CLOSE PROX MLG GA-SMA ON APCH TO BFL.

Narrative

DURING AN ILS APCH TO BOEING FIELD; A LIGHT SINGLE ENG ACFT WAS SPOTTED AND EVASIVE ACTION TAKEN TO AVOID COLLISION. I WAS PIC AND WAS MANUALLY FLYING AT THE TIME. THE APCH WAS IN EXCELLENT VISUAL FLT CONDITIONS AND I WAS FLYING ILS RAW DATA FOR REF; BUT PRIMARILY USING VISUAL CUES TO FLY THE APCH. THE FINAL PORTION OF THE APCH IS OVER A HEAVILY INDUSTRIALIZED AREA. THE COPLT CALLED TFC DEAD AHEAD. ALTHOUGH I HAD JUST LOOKED STRAIGHT AHEAD; I HAD NOT SEEN THE ACFT. I LOOKED AGAIN AND WAS THEN ABLE TO SEE HIM AGAINST THE MULTIPLE PATTERNED BACKGROUND. THE COLORING OF HIS ACFT DID NOT CONTRAST WITH THE GND AND SINCE WE WERE ON AN OVERTAKING COLLISION COURSE (HE WAS ALSO ON THE ILS LOC); THERE WAS LITTLE RELATIVE MOTION. WE LEVELED OFF AND TURNED RIGHT TO AVOID COLLISION. AFTER EVASION; WE RE-ESTABLISHED ON THE ILS AND COMPLETED THE APCH. AFTER THE INCIDENT; WE SPOKE WITH THE TWR PERSONNEL. THEY STATED THAT THE SMA WAS IN RADIO CONTACT WITH TWR AT THE TIME OF THE INCIDENT. HE HAD NO TRANSPONDER AND TWR WAS TRYING TO JUDGE RELATIVE DISTANCES BTWN US VISUALLY WITH THE AID OF BINOCULARS. THE TWR CTLR DID NOT PERCEIVE A POTENTIAL CONFLICT SINCE HE SAW THE SMA AS BEING BEHIND US. NEITHER THE TWR NOR THE PLT OF THE SMA WAS AWARE OF A PROB UNTIL WE TOOK EVASIVE ACTION. AT THAT TIME; I ESTIMATE THAT THERE WAS APPROX 20 SECS UNTIL IMPACT. ALTHOUGH BOTH MYSELF AND MY COPLT WERE SCANNING OUTSIDE; ONLY HE SAW THE ACFT IN TIME TO PREVENT THE DEVELOPMENT OF AN EXTREMELY DANGEROUS SITUATION. PERHAPS NON IFR ACFT SHOULD USE THE STANDARD VISUAL ENTRIES TO THE FIELD AND NOT THE INSTRUMENT APCH.