On October 23, 2006, at 1015 mountain standard time, a Cessna U206G airplane, N206LL, was substantially damaged during a porpoise and ground loop while landing on runway 3 at the Cochise County Airport, Willcox, Arizona. The private pilot, who was the sole occupant, was not injured in the event. The pilot operated the airplane under the provisions of 14 CFR Part 91 as a personal flight. The local flight began about 5 minutes prior to the accident. Visual meteorological conditions prevailed, and a flight plan had not been filed. According to the pilot, he flew the aircraft on the day prior to the accident with the intent to conduct touch-and-go takeoff and landings. He said that during the first landing, the airplane "started to jump all over." The pilot applied full throttle and aborted the landing. He returned the airplane for a full stop landing and came in "as low and slow" as possible. When the airplane touched down it began to "jump around" again and eventually came to a stop. The pilot noticed that the airplane "did not want to steer straight" so he asked the local mechanic to examine the aircraft. The following morning, the day of the accident, the mechanic finished his work on the airplane. According to the statement provided by the mechanic, he corrected the oscillation problem by checking the strut, attachments, wheel and tire, adding air pressure to the strut and tire, and greasing the scissor link. The pilot reported that he received the airplane from the mechanic and decided to conduct more touch-and-go takeoff and landings. During the second touch-and-go landing, upon touchdown, the airplane began "jumping again, striking the prop and going out of control." In a telephone interview with the investigator-in-charge, a witness described the landing as "flat attitude with power." He reported that the airplane began to oscillate, and after several porpoises, the propeller and nose tire impacted the runway simultaneously. The airplane veered to the right, and the left wing and propeller contacted the runway. The pilot added in his statement that the mechanic had installed a rubber bumper on the nose landing gear strut to prevent it from bottoming out. He also indicated that the mechanic installed the bumper without consulting him. The rubber bumper was installed in June 2006 and the airplane had been flown several times prior to the accident. The airplane sustained structural damage to the firewall.