On June 9, 2000, approximately 1215 Pacific daylight time, a Cessna 182H, N8321S, collided with the terrain after running off the end of the runway during a landing attempt at Stark's Twin Oaks Airport, Hillsboro, Oregon. The private pilot and his passenger received minor injuries, and the aircraft, which was owned and operated by Stark's Twin Oaks Airport, Inc., sustained substantial damage. The 14 CFR Part 91 work use flight, which departed Troutdale Airport, Portland, Oregon, about thirty minutes earlier, was being operated in visual meteorological conditions. No flight plan had been filed, and there was no report of an ELT activation. The pilot of the subject aircraft had flown from Stark's Twin Oaks Airpark to Troutdale Airport to pick up a passenger. Because of scattered rainy conditions and apparent high humidity, the pilot applied carburetor heat during taxi at Troutdale and a number of times while en route to Portland-Hillsboro Airport. Although he did not detect any ice accumulation, his cruise indicated airpeed was about 10 to 12 knots slower than he thought it should have been based upon the throttle and propeller rpm setting. According to the pilot, he felt that the engine might not be producing full power, and he therefore continued to be concerned about ice accumulation in the carburetor throat. Upon reaching Portland-Hillsboro, the pilot was unable to make contact with the tower, and therefore elected to land back at Twin Oaks Airpark, which is about six miles southeast of the Hillsboro Airport. Upon arrival at Twin Oaks, the pilot entered the pattern for runway 02. Although that presented him with a quartering tailwind on final, he elected to land in that direction because local terrain avoidance procedures, and the narrative in the Oregon Airport Guide, dictate landing on runway 02 until the tailwind is 10 knots or more. While on final approach, the pilot continued to find it necessary to use considerably more power than he expected in order to maintain an airspeed above what he thought would cause the aircraft to stall. Because he was concerned about the apparent reduction in power available, the pilot came in high, and elected to execute a go-around about 200 feet above ground level (AGL) when he became uncomfortable with the approach. After the go-around, the pilot initiated a second approach, and once again found that he had to apply excessive amounts of throttle in order to maintain an airspeed above published stall speed. On this second attempt, the pilot intentionally came in high with only 20 degrees of flaps, in case he had to go around again. He reported that although he came in high, he was able to maintain an acceptable airspeed by applying power while descending toward the runway. As the aircraft crossed the threshold, it became obvious to the pilot that although his indicated airspeed was near what he desired, his ground speed was excessive. At that point, believing he was experiencing a deteriorating engine condition, the pilot elected to continue his attempt to land instead of executing a second go-around. Because of the excess speed, the aircraft floated and did not touch down until it was about half way down the 2,150-foot treated-gravel wet runway. The pilot then applied maximum braking, but was unable to bring the aircraft to a stop before it slid off the end of the runway and went over an embankment. In a post-accident phone interview, the pilot stated that just as he was touching down he realized that he would probably go off the end of the runway, but he thought that if he tried a go-around, he might collide with the trees on the rising terrain just off the departure end of the runway. Although a post-accident engine inspection revealed no evidence of any anomaly or malfunction that would have precluded normal engine operation, a visual inspection of the pitot mast revealed system contamination. The contamination consisted of a one-centimeter ball of mud and insect parts in the lower portion of the mast, and what appeared to be insect wings or other organic material higher in the mast near the top fitting. The size and structure of the contaminants were such that they could interfere with the correct display of the aircraft's indicated airspeed.