On June 19, 1999, approximately 0815, a Piper PA-28-140, N9887W, sustained substantial damage during landing at Zillah, Washington. The commercial pilot and his two passengers were uninjured. No flight plan was filed for the flight, which initiated at Prosser, Washington, about 20 minutes earlier. Visual meteorological conditions prevailed at the time of the accident. There was no fire, and no report of an ELT actuating. In a written report, the pilot noted that the winds were calm. He approached to the east, into the morning sun, in order to land upslope on the grass airstrip. He said he positioned the plane in the center of the light green fresh mowed area, which he learned after landing was not all runway. He stated that the sun was in his eyes on final approach, obscuring details of the landing area. He touched down about 250 feet from the approach end with full flaps and a touch of power, using standard soft field techniques. The airplane slid down the side slope to the left and would not respond to right rudder inputs, until the left wing contacted a large wooden fence post and about four feet of the left wingtip separated. He stated that the day before the flight he obtained an airport briefing from a local pilot familiar with the airport. Sun azimuth at the time of the accident was 109.9; elevation was 49.4 degrees.