On May 1, 1995, about 1035 hours Pacific daylight time, N2548Y, a Cessna 172, registered to Methow Valley Flyers, Inc., Twisp, Washington, nosed over during landing at the Twisp Airport and was substantially damaged. The private pilot, the sole occupant, received minor injuries. Visual meteorological conditions prevailed and no flight plan had been filed. The personal flight was conducted under 14 CFR 91. The pilot stated that she performed several touch-and-go landings at another airport without any reported problems. She then flew to Twisp Municipal Airport to practice more landings. She entered the traffic pattern and landed uneventfully on runway 28. She then back taxied the airplane for another takeoff. According to the pilot: The takeoff roll seemed normal. I flew the pattern, touched down just beyond the displaced threshold as I'd planned, lowered the nose gear, and couldn't control the direction of the plane.... It veered onto the left and skidded into the dirt. The nose wheel stopped. The rest of the plane kept going and flipped over. According to an FAA certified airframe and powerplant mechanic who was visiting the area at the time of the accident, the nose gear was cocked 90 degrees to one side. Further examination revealed that the nose landing gear assembly center scissor bolt was missing. The bolt secures the connection of the two scissor castings of the nose gear assembly. The bolt was later found about 1,000 feet from the threshold of the departure end of the runway, and 800 feet prior to the final resting site of the wreckage. According to the mechanic, the bolt did not exhibit any mechanical abnormalities. Closer inspection of the bolt revealed that the cotter key was missing, and the cotter key hole was filled with grease, oil residue, and dirt. The mechanic further stated that no cotter key appeared to have been installed on the bolt.