A C172 PLT EXPERIENCED AN NMAC WITH A DECATHLON IN VFR CONDITIONS IN THE VICINITY OF 9G3. WHILE THE C172 PLT WAS TRYING TO SPOT THE DECATHLON HE DSNDED TO 500 FT AGL.

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I HAD BEEN PRACTICING VFR FLT 10-15 MI NE OF 9G3; REMAINING IN CONSTANT CONTACT WITH LCL APCH CTL FOR TA'S; AND WAS DSNDING WBOUND TOWARD THE ARPT FOR A LNDG. THERE WAS A DECATHLON SOME FEW MI N OF THE ARPT; ALSO TALKING TO APCH; FLYING SBOUND AND STATING HIS INTENTION TO LAND AT THE SAME ARPT. AS I DSNDED TOWARDS THE ARPT; I HEARD APCH TELL THE DECATHLON THAT THEY HAD LOST HIS ALT READOUT (OR POSSIBLY HIS ENTIRE XPONDER RETURN; I DON'T RECALL EXACTLY WHICH); AND SOON THEREAFTER; HE STOPPED ANSWERING THEIR CALLS; ALTHOUGH THEY HAD (AT LEAST) A PRIMARY RADAR RETURN ON HIM. I ASKED APCH TO STAY WITH ME UNTIL HE WAS NO FACTOR; AND APCH COMPLIED. APCH AND I HAD A FAIRLY LIVELY SERIES OF XMISSIONS; WITH APCH TRYING TO CONTACT THE DECATHLON WHILE SIMULTANEOUSLY TRYING TO POINT HIM OUT TO ME WITH 'ALT UNKNOWN.' THE APCH CTLR WAS EXCELLENT -- VERY PROFESSIONAL AND EXTREMELY HELPFUL. THE DECATHLON THEN SWITCHED TO CTAF AT 9G3; AND ANNOUNCED HIS INTENTION TO LAND THERE (I WAS LISTENING ON BOTH APCH AND CTAF). I TURNED S TO GET AWAY FROM THE ARPT TFC PATTERN (AND THE DECATHLON; PRESUMABLY) AND TO BE PREPARED TO ENTER THE PATTERN DOWNWIND AFTER THE DECATHLON LANDED. I CALLED HIM ON CTAF TO GET HIS LOCATION; AS I WAS UNABLE TO VISUALLY LOCATE HIM; BUT GOT NO ANSWER. I ASKED APCH FOR VECTORS AWAY FROM HIM; AS THE DECATHLON AND I WERE PROBABLY CLOSE BUT I COULDN'T FIND HIM VISUALLY. THEY GAVE ME A VECTOR; AND I FLEW IT; BUT WHEN I ASKED APCH WHERE HE WAS IN RELATION TO ME; APCH TOLD ME HE WAS RIGHT IN FRONT OF ME! I STILL COULDN'T FIND HIM. AT THIS POINT I WAS VERY STRONGLY CONSIDERING THE POSSIBILITY OF AN IMMINENT MIDAIR COLLISION WITH AN ACFT I KNEW WAS THERE BUT COULD NOT FIND; AND BEGAN A R TURN TO THE N TOWARD THE ARPT TO TURN AWAY FROM WHAT I BELIEVED WAS UNSEEN TFC RIGHT IN FRONT OF ME. AS I ALMOST FINISHED MY TURN; I SPOTTED HIM ABOUT 200 FT BELOW ME AND ABOUT 500 FT TO THE E (OFF MY R WING) HEADING SBOUND. I DECIDED THE SAFEST THING TO DO WAS TO CONTINUE N TO ENTER DOWNWIND AS FAST AS POSSIBLE TO KEEP AS MUCH DISTANCE AS POSSIBLE BTWN MYSELF AND THIS PLT WHO WASN'T TALKING TO APCH OR ME. I LEFT THE APCH FREQ; AFTER THANKING THE CTLR FOR ALL HIS HELP; AND I ENTERED DOWNWIND; MADE MY ANNOUNCEMENT; AND CALLED THIS PLT AGAIN -- THIS TIME HE ANSWERED ME. HE TOLD ME HE HAD ME IN SIGHT AND WOULD FOLLOW ME IN. I WAS VERY RELIEVED; TO SAY THE LEAST; THAT HE COULD NOW AT LEAST SEE AND AVOID ME. AS I REGAINED MY COMPOSURE; I NOTICED THAT I WAS ABOUT 500 FT AGL ON DOWNWIND; WHERE I SHOULD HAVE BEEN AT 1000 FT AGL. WITH HIM HAVING ME IN SIGHT; AND MY BEING