANNOT OVEREMPHASIZE THE CARELESSNESS AND RECKLESSNESS OF THE COMMUTER CAPT IN GETTING SO CLOSE TO A MUCH LARGER AIRPLANE -- HIS TCASII HAD TO BE GOING NUTS ALSO! THIS WAS EXTREMELY CLOSE. WHEN THE COMMUTER CAPT PASSED US HE COULD NO LONGER HAVE KEPT VISUAL SEPARATION. HIS VIEW OF VISUAL SEPARATION DOESN'T COME CLOSE TO MINE. WE NEED A BETTER DEFINITION THAN WHAT WE HAVE NOW. WE NEED BETTER PROCS AT ARPTS WITH CLOSE PARALLEL RWYS. THIS CAN BE DONE WITH AN ATC REG OR LCL PROC. THE TWR SUPVR HAD NO IDEA HOW CLOSE WE WERE.