On May 19, 2004, approximately 1928 central daylight time, a Beech A23A single-engine airplane, N6943Q, sustained minor damage following an in-flight separation of the propeller assembly during cruise flight near Kress, Texas. The flight instructor and student pilot were not injured. The airplane was registered to and operated by the student pilot. Visual meteorological conditions prevailed, and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 instructional flight. The cross-country flight originated from the Buffalo Airport (1E7), near Amarillo, Texas, at 1850, and was destined for the Hale County Airport (PVW), near Plainview, Texas. The 1,911-hour certificated flight instructor (CFI) reported in a written statement that during cruise flight at 5,500 feet mean seal level (msl), he noticed the windscreen was "covered with a very light oil spray from the engine." A few minutes later, "the engine began to vibrate" and the CFI reduced power. Subsequently, the propeller "come off the nose, hit the cowling, and flew up and to the right over the airplane." At this point the CFI took control of the airplane, established the proper glide speed, and executed a forced landing to the Vaughn Airport (29F), near Kress, Texas, without further incident. The owner of the airplane reported that he was not aware of any anomalies leading to the failure of the propeller assembly, there were no pre-existing vibrations, no propeller changes, and to his knowledge, "no prior incursion or indications of this occurrence." Examination of the airplane by an Federal Aviation Administration (FAA) inspector, who responded to the accident site, revealed the engine crankshaft sheared just aft of the propeller flange. The propeller assembly and portion of the crankshaft were not located. The airplane was powered by a fuel injected four cylinder Continental IO-346-A horizontally opposed engine, serial number 100446-5-A, rated at 165 horsepower, driving an all-metal fixed-pitch Sensenich propeller, model number M74DC-0-60. According to the aircraft maintenance records, an annual inspection was completed on July 7, 1981, prior to the airplane being placed into storage. The most recent annual inspection was completed on January 9, 2004, at a total airframe time of 2,389.8 hours. As of the last annual inspection, it was reported that the engine had accumulated approximately 936 hours since its last major overhaul. Examination of the engine was conducted on August 23, 2004, at the facilities of Teledyne Continental Motors (TCM) near Mobile, Alabama, under the supervision of an FAA inspector. The examination revealed that the crankshaft was sheered just aft of the propeller flange. The connecting rod journals, main journals, and thrust surfaces were undamaged. No visible evidence of heat distress was noted. All engine accessories remained intact and free of anomalies. A metallurgical examination of the fractured area of the crankshaft revealed the fracture (consistent with fatigue) within the nitrided layer of the crankshaft originated from a small circumferential crack (consistent with overload) and progressed to the identification stamp. The origination of the original crack is undetermined. The hardness of the nitrided layer was tested and found to be within TCM's manufacturer specifications.