A small aircraft pilot descended below the MDA for Petaluma airport's VOR approach shortly after sunset. A near CFIT event occurred but a nearby tree line provided a visual cue that made him climb back to the VASI.

Narrative

Part 91 flight from ZZZ to O69. Uneventful until the last 2 minutes of flight. The sun set about 20 minutes before landing at O69; and there was a thin broken layer over and south of the airport; with tops at perhaps 1;200-1;400 FT and bases at 1;000-1;200 FT. With the cloud cover; and the surrounding hills reaching to about 1;500 FT MSL; the airport (82 FT MSL) and environment was dark. Cleared for VOR 29 approach to O69; no procedure turn; I flew the approach normally; with a step descent from 1;900 FT MSL at AFTIN down to the MDA of 1;100 FT. That put me right at the base of the clouds. The airport was in sight at a distance of about 5 miles; and so I continued my descent to remain clear of the clouds and kept the airport in clear sight. Then I noticed something; a dark shape; to the left in my peripheral vision. I looked and saw it was a large eucalyptus tree on a small hill about 3 miles out on the extended centerline of the Petaluma Airport's runway. The tree was at most 200 FT below me. I had continued my descent to about 400 FT MSL. I added power and climbed back to about 800 FT; at which point I was back on the VASI 3 degree GS. I have flown this approach many times before. I have taken simulator training which graphically presents the danger of an approach over a dark terrain toward a lighted runway. In spite of that background; I descended well below the visual glide path to 'make sure' I did not re-enter the clouds; and nearly entered the terrain instead. I have for a decade had a self-imposed rule that I will not fly circle-to-land approaches in IMC. This approach made me realize that a straight-in non-precision approach at night has its own hazards. I; therefore; decided that I would add to my personal operations manual this: I will not fly non-precision approaches at night unless there is some form of vertical guidance -- either VASI or GPS-derived; and I will not descend below that glide path reference until reaching the runway. There is nothing particularly new or novel about the incident reported here. I have often read of pilots descending well below the MDA with sad consequences; and wondered why they would do so. Now I see that I; too; can be lulled into complacency and did the same thing. I'll stick to my newly added operations rule in the future.