C414 EXPERIENCED CONFLICT DURING VISUAL APCH TO PDK RWY 34 WITH VFR CITATION DEP FROM RWY 20L; WX WAS A FACTOR.

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ARRIVING IN THE AREA OF THE DEST ARPT; WE WERE INITIALLY BEING VECTORED FROM THE SE FOR THE ILS RWY 20L APCH. ATL APCH THEN ADVISED US THERE WAS 'EXTREME' PRECIP ALONG THE FINAL APCH COURSE; AND ASKED IF WE THOUGHT WE COULD 'GET A VISUAL' IF WE WERE CLOSER TO THE ARPT. I REPLIED IN THE AFFIRMATIVE AND THE CTLR CLRED US 'DIRECT PDK; MAINTAIN 3000 FT.' AS WE MOVED TO WITHIN ABOUT 5 MI OF PDK; BOTH I AND MY COPLT (CMEL; CFII; +/-2000 TOTAL TIME) AGREED WE HAD VISUAL CONTACT WITH THE ARPT AND I RPTED 'PDK IN SIGHT;' WHEREUPON WE WERE 'CLRED FOR THE VISUAL APCH' AND HANDED OFF TO PDK TWR. TUNING IN THE TWR FREQ; THERE WAS EXTENDED CONVERSATION REGARDING AN APPARENT ABORTED TKOF WITH AN ACFT ON THE ACTIVE RWY; ANOTHER ACFT SUBSEQUENTLY DIRECTED TO EXECUTE A MISSED APCH; AND A THIRD ACFT ATTEMPTING TO CANCEL IFR AND DEPART VFR. WE CONTINUED TOWARD THE ARPT; EXECUTING THE LNDG CHKLIST; AND ESTABLISHED A LNDG CONFIGN AND +/-700 FPM DSCNT; AND WHEN THE FIRST OPPORTUNITY AROSE; WE CHKED IN WITH THE TWR AND WERE CLRED TO LAND #2 BEHIND A CITATION ON A CLOSE-IN L BASE. WE WERE TOLD TO SQUARE THE TURN TO FINAL (WE WERE ON ABOUT A 230 DEG HDG TO JOIN THE FINAL). WE DID SO AND WERE ASKED IF WE HAD THE CITATION ON FINAL IN SIGHT; WE RPTED WE DID NOT AND WERE SEARCHING. PDK TWR THEN ADVISED US (IN AN URGENT TONE OF VOICE) OF TFC AT 10 O'CLOCK POS; LESS THAN 1 MI (THIS TFC CALL WAS IN MY OPINION THE KEY ELEMENT IN MY AVOIDING A MIDAIR COLLISION). I LOOKED IN THAT DIRECTION AND SAW A CITATION (NOT THE ONE WE WERE TO FOLLOW) AT MY ALT WITH LNDG LIGHTS ON AND GEAR RETRACTED COMING STRAIGHT AT ME; EXTREMELY CLOSE. I TOOK CTL OF THE ACFT AND EXECUTED AN IMMEDIATE AND SIGNIFICANT NEGATIVE 'G' DIVE TO AVOID THE CITATION. I TOLD THE TWR I WAS RPTING A 'NEAR MISS' AND PROCEEDED TO LAND UNEVENTFULLY. AFTER SHUTDOWN; I DETERMINED THAT NONE OF MY 4 PAX WERE INJURED; AND I WAS SHOCKED AT THE DISARRAY IN THE CABIN; WHICH THE PAX TOLD ME WAS CAUSED BY THE RAPID DSCNT. ONE PAX; WHOSE HUSBAND HAS BEEN A PLT FOR MANY YRS; TOLD ME SHE LOOKED OUT THE L WINDOW DURING THE DSCNT AND SAW 'A BIG JET; VERY CLOSE.' WX AT THE TIME WAS NEAR MVFR AND DETERIORATING WITH A GUST FRONT TYPE TSTM LINE APCHING FROM THE W. ARPT WINDS RPTED BY THE TWR WERE USUALLY FROM ABOUT 300 DEGS WITH 25-35 KT GUSTS. RWY 20L WAS APPARENTLY IN USE FOR DEPS. AFTER LNDG (RWY 34) AND DURING A TELEPHONE CONVERSATION WITH PDK TWR PERSONNEL; I LEARNED THAT ATL APCH HAD CHOSEN NOT TO ISSUE AN IFR RELEASE TO THE CITATION BECAUSE HIS RTE OF FLT WOULD HAVE TAKEN HIM IN TO THE SEVERE WX. THAT WAS APPARENTLY HIS REASON FOR DEPARTING VFR. I WOULD NOT HAVE DEPARTED IN TO THAT WX IN ANY ACFT WITH ALL THE EXPERIENCE IN THE WORLD ON MY SIDE. WE COULD HAVE AVOIDED THIS NMAC IF THE PF HAD MAINTAINED VISUAL CONTACT WITH THE RWY AND SEARCHED FOR THE ACFT TO FOLLOW; WHILE THE PNF HAD MAINTAINED A GENERAL VISUAL SCAN FOR OTHER TFC IN THE AREA. THIS DID NOT HAPPEN IN MY OPINION BECAUSE BOTH PLTS WERE SEASONED CFII'S ACCUSTOMED TO FLYING EITHER SINGLE PLT OR WITH PLTS OF MARGINAL OR NO EXPERIENCE; FORCING US IN TO A 'SINGLE PLT' THOUGHT/BEHAVIOR PATTERN WHERE WE WERE BOTH FOCUSED ON THE RWY AND THE TFC TO FOLLOW. EXPERIENCED CFII'S WHO ARE MOVING (OR HAVE ALREADY MOVED) FROM THE FLT SCHOOL ENVIRONMENT TO MORE CONVENTIONAL FLYING EMPLOYMENT SHOULD BE STRONGLY ADVISED TO BE AWARE OF THIS TRAP AND HOW TO AVOID IT.