Washington (CNN) -- The pilot of the P-51 Mustang that crashed at the Reno Air Races last September experienced overwhelming g-forces at the outset of the incident, and likely was incapacitated almost instantly, the National Transportation Safety Board said Tuesday.
The safety board said the pilot rapidly experienced more than 9 g's of acceleration, enough to decrease blood flow to his brain and render him unconscious. Photographs show the violent force deformed the plane's fuselage, forced the tail wheel to deploy and likely resulted in the plane's trim tab -- a piece of the tail -- to fly off, the safety board said.
The safety board released the details at a news conference in Reno, Nevada, not far from the crash site. Board chairwoman Deborah Hersman said it would be months before the board determines the probable cause of the accident.
Nonetheless, the board issued seven recommendations to make conditions safer at the next Reno air race, scheduled for September.
Foremost among the recommendations: the safety board said all of the unlimited class aircraft like the P-51 Mustang should be made to demonstrate their airworthiness at racing speeds before participating in a public air race.
The P-51 aircraft, The Galloping Ghost, was flying the fastest it had ever flown on the Reno course since the plane had been modified in 2009, the safety board said.
"This pilot, in this airplane, had never flown this fast, on this course," Hersman said.
Tuesday's news conference cast strong doubts on at least two widely-held beliefs about the accident.
The first was that 74-year-old pilot James "Jimmy" Leeward took last-minute actions to avoid hitting the crowded grandstands. The NTSB said that Leeward was likely incapacitated in the very first second of the accident sequence. The plane experienced g-forces exceeding the 9-g limit of the plane's accelerator, Hersman said. It is difficult for trained pilots to remain conscious with even 5 g's, Hersman said. "But more importantly is the rapid onset in less than a second of this increased load," she said.
Photos show the pilot is not visible in the canopy just two seconds into the accident sequence and is seen bent forward and leaning to the right in a later photo, Hersman said, indicating he lost consciousness early in the mishap.
The NTSB also cast doubt on speculation the loss of the plane's left trim tab caused the plane crash. Photos show the trim tab departing the plane six seconds into the accident sequence, meaning the break may have been a result of the mishap, not its cause.
Hersman noted the aircraft was highly modified to improve its speed. Its 37-foot wingspan had been reduced to 29 feet. In addition to other alterations, the right trim tab was locked in a faired position, in aligned with the tail wing.
Log books indicate a mechanic certified the plane had been tested and "throughout its normal range of speeds" and maneuvers. But that statement, the NTSB said, "does not necessarily mean that the airplane... was evaluated while operating at speeds it would encounter on the race course.
The plane was traveling about 530 mph when it veered off course, entered a steep climb maneuver and then spiraled down to a box seat area filled with spectators.
The NTSB recommended the National Air Racing Group Unlimited Division require aircraft owners in the unlimited class to provide an engineering evaluation that includes flight demonstrations and analysis prior to a race. It also recommended the group provide high g training to pilots and study whether pilots should wear g suits.
The NTSB recommended race sponsors evaluate the course to minimize potential conflicts with spectators. The NTSB said it found numerous discrepancies, errors and instances of outdated information" in Federal Aviation Administration documents that provide guidance for air races and course design. In one noteworthy instance, one document requires a 500-foot distance between spectators and the race course, while another requires 1,000 feet. At Reno, a 500-foot separation was used.
Hersman said a lot of work remains to be done before the safety board rules on the probable cause of the accident.
"This is an ongoing investigation," Hersman said. "What we're seeing is a lot of very heavy forces on this aircraft and this pilot and what we're working on now is what precipitated that."
The pilot and 10 spectators were killed in the Sept. 16, 2011, crash. In addition, more than 60 spectators were injured.
"We are not here to put a stop to air racing," Hersman said in a statement. "We are here to make it safer."
A representative of the Reno Air Racing Association did not immediately return a call for comment on the recommendations.