On December 31, 1993, at 1930 central standard time, a Hughes 369HS helicopter, N7092Q, was destroyed on impact following an uncontrolled descent approximately 12 miles southwest of Mason, Texas. The pilot received serious injuries, the student pilot passenger minor injuries, and the helicopter was destroyed. The flight departed Kimble County Airport, Junction City, Texas, approximately 20 minutes prior to the accident. A VFR flight plan was filed and visual meteorological conditions prevailed for this personal transportation flight with a destination of Orlando, Florida. According to the pilot, the helicopter began to side slip and he interpreted this to be a tail rotor failure. He said he entered autorotation and impacted the ground during the flare. Information in this paragraph is based on the passenger's statement. The helicopter began to turn to the right and he asked the pilot what was going on. The pilot did not respond and he was unresponsive and appeared to be in a dazed state. The helicopter then pitched nose down followed by a nose up pitch to near vertical. The helicopter then entered a spiral. During the spiral, the passenger took control of the helicopter and entered autorotation. He noted that during the autorotation, all controls responded normally. During the flare, the pilot grasped the controls and rolled the helicopter into a left wing down attitude and it struck the ground and began to burn. The passenger pulled the pilot from the burning helicopter, made him comfortable, and proceeded to walk for help. (See passenger statement). Due to a chip light indication on the tail rotor gear box the previous day, and maintenance performed as a result of the light, an examination of the tail rotor assembly and tail rotor gear box was conducted at the facilities of McDonnell Douglas on March 14, 1994. The examination was witnessed by an FAA airworthiness inspector from the Scottsdale, Arizona, Flight Standards District Office. No evidence of preimpact failure or malfunction of the tail rotor drive, assembly, or gear box was found. Based on information provided by the student pilot passenger, discussions were held with Dr. DeJohn at The FAA Civil Aviation Medical Institute (CAMI), regarding possible pilot incapacitation. As a result of those discussions, Dr. De John expressed the opinion that the pilot displayed symptoms of atypical epileptic form seizure.