NMAC BTWN A C150 IN LEVEL FLT AT 5500 FT MSL NBOUND AND A C172 NWBOUND IN A SLOW DSCNT UNDER FLT FOLLOWING PROVISIONS. TA ISSUED TO C172 TO WHICH THE PLT RPTED THE C150 IN SIGHT.

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HOW THE PROB AROSE: ACFT #1 (MINE) WAS NBOUND; TRACKING V-39 TO GVE VOR. MC 032 AT 5500 FT. WINDS WERE FORECAST (AND APPEARED) WESTERLY NEAR 20 KTS; SO MY MAGNETIC HDG WAS ABOUT 010 DEGS. #1 WAS MONITORING BUT NOT IN CONTACT WITH RICHMOND APCH 132.85 SECTOR. ACFT #2 APPARENTLY WAS NWBOUND; LEVEL ABOVE (OR DSNDING VERY SLOWLY INTO) THE SAME ALT; AND EITHER IFR OR VFR ADVISORIES WITH RIC APCH. COURSES CROSSED AT NEAR R ANGLES. HOW THE PROB WAS DISCOVERED: #1 HEARD RIC APCH CALL TFC FOR #2 'AT YOUR 9 O'CLOCK; 1 MI...HDG N AT 5500 FT' OR WORDS TO THAT EFFECT. AFTER A SHORT DELAY; I HEARD #2 INFORM RIC APCH THAT THEY HAD 'TFC IN SIGHT.' RECOGNIZING THAT MY DIRECTION AND ALT MATCHED THE RIC DESCRIPTION; I IMMEDIATELY BEGAN LOOKING FOR #2 ON MY R (E) SIDE; AS I ASSUMED TFC MIGHT BE OUT OF RIC. I ROLLED MY R WING UP BUT WAS UNABLE TO LOCATE #2. I THEN BEGAN LOOKING BEHIND AND BELOW ME; UNDER BOTH WINGS. STILL UNABLE TO LOCATE TFC (I HAD NO KNOWLEDGE OF #2'S ALT); I RETURNED TO FORWARD SCAN AND IMMEDIATELY SPOTTED #2 AT MY 1-2 O'CLOCK POS; ROUGHLY 1/3 MI AHEAD AND ABOVE ME +100-150 FT. ACFT #2 APPEARED TO BE IN NEAR LEVEL FLT ON A NEAR R ANGLE XING COURSE. GIVEN MY HDG AND ALT (ABOUT 5600 FT; MY MODE C READS SLIGHTLY LOW); I LATER CONCLUDED #2 WAS HDG ABOUT 300 DEGS (ASSUMING A SMALLER WCA) AT AN ALT OF ABOUT 5750 FT. I WOULD HAVE EXPECTED LEVEL VFR XING TFC AT AN EVEN-PLUS ALT; YIELDING A 1000 FT SEPARATION. IFR WOULD (SHOULD) HAVE HAD BETTER SEPARATION -- UNLESS DSNDING ENRTE. (POSSIBLE BUT UNLIKELY: THERE ARE MOUNTAINS W OF; AND NO PUBLIC AIRFIELDS ANYWHERE NEAR THIS LOCATION.) CORRECTIVE ACTIONS NEEDED: IT PROBABLY TOOK AT LEAST 3-4 SECONDS TO ASSESS AND CONFIRM A POTENTIAL COLLISION COURSE (IE; CONSTANT ANGLE OF CONVERGENCE); DURING WHICH BOTH ACFT CONTINUED TO CLOSE RAPIDLY. I WENT NOSE DOWN; TO IMMEDIATELY INCREASE VERT SEPARATION; PREPARATORY TO A R TURN TO PASS CLR AFT OR; AND GIVE WAY TO; ACFT #2. AS I DID SO; AND BEFORE MY R TURN WAS EXECUTED; #2 SUDDENLY DIVED THROUGH MY ALT WHILE PASSING R TO L (W) AHEAD OF ME; AT A DISTANCE OF NOT MORE THAN 250 YARDS. I ROLLED R (E) IMMEDIATELY AND PASSED CLR; AFT OF #2 AT NOT MORE THAN 200 YARDS HORIZ; AND -200 FT VERT SEPARATION (#2 NOW BELOW #1). THEREAFTER; I LOST CONTACT WITH #2 AND HEARD NOTHING MORE ON 132.85; FROM EITHER #1 OR RIC. HUMAN PERFORMANCE FACTORS; PERCEPTIONS; JUDGEMENTS; DECISIONS AND ACTIONS OR INACTIONS: GIVEN THAT #2 RPTED 'TFC IN SIGHT;' I WAS SURPRISED THE SIT DEVELOPED AS IT DID. UNDER THE CIRCUMSTANCES; I WAS THE BURDENED ACFT; REQUIRED TO GIVE WAY. TO DO THAT; HOWEVER; ONE MUST HAVE THE OTHER ACFT IN SIGHT; WHICH I DID NOT (IMMEDIATELY). MY FIRST REACTION ON ACQUIRING 32 SO CLOSE BY WAS SURPRISE (AND; PERHAPS; DISBELIEF). THIS ONLY INCREASED RESPONSE DELAY. ONCE I MADE CONTACT AND ASSESSED THE SIT; I PROBABLY SHOULD HAVE TURNED R; BEFORE DROPPING THE NOSE; SO AS TO SIGNAL CLRLY MY SIGHTING OF #2 AND INTENTIONS (TO PASS AFT OF IT). LEARNINGS: FIRST; I LEARNED HOW QUICKLY 'PROX' CAN TURN INTO 'NEAR MISS.' SECOND; WHENEVER OPERATING NEAR N OR S (IE; ACROSS DIRECTIONAL ALT DEMARCATIONS); COURSE/HDG CHANGES; FOR WHATEVER REASON; MAY CREATE CONFLICTS WITH OTHER ACFT AND/OR CONFUSE A RADAR CTLR. ONCE STABILIZED ON TRACK; IT IS IMPORTANT TO HOLD COURSE AS STEADY AS POSSIBLE AT THE APPROPRIATE ALT. THIRD; WHEN ENCOUNTERING CONFLICTING TFC; IT IS IMPORTANT TO HOLD COURSE AS STEADY AS POSSIBLE AT THE APPROPRIATE ALT. THIRD; WHEN ENCOUNTERING CONFLICTING TFC; IT IS IMPORTANT TO MANEUVER SO AS TO GIVE CLR INDICATIONS OF INTENDED ACTIONS TO THE OTHER ACFT. DELAY OR INDECISION ONLY ADDS UNCERTAINTY -- AND RISK. FOURTH; WHENEVER RPTING 'TFC IN SIGHT' IT IS IMPORTANT TO REMEMBER THAT SAME MAY NOT BE THE CASE FOR THE OTHER ACFT. RIGHT-OF-WAY DOES NOT RELIEVE ONE OF RESPONSIBILITY TO AVOID INCIDENTS. I WILL MAKE IT A POINT TO MANEUVER WELL CLR; REGARDLESS. FINALLY; I HAVE REVIEWED SCAN PROCS. THIS WAS SIMPLY TOO CLOSE FOR COMFORT.