On October 25, 2003, about 1150 Pacific daylight time, a Piper PA20, N7409K, ground looped during landing at Petaluma Municipal Airport (O69), Petaluma, California. The pilot was operating the airplane under the provisions of 14 CFR Part 91. The private pilot, the sole occupant, was not injured; the airplane sustained substantial damage. The personal cross-country flight departed Sonoma Skypark (0Q9), Sonoma, California, about 1130, en route to Petaluma Municipal. Visual meteorological conditions prevailed, and no flight plan was filed. The pilot reported in a written statement that he entered the traffic pattern on the left downwind leg for runway 11 at a 45-degree angle. He configured the airplane to land with full flaps and the engine running at idle. On the landing roll the airplane ground looped to the right and came to rest about 70 degrees from the runway heading. The left wing tip and propeller came into contact with the ground, and the left main landing gear collapsed. The pilot reported that a light, quartering headwind existed, with winds from 170 degrees at 4 knots and gusting to 8 knots. He noted that the airplane had no mechanical discrepancies. A routine aviation weather report (METAR) generated by an Automated Surface Observation System (ASOS) at Napa County Airport (APC), Napa, California (located 15.5 nautical miles east from the accident site), indicated that about 4 minutes after the accident winds were variable at 5 knots.