By Stephen Ward
RENO, Nevada (Reuters) - U.S. safety regulators investigating the crash of a World War Two-era plane at a Nevada air race that killed 11 people said on Tuesday they found evidence the pilot exceeded tolerable G-force limits and likely was incapacitated seconds before the accident.
The cause of the crash in September at the 48th annual Reno National Championship Air Races, where a vintage P-51 Mustang fighter dubbed the Galloping Ghost plowed into a seating area in front of the grandstand, remains under investigation.
But the National Transportation Safety Board said that evidence indicates the aircraft exceeded the 9-G limit on its own accelerometer, a devices that measures the plane's change in velocity. A 9-G force is equivalent to nine times the force of gravity.
"While the investigation into g-forces and g-tolerance is ongoing, the photographic and telemetry evidence indicates that both the airplane and pilot experienced an unanticipated, rapid onset of high g-forces and appears to support pilot incapacitation," the agency said in a statement.
The NTSB previously has said that eyewitness accounts and photographic evidence showed that a piece of the aircraft broke off before the highly modified plane crashed into spectators after making several laps around the race course.
The plane's pilot, real estate developer Jimmy Leeward, 74, and 10 people on the ground were killed, and more than 60 spectators were injured in the crash.
Leeward had said in a May 2011 interview with Sