?On September 16, 2011, about 1625 Pacific daylight time, an experimental, single-seat North American P-51D, N79111, collided with the airport ramp in the spectator box seating area following a loss of control during the National Championship Air Races unlimited class gold race at the Reno/Stead Airport (RST), Reno, Nevada. The airplane was registered to Aero-Trans Corp (dba Leeward Aeronautical Sales), Ocala, Florida, and operated by the commercial pilot as Race 177, The Galloping Ghost, under the provisions of 14 Code of Federal Regulations Part 91. The pilot and 10 people on the ground sustained fatal injuries, and at least 64 people on the ground were injured (at least 16 of whom were reported to have sustained serious injuries). The airplane sustained substantial damage, fragmenting upon collision with the ramp. Visual meteorological conditions prevailed, and no flight plan had been filed for the local air race flight, which departed RTS about 10 minutes before the accident. The accident airplane was in third place during the third lap of the six-lap race and was traveling about 445 knots when it experienced a left roll upset and high-G pitch up. Subsequently, the airplane entered a right-rolling climb maneuver. During these events, the vertical acceleration peaked at 17.3 G, and, a few seconds later, a section of the left elevator trim tab separated in flight. The characteristics of the airplane's pitch changes during the upset were such that the pilot's time of useful consciousness was likely less than 1 second. As a result, the pilot soon became completely incapacitated, and the airplane's continued climb and helical decent occurred without his control. The accident airplane had undergone many structural and flight control modifications that were undocumented and for which no flight testing or analysis had been performed to assess their effects on the airplane's structural strength, performance, or flight characteristics. The investigation determined that some of these modifications had undesirable effects. For example, the use of a single, controllable elevator trim tab (installed on the left elevator) increased the aerodynamic load on the left trim tab (compared to a stock airplane, which has a controllable tab on each elevator). Also, filler material on the elevator trim tabs (both the controllable left tab and the fixed right tab) increased the potential for flutter because in increased the weight of the tabs and moved their center of gravity aft, and modifications to the elevator counterweights and inertia weight made the airplane more sensitive in pitch control. It is likely that, had engineering evaluations and diligent flight testing for the modifications been performed, many of the airplane's undesirable structural and control characteristics could have been identified and corrected. The investigation determined that the looseness of the elevator trim tab attachment screws (for both the controllable left��tab and the fixed right tab) and a fatigue crack in one of the screws caused a decrease in the structural stiffness of the elevator trim system. At racing speeds, the decreased stiffness was sufficient to allow aerodynamic flutter of the elevator trim tabs. Excitation of the flutter resulted in dynamic compressive loads in the left elevator trim tab's link assembly that increased beyond its buckling strength, causing a bending fracture. The flutter and the failure of the left elevator trim tab's link assembly excited the flutter of the right elevator trim tab, increasing the dynamic compressive loads in the right elevator trim tab's fixed link assembly beyond its buckling strength, causing a bending fracture. The investigation found that the condition of the trim tab attachment screws' locknut inserts, which showed evidence of age and reuse, rendered them ineffective at providing sufficient clamping pressure on the trim tab attachment screws to keep the hinge surfaces tight.