AT THE POINT WHERE I SHOULD TURN A SHORT BASE LEG; I DECIDED TO STAY AT THIS IMPROPERLY LOW ALT; RATHER THAN CLB UP; POSSIBLY INTO THIS PLANE WHICH WAS BEHIND (AND PRESUMABLY ABOVE) ME BUT WHICH I COULDN'T SEE. I MADE AN UNEVENTFUL LNDG; CLRING THE RWY AT MIDFIELD JUST AS THE DECATHLON WAS TOUCHING DOWN -- WHICH MAKES ME THINK HE HAD BEEN PRETTY CLOSE BEHIND ME. THE AREA OVER WHICH I FLEW ON DOWNWIND IS NOT CONGESTED; BUT DOES HAVE A FEW HOUSES THERE. I'M NOT SURE HOW I GOT THAT LOW -- I CLRLY LOST CONCENTRATION ON MY ALT WHILE TRYING TO AVOID A COLLISION. ONCE I REALIZED WHAT MY ALT ON DOWNWIND WAS; I THINK MY DECISION TO REMAIN AT THAT ALT WAS CORRECT; GIVEN THE UNSEEN TFC BEHIND ME; THE FACT THAT HE STATED HE COULD NOW SEE ME; AND THE PROX TO THE POINT AT WHICH THAT ALT WOULD SOON BE CORRECT. THE OTHER PLT (A VERY INTELLIGENT GUY -- WELL KNOWN TO ME AND OTHERS ON THE FIELD) AND I HAD A VERY POLITE DISCUSSION AT THE FBO. MY STATEMENT ABOUT HIS NOT RESPONDING TO APCH OR MY CALLS TOOK HIM BY SURPRISE; AS DID THE FACT THAT APCH LOST HIS XPONDER EITHER PARTIALLY OR TOTALLY. HE CLAIMS HE NEVER HAD A PROB WITH HIS RADIOS BEFORE; AND WOULD LOOK INTO THE MATTER. I BELIEVE HIM. WHILE I KNOW MY DEV TO A LOWER THAN STANDARD ALT ON DOWNWIND SHOULD NOT HAVE HAPPENED; AND I KNOW I'M SUPPOSED TO 'AVIATE; NAVIGATE; COMMUNICATE' IN THAT ORDER; THE MENTAL IMAGE OF AN UNSEEN ACFT DESCRIBED BY APCH AS 'RIGHT IN FONT OF YOU' IS A POWERFUL DISTR. MY GUESS -- AND IT'S ONLY A GUESS -- IS THAT WHEN I DIDN'T SEE HIM IN FRONT OF ME; I LOWERED THE NOSE A BIT TO SEE LOWER; AND NEVER RAISED IT; OR UNCONSCIOUSLY KEPT IT A BIT LOW JUST TO INCREASE MY VIEW BELOW ME; AS I DID NOT SEE HIM EITHER IN FRONT OF ME OR ABOVE ME. ANOTHER POSSIBILITY IS THAT WHEN I FINALLY DID SEE THE DECATHLON HEADING IN THE OPPOSITE DIRECTION BELOW MY R WING; AND SAW THE RWY AHEAD OF ME; I UNCONSCIOUSLY LOWERED THE NOSE TO INCREASE MY AIRSPD. I STILL WONDER WHY I COULDN'T SEE THIS DECATHLON MORE READILY AGAINST THE GND OR SKY. I WAS CONSCIOUSLY SEARCHING ONE SECTOR OF SKY OR GND AT A TIME; BUT WAS ONLY ABLE TO FIND HIM ONCE; AS I STATED ABOVE. WHAT WOULD I DO DIFFERENTLY NEXT TIME? I'M NOT SURE; BUT I THINK I'D TRY TO STAY HIGHER -- WELL ABOVE THE TFC PATTERN ALT; AND PROBABLY GO FURTHER AWAY FROM THE ARPT NEXT TIME; BUT DOING THIS HAS ITS OWN SET OF RISK FACTORS ALSO. IT'S EASY TO SAY THAT HAD THE DECATHLON ENTERED UPWIND OR A MIDFIELD DOWNWIND AS HE APCHED THE RWY (17/25) FROM THE N; THIS PROB WOULDN'T HAVE HAPPENED. ON THE OTHER HAND; I ALWAYS ASSUME THAT THERE IS SOMEBODY WITH NO XPONDER OR RADIO FLYING WHERE HE SHOULDN'T BE AND IT'S MY JOB TO SEE AND AVOID HIM WHILE NOT GETTING DISTR FROM FLYING MY AIRPLANE. ON THIS FLT; I DID EVENTUALLY SEE AND AVOID; BUT NOT WITHOUT GETTING DISTR FROM FLYING THE AIRPLANE.