PILOT IN S50 TRAFFIC PATTERN REPORTS CRITICAL CONFLICT WITH HIGH SPEED SOUTHBOUND SMA.

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THIS FLT WAS A LCL PROFICIENCY FLT. FOLLOWING DEP; I FLEW OUT TO THE PRACTICE AREA AND PERFORMED A FEW STEEP TURNS AND RETURNED TO THE PATTERN AT AUBURN MUNICIPAL (S50) FOR TOUCH-AND-GOES ON RWY 34R. AFTER THE THIRD TOUCH-AND-GO I WAS ON THE XWIND LEG; JUST ABOUT TO TURN DOWNWIND; AT APPROX 900 FT MSL WHEN A VERY SHARP SHADOW PASSED OVER ME. I TURNED DOWNWIND TO SEE A SBOUND SMA ONLY SLIGHTLY ABOVE MY ALT TRACKING STRAIGHT DOWN THE DOWNWIND TRACK FOR RWY 34R. THE ACFT'S TRACK WAS SBOUND PARALLEL TO AUBURN'S RWY AND BTWN THE EMERALD DOWNS RACETRACK AND HWY 167 HOWEVER; THE ACFT WAS IN THE CLEAN CONFIGN; APPEARED TO BE AT CRUISE SPD AND DEPARTED THE ARPT TO THE S. ESTIMATED VERT SEPARATION WAS LESS THAN 500 FT AND LATERALLY APPROX 500 FT. ACTUAL MISS DISTANCE IS DIFFICULT TO DETERMINE AS THE INTRUDER WAS NOT SIGHTED UNTIL 10-15 SECONDS AFTER PASSING. I WAS MONITORING AND XMITTING ON THE S50 FREQ; DURING THE ENTIRE FLT AND HAD JUST RPTED THE XWIND LEG WHEN THE NEAR COLLISION OCCURRED. ONLY 1 OTHER ACFT; A CESSNA SKYLANE APPROX 7 MI SE INBOUND; WAS ON THE FREQ FOR S50 AND THE FREQ WAS RELATIVELY QUIET AT THE OTHER 2 OR 3 ARPTS THAT ARE IN RECEPTION RANGE FROM THE S50 PATTERN. NO XMISSIONS WERE HEARD FROM THE SMA AT ANY TIME; NOR DID IT RESPOND TO QUERIES ON THE CTAF AFTER THE CONFLICT. TFC TRAVELING N OR S ALONG THE VALLEY IS COMMON. HELI TFC OFTEN FOLLOWS HWY 167; BUT USUALLY AT 500 FT AGL; AND FIXED WING TFC SOMETIMES PASSES OVER THE W SIDE OF THE PATTERN AT 1500 FT MSL. TFC OVER THE FIELD IS RESTR BY THE SEATTLE CLASS B FLOOR WHICH IS 1600 FT JUST TO THE W OF S50. THE MAJORITY OF N AND S FIXED WING TFC USUALLY TRACKS E OF S50. MOST OF THE TRANSITING TFC ANNOUNCES ON CTAF; BUT NOT ALL. I AM NOT AWARE OF ANY PUBLISHED VFR TRANSIT RTE FOR THE AUBURN AREA; BUT THIS IS A CONTINUING THREAT TO TFC OPERATING AT S50.