Narrative:

I changed my squawk to VFR and changed to CTAF; reporting 10 miles out and then reporting left downwind for runway 20. There was a learjet on the frequency approaching from the west; subsequently reporting right base for runway 20. A helicopter; not landing at the airport; reported west of the airport and transiting the area. I picked up the lear visually and extended my downwind slightly in order to follow it to the runway. I reported such over the unicom frequency. I thought that the helicopter had reported both the lear and myself in sight and wrongly assumed that he would cross the airport clear of the traffic pattern. Although I was searching for the helicopter; I did not see it. When I was about 300-400 AGL and established on final my passenger saw the helicopter low and to our right and pointed it out to me at which time I pulled up hard to go above the helicopter. Without the evasive action we would have passed very close. I made a radio call announcing that I was maneuvering to avoid the helicopter. The helicopter announced on the radio that he had me in sight. The helicopter pilot had chosen a very bad location to cross the final approach course. He was aware of the runway in use and that there were at least two airplanes; the lear jet and myself; approaching the airport.although I had flight planned for this flight and realized that there was right traffic for runway 20; by the time I entered the traffic pattern I had forgotten and entered the left downwind. Only three days later when I was wondering why the lear had made a 'non-standard' right base did I remember that I was the one who was non-standard when flying a 'standard' left hand pattern.

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Original NASA ASRS Text

Title: A PA32 pilot; mistakenly flying a left hand vice the published right hand pattern at an non-towered airport experienced an NMAC with a helicopter on short final.

Narrative: I changed my squawk to VFR and changed to CTAF; reporting 10 miles out and then reporting left downwind for Runway 20. There was a Learjet on the frequency approaching from the west; subsequently reporting right base for Runway 20. A helicopter; not landing at the airport; reported west of the airport and transiting the area. I picked up the Lear visually and extended my downwind slightly in order to follow it to the runway. I reported such over the UNICOM frequency. I thought that the helicopter had reported both the Lear and myself in sight and wrongly assumed that he would cross the airport clear of the traffic pattern. Although I was searching for the helicopter; I did not see it. When I was about 300-400 AGL and established on final my passenger saw the helicopter low and to our right and pointed it out to me at which time I pulled up hard to go above the helicopter. Without the evasive action we would have passed very close. I made a radio call announcing that I was maneuvering to avoid the helicopter. The helicopter announced on the radio that he had me in sight. The helicopter pilot had chosen a very bad location to cross the final approach course. He was aware of the runway in use and that there were at least two airplanes; the Lear jet and myself; approaching the airport.Although I had flight planned for this flight and realized that there was right traffic for Runway 20; by the time I entered the traffic pattern I had forgotten and entered the left downwind. Only three days later when I was wondering why the Lear had made a 'non-standard' right base did I remember that I was the one who was non-standard when flying a 'standard' left hand pattern.

Data retrieved from NASA's ASRS site as of July 2013 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.