Narrative:

Departed atl IFR, cruising at assigned 4000' for approximately 15 mins. While scanning for other aircraft, caught sight of a white with blue trim small aircraft descending from the 8 O'clock high position and overtaking my aircraft (small transport). My cruise speed was 178 KTS indicated. A collision appeared imminent. An immediate dive to 3500' was executed and that was followed by a 360 degree turn to the right in an attempt to locate the small aircraft. When located the small transport was level at 3500' and still on his original heading of approximately 220 degrees. Approach control was informed of the incident and asked if they had the small aircraft under their control. Approach said there was no other aircraft in the immediate area giving a transponder code. Shortly after approach control said there was a primary return at my 6 O'clock position about 1 mi. I had passed the aircraft by then and climbed back to 4000'. I listened to atl approach control for another approximately 4 mins and was then cleared to contact the columbus, GA, approach control on 126.0 VHF. Shortly after contacting columbus approach control, I heard a same model small aircraft (full call sign) call columbus and was given an expected visibility approach to runway 23 at columbus. I continued to my destination csg and heard the small aircraft change destination to pine mountain, GA. I believe the pilot of the small aircraft was flying either in violation of the atl TCA or very close to it, and for some unknown reason had his transponder turned off to avoid detection. His diving intercept made me think I was back in combat. Ps: the small aircraft had little tip tanks and the small registration #'south . This incident could have been avoided if: a) the pilot had been visually scanning for other traffic, B) the pilot had been in contact with atl approach control, and C) the pilot had been transmitting 1200 code on his transponder. Callback conversation with reporter revealed the following: reporter states he was grateful for his fgt pilot training as he instinctively knew how to avoid the other aircraft. His main concern was the aircraft letting down through the TCA west/O contact or use of the transponder. Safety procedures become non existent. Second concern was that controller did not see the traffic. Consequently no traffic call was made. He did call approach and listen to the tape, but took no further action.

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Original NASA ASRS Text

Title: CORP PLT ON IFR FLT PLAN 'ATTACKED' BY SMA BELOW TCA FLOOR AND NOT IN CONTACT WITH ATC. NMAC.

Narrative: DEPARTED ATL IFR, CRUISING AT ASSIGNED 4000' FOR APPROX 15 MINS. WHILE SCANNING FOR OTHER ACFT, CAUGHT SIGHT OF A WHITE WITH BLUE TRIM SMA DSNDING FROM THE 8 O'CLOCK HIGH POS AND OVERTAKING MY ACFT (SMT). MY CRUISE SPD WAS 178 KTS INDICATED. A COLLISION APPEARED IMMINENT. AN IMMEDIATE DIVE TO 3500' WAS EXECUTED AND THAT WAS FOLLOWED BY A 360 DEG TURN TO THE RIGHT IN AN ATTEMPT TO LOCATE THE SMA. WHEN LOCATED THE SMT WAS LEVEL AT 3500' AND STILL ON HIS ORIGINAL HDG OF APPROX 220 DEGS. APCH CTL WAS INFORMED OF THE INCIDENT AND ASKED IF THEY HAD THE SMA UNDER THEIR CTL. APCH SAID THERE WAS NO OTHER ACFT IN THE IMMEDIATE AREA GIVING A XPONDER CODE. SHORTLY AFTER APCH CTL SAID THERE WAS A PRIMARY RETURN AT MY 6 O'CLOCK POS ABOUT 1 MI. I HAD PASSED THE ACFT BY THEN AND CLBED BACK TO 4000'. I LISTENED TO ATL APCH CTL FOR ANOTHER APPROX 4 MINS AND WAS THEN CLRED TO CONTACT THE COLUMBUS, GA, APCH CTL ON 126.0 VHF. SHORTLY AFTER CONTACTING COLUMBUS APCH CTL, I HEARD A SAME MODEL SMA (FULL CALL SIGN) CALL COLUMBUS AND WAS GIVEN AN EXPECTED VIS APCH TO RWY 23 AT COLUMBUS. I CONTINUED TO MY DEST CSG AND HEARD THE SMA CHANGE DEST TO PINE MOUNTAIN, GA. I BELIEVE THE PLT OF THE SMA WAS FLYING EITHER IN VIOLATION OF THE ATL TCA OR VERY CLOSE TO IT, AND FOR SOME UNKNOWN REASON HAD HIS XPONDER TURNED OFF TO AVOID DETECTION. HIS DIVING INTERCEPT MADE ME THINK I WAS BACK IN COMBAT. PS: THE SMA HAD LITTLE TIP TANKS AND THE SMALL REGISTRATION #'S . THIS INCIDENT COULD HAVE BEEN AVOIDED IF: A) THE PLT HAD BEEN VISUALLY SCANNING FOR OTHER TFC, B) THE PLT HAD BEEN IN CONTACT WITH ATL APCH CTL, AND C) THE PLT HAD BEEN XMITTING 1200 CODE ON HIS XPONDER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR STATES HE WAS GRATEFUL FOR HIS FGT PLT TRNING AS HE INSTINCTIVELY KNEW HOW TO AVOID THE OTHER ACFT. HIS MAIN CONCERN WAS THE ACFT LETTING DOWN THROUGH THE TCA W/O CONTACT OR USE OF THE XPONDER. SAFETY PROCS BECOME NON EXISTENT. SECOND CONCERN WAS THAT CTLR DID NOT SEE THE TFC. CONSEQUENTLY NO TFC CALL WAS MADE. HE DID CALL APCH AND LISTEN TO THE TAPE, BUT TOOK NO FURTHER ACTION.

Data retrieved from NASA's ASRS site as of August 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.