On December 21, 2004, about 1330 central standard time, a Cessna 421C twin-engine airplane, N369RK, was substantially damaged when the nose landing gear collapsed during taxi, following a landing at the Rusk County Airport (F12), near Henderson, Texas. The two instrument-rated commercial pilots were not injured. The airplane was registered to Yates Air LLC, of Henderson, Texas. Visual meteorological conditions prevailed, and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 maintenance flight. The local flight originated from the Tyler Pounds Regional Airport (TYR), near Tyler, Texas, approximately 20 minutes earlier. According to the pilot, while on a maintenance flight, a rough running engine developed and he elected to make a precautionary landing at Rusk County Airport. After a "normal" landing on runway 16, he back taxied and was turning onto taxiway bravo, when the airplane made an uncontrolled right turn, followed by the collapse of the nose landing gear. The pilot proceeded to shut down and secure the engines, and both pilots exited the airplane. A witness, who observed the incident, reported the airplane was taxiing at a slow speed when the nose landing gear collapsed. An aircraft mechanic (A&P) from the Waco Regional Airport inspected the airplane. The A&P reported that the nose gear trunnion lug had fractured, resulting in the nose gear collapse and structural damage to the nose gear well. Inspection of the failed nose gear trunnion lug was addressed in Cessna Service Bulletin MEB88-5 Revision 2. The stated purpose of the Service Bulletin is as follows: "Field reports of cracks in the nose landing gear trunnion lugs on some airplanes indicate the need for an inspection of the trunnion to ensure cracks are not present. Non-compliance with this service bulletin may result in failure of the nose landing gear trunnion which could cause substantial damage to the airplane and possible injury to the airplane occupants and/or ground personnel." A review of the airplane logbooks by an A&P mechanic on behalf of the National Transportation Safety Board (NTSB), revealed that Service Bulletin, MEB88-5 Revision 2, had not been complied with. At 1353, the weather observation facility at Tyler Pounds Regional Airport, located approximately 30.7 miles northwest of the accident site, reported wind from 190 degrees at 18 knots gusting to 22 knots, visibility 10 statute miles, clouds scattered at 3,400 feet, temperature 72 degrees Fahrenheit, dew point 55 degrees Fahrenheit, and a barometric pressure setting of 29.81 inches of Mercury.