Narrative:

I made a 45 NM flight from N57 to N76, under radar coverage from harrisburg approach. 4 NM from N76, the controller cancelled coverage with no comments about traffic in the vicinity of N76. I switched to CTAF 123.0 and called 4NM out, then since I was in position to directly enter left base runway 29, I did so and called my position. I saw no traffic in the pattern, although my new tpas warned me of an aircraft within 2NM. As the tpas gives only range and not bearing, I suspected the traffic might be heading to a nearby airport since no one was responding on N76's CTAF. I turned final and made the radio call, still no response from other traffic. Now, the tpas began to display a rapidly decreasing range, down to >0.4NM. Suspecting I was descending on another plane on final, I leveled off went around, not climbing in case the traffic was above me. At midfield and at 100 ft AGL, I heard a helicopter make a radio call of short final, then I saw him as I turned crosswind. He completed his touch and go, then flew another tight and very low pattern, completed another touch and go then left the area. Neither I nor the other pilot commented over the radio about this. I suspect that his radio was off until he saw me pass over him on final at this infrequently used rural airport. I see 3 factors contributing to this event and one factor that may well have prevented a tragedy. First, I failed to see and avoid traffic in the pattern, although it was difficult to see a small helicopter flying a nonstandard pattern. Second, the chopper pilot was not using his radio, assuming he was the only one around. Third, the harrisburg controller probably saw the traffic but did not mention it. The major factor in avoiding a collision was the tpas, it made me aware of traffic that I otherwise would not have seen. While it is relatively unsophisticated, giving only approximately range with no bearing and depends on active transponders in the other aircraft, it has nevertheless proved it's value to me. Still, it is not a substitute for 'see and avoid', I need to be more observant.

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Original NASA ASRS Text

Title: AA5 LNDG NON-CTLED ARPT EXPERIENCES CLOSE ENCOUNTER WITH HELI.

Narrative: I MADE A 45 NM FLT FROM N57 TO N76, UNDER RADAR COVERAGE FROM HARRISBURG APCH. 4 NM FROM N76, THE CTLR CANCELLED COVERAGE WITH NO COMMENTS ABOUT TFC IN THE VICINITY OF N76. I SWITCHED TO CTAF 123.0 AND CALLED 4NM OUT, THEN SINCE I WAS IN POSITION TO DIRECTLY ENTER LEFT BASE RWY 29, I DID SO AND CALLED MY POS. I SAW NO TFC IN THE PATTERN, ALTHOUGH MY NEW TPAS WARNED ME OF AN ACFT WITHIN 2NM. AS THE TPAS GIVES ONLY RANGE AND NOT BEARING, I SUSPECTED THE TFC MIGHT BE HEADING TO A NEARBY ARPT SINCE NO ONE WAS RESPONDING ON N76'S CTAF. I TURNED FINAL AND MADE THE RADIO CALL, STILL NO RESPONSE FROM OTHER TFC. NOW, THE TPAS BEGAN TO DISPLAY A RAPIDLY DECREASING RANGE, DOWN TO >0.4NM. SUSPECTING I WAS DSNDING ON ANOTHER PLANE ON FINAL, I LEVELED OFF WENT AROUND, NOT CLIMBING IN CASE THE TFC WAS ABOVE ME. AT MIDFIELD AND AT 100 FT AGL, I HEARD A HELICOPTER MAKE A RADIO CALL OF SHORT FINAL, THEN I SAW HIM AS I TURNED XWIND. HE COMPLETED HIS TOUCH AND GO, THEN FLEW ANOTHER TIGHT AND VERY LOW PATTERN, COMPLETED ANOTHER TOUCH AND GO THEN LEFT THE AREA. NEITHER I NOR THE OTHER PLT COMMENTED OVER THE RADIO ABOUT THIS. I SUSPECT THAT HIS RADIO WAS OFF UNTIL HE SAW ME PASS OVER HIM ON FINAL AT THIS INFREQUENTLY USED RURAL ARPT. I SEE 3 FACTORS CONTRIBUTING TO THIS EVENT AND ONE FACTOR THAT MAY WELL HAVE PREVENTED A TRAGEDY. FIRST, I FAILED TO SEE AND AVOID TFC IN THE PATTERN, ALTHOUGH IT WAS DIFFICULT TO SEE A SMALL HELICOPTER FLYING A NONSTANDARD PATTERN. SECOND, THE CHOPPER PLT WAS NOT USING HIS RADIO, ASSUMING HE WAS THE ONLY ONE AROUND. THIRD, THE HARRISBURG CTLR PROBABLY SAW THE TFC BUT DID NOT MENTION IT. THE MAJOR FACTOR IN AVOIDING A COLLISION WAS THE TPAS, IT MADE ME AWARE OF TFC THAT I OTHERWISE WOULD NOT HAVE SEEN. WHILE IT IS RELATIVELY UNSOPHISTICATED, GIVING ONLY APPROX RANGE WITH NO BEARING AND DEPENDS ON ACTIVE TRANSPONDERS IN THE OTHER ACFT, IT HAS NEVERTHELESS PROVED IT'S VALUE TO ME. STILL, IT IS NOT A SUBSTITUTE FOR 'SEE AND AVOID', I NEED TO BE MORE OBSERVANT.

Data retrieved from NASA's ASRS site as of July 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.