Washington (CNN) -- A lethal combination of worn aircraft parts and unprecedented speed caused last year's Reno Air Races crash, which killed a veteran pilot and 10 spectators, and injured more than 60 others, federal accident investigators concluded Monday.
The main culprits, according to the National Transportation Safety Board, were several lock nuts on the left trim tab -- an aerodynamic surface on the horizontal part of the plane's tail -- nuts that had not been replaced in at least 26 years.
The worn nuts allowed screws to loosen -- possibly years ago, without apparent consequence, the NTSB said. But when pilot Jimmy Leeward pushed the highly modified P-51 Mustang to a speed of 445 knots -- 35 knots faster than it had ever flown before on the Nevada race course -- the flaw caused flutter vibrations in the plane's trim tabs. The plane pitched up at about 17 Gs of acceleration, incapacitating Leeward.
The plane rolled, plunged towards the ground and slammed into a box seating area, showering the area with debris.
When the incident happened last September, it threatened to bring an end to the National Championship Air Races in Reno, the only race of its kind in the nation. But organizers have compiled a complete schedule of races from September 12 through 16, saying they have made numerous safety improvements recommended by the NTSB.
Among the improvements: race planes will be required to demonstrate their airworthiness at racing speeds before participating in races, spectators will be seated further from the aerial race course and the Reno Air Races has "flattened" the tight turn where last year's accident occurred.
"Pilots know that they are taking risks, but air race pilots expect that the risks taken are theirs alone," said NTSB Chairman Deborah A.P. Hersman. "The spectators assumed that their safety had been assessed and addressed."
Hersman said the NTSB made its recommendations in April to give the Reno Air Racing Association time to implement the changes for this year's race.
The NTSB said major modifications to the aircraft, intended to make the plane faster, contributed to the accident. Its 37-foot wingspan had been reduced to 29 feet. Some of the changes were undocumented and had a detrimental effect on the World War II-era aircraft, which was known as "The Galloping Ghost." For instance, investigators said, the plane's right trim tab -- a piece of the tail -- was locked in position aligned with the tail.
"On this airplane, you should have both trim tabs," investigators said. "This pilot put a screw through there to fix it in position, and when you do that, he lost redundancy. You have put all the eggs in the basket of hoping the left elevator will not fail. And it did fail."
Leeward's age, 74, was not found to be a factor in the crash. The 17 g forces that resulted from the failure greatly exceeded human tolerances, the NTSB said. It is difficult for trained pilots to remain conscious with even 5 Gs, the NTSB has said.
The accident investigation was greatly aided by numerous movies and photographs by spectators, according to the NTSB.
Since the accident, several victims and family members have filed suit, one alleging that the crash was not a "freak accident," but was "the predictable result of a reckless drive for speed by a risk taking pilot and crew, coupled with an insatiable drive for profit" by race organizers.
The Reno racing association, meanwhile, announced last week it has set up a $77 million fund to compensate victims or heirs. The program is being administered by Kenneth Feinberg who administered funds for victims of the 9/11 terrorist attacks and the BP Deepwater Horizon disaster.
The Reno crash focused attention on the dangers of air races and air shows. In addition to the 11 people killed at the Reno races, five performers died at air shows elsewhere last year. Federal regulators and air show organizers vouch for the safety of the system, saying rules for U.S. air shows are stricter than those in other countries.