HELICOPTER PILOT DEPARTING BFI CLASS D TO THE NORTHWEST REPORTS NMAC WITH PA28 AT 1000 FT.

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DEPARTED HELI PAD WITH A CLRNC FROM BOEING TWR (BFI) TO DEPART TO THE N AND TURN WBOUND OVER ELLIOTT BAY. ACFT X CLBED TO 1000 FT MSL AND TURNED WBOUND OVER ELLIOTT BAY AFTER CLRING THE DOWNTOWN AREA HIGH BUILDINGS AND OBSTRUCTIONS. ACFT X RECEIVED AND ACKNOWLEDGED A TFC CALL CONCERNING HELI TFC LOW TO THE R FRONT AND AHEAD. AS ACFT X CROSSED THE SHORELINE; A PIPER CHEROKEE LOW WING ACFT APPEARED SLIGHTLY ABOVE AND TO THE R CREATING A POTENTIAL MIDAIR. ACFT X TOOK EVASIVE ACTION TURNING TO THE R AND DSNDING. THE CLOSEST APPROX DISTANCE BTWN ACFT WAS 300 FT. THE OTHER ACFT MADE NO TURNS NOR EVASIVE MANEUVERS. THE EVENT TOOK PLACE JUST N OF BOEING FIELD (BFI) CLASS D SURFACE AREA. ONE CREW MEMBER WAS SEATED IN THE L FRONT SEAT NEXT TO THE PLT AND THE OTHER IN THE CABIN; AFT OF THE PLT ON THE R SIDE. THE CHEROKEE DID NOT BECOME VISIBLE UNTIL VERY LATE BECAUSE IT WAS VEILED BY THE WINDSCREEN FRAME ABOVE THE WINDSCREEN OF ACFT X. AFTER THE NEAR MIDAIR WHEN BACK AT BASE; THE PLT CALLED BOEING TWR TO DETERMINE ANY ADDITIONAL HELPFUL INFO FROM THE TAPES. ACCORDING TO BOEING TWR; A CALL WAS MADE THAT GAVE THE BASIC INFO TO ACFT X AS TO THE ALT; DISTANCE; AND LOCATION OF THE CHEROKEE. NEITHER THE PLT NOR THE CREW RECALL HEARING THE COM AS TO THE WHEREABOUTS AND PROX OF THE CHEROKEE; OTHER THAN THAT THERE WAS ADDITIONAL TFC (NO SPECIFIC DISTANCE OR LOCATION) IN THE AREA SBOUND AT 1200 FT MSL. ONE DEBRIEF ASSUMPTION IS THAT THE LATTER HALF OF THE TFC XMISSION WAS MASKED WHEN THE ACFT X PLT ACKNOWLEDGED THE FIRST TFC CALL CONCERNING THE OTHER HELI. STERILE COCKPIT WAS BEING OBSERVED AT THE TIME OF THE NEAR MIDAIR.