The National Transportation Safety Board (NTSB) is faulting deteriorated locknut inserts found in a “highly modified” World War II-era North American P-51D for the Sept. 16, 2011, crash during the National Championship Air Races in Reno, Nev.

The crash which killed the pilot and 10 other people on the ground and caused and injuries to 60 other people, captured national attention and has led to evaluations of the safety of experimental aircraft and air racing, along with alterations of the Reno course.

NTSB says the warn locknut inserts enabled the trim tab attachment screws to loosen, reducing the stiffness in the elevator trim system and causing an aerodynamic flutter. When the aircraft was at racing speed, the flutter broke the trim tab linkages, and the pilot lost control.

The aircraft, The Galloping Ghost, reached about 445 kt. in the third lap of the six-lap race, experienced a left-roll upset and “high-g” pitch up after it passed Pylon 8. NTSB believes the aircraft’s vertical acceleration reached 17.3g, incapacitating the pilot, and that the left elevator trim tap separated in flight. The aircraft then crashed into the ramp in the spectator box seating area.

NTSB cites as contributing factors the undocumented and untested modifications to the airplane. The airplane incorporated shortened wings, a boil-off cooling system installed on the engine, increased elevator counterweights, modification to the pitch trim system and a changed incidence of the horizontal and vertical stabilizers.

While FAA requires written notification of major changes before an aircraft can be flown, safety investigators could find no records of the aircraft’s modification except for the boil-off cooling system.

NTSB further points to the pilot’s operation of the airplane for air racing without adequate flight testing.

The safety agency in April had registered its concerns about the use of modified aircraft for air racing that had not been properly tested, and made a series of recommendations to better ensure their safety.

NTSB, which released the probable cause Aug. 27, did not make any new recommendations. It did note that a number of the recommendations made earlier already had either been addressed or that work was underway to address them.

“In Reno, the fine line between observing risk and being impacted by the consequences when something goes wrong was crossed,” says NTSB Chairman Deborah A. P. Hersman. “The pilots understood the risks they assumed; the spectators assumed their safety had been assessed and addressed.”