PART 135 ACR FAILS TO MAINTAIN VISUAL ON TFC WHILE MAKING PARALLEL VISUAL APCHS.

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WE WERE FLYING N FROM AUBURN ON THE CHINS ARR COMING FROM YKM TO SEA. WE WERE CLRED VISUAL APCH RWY 16L TO FOLLOW A COMPANY DORNIER BEHIND US AT 8 O'CLOCK POS GOING TO 7 O'CLOCK POS ON FINAL FOR RWY 16L. WE WERE THEN TOLD TO MAINTAIN VISUAL SEPARATION ON AN ACR DC-8 TO N AT 12 O'CLOCK POS (XING R TO L FROM THE E). THE DC-8 WAS ABOUT TO TURN FINAL FOR THE R RWY 16R. SO WE'RE HDG N WHILE DIRECTED TO FOLLOW AN ACFT TO THE S WHICH USUALLY ISN'T A PROB SINCE WE WOULD NORMALLY TURN TO FOLLOW. HOWEVER; WE EXTEND N TO AVOID FLYING IN FRONT OF THE DC-8 TURNING FINAL. WE REDUCE PWR AND DROP INSIDE HIS TURN MAINTAINING VERT AND LATERAL SEPARATION STAYING AFT OF HIS WINGLINE. NATURALLY OUR POS RELATIVE TO THE DC-8 MOVES FROM HIS 7 O'CLOCK POS TO 8 O'CLOCK POS TOWARDS 9 O'CLOCK POS AS BOTH ACFT ROLL OUT ON FINAL. THE DC-8 CREW OBSERVES US ON THEIR TCASII AND THE CTLR MENTIONS OUR POS AS CHANGING FROM 7 O'CLOCK POS TO 8 O'CLOCK POS TOWARDS 9 O'CLOCK POS OR 10 O'CLOCK POS. THE MENTION OF THIS APPARENTLY UPSETS THE DC-8 CREW; AND DISCUSSION FOLLOWS ABOUT STAYING BEHIND AN ACFT WHEN MAINTAINING VISUAL SEPARATION. YOU MIGHT GUESS WHAT HAPPENS NEXT. HE DROPS HIS GEAR; WHICH WE HAVE YET TO DO WITH THE DISTRS; AND WE MOVE OUT IN FRONT. THIS IS THE VERY THING THAT EACH OF US WANTED TO AVOID IN THE FIRST PLACE. I SPENT SOME TIME ON THE PHONE OVER THIS; BUT DIDN'T HEAR FROM OTHER PLT. TALKING OVER THE PHONE WE AGREED THAT WHAT'S MOST IMPORTANT IS THAT WE'RE ALL AWARE OF THE CHANCE FOR; AND THE CAUSES OF; THESE POTENTIAL PROBS. SECONDLY; THAT WE WORK TOGETHER SINCE WE'RE ALL IN THE SAME BOAT; REGARDLESS OF WHICH SEAT YOU OCCUPY: PLTS; CTLR; SUPVR; EVEN COMPANY OPS; ETC. I'M NOT UP ON ALL THE ASRS CALLBACK LETTERS/RPTS; BUT THIS ONE BEARS MENTIONING. IF NOTHING ELSE TO AVOID VIOLATIONS. HAVING AN FO LOOKING OVER HIS SHOULDER TO MAINTAIN A VISUAL TAKES HIM OUT OF THE COCKPIT AWAY FROM THE TASK IN FRONT OF US (IE; LNDG AND FORWARD VISUAL LOOKOUT). FURTHER STAYING BEHIND HELPS MAINTAIN VISUAL SEPARATION DESPITE THE LIKELY TCASII DISTR IN THE ADJACENT ACFT. FINALLY WHEN IT COMES TO MAINTAINING CTL OF AN ACFT IT IS IMPORTANT TO REALIZE WHEN SOMETHING IS DRAWING YOUR ATTN AWAY FROM THE TASK AT HAND. WHEN AN ACFT DOESN'T RESPOND PROPERLY TO A CTL INPUT (IE; PWR REDUCTION TO SLOW) CHK YOUR CONFIGN! CHK YOUR CONFIGN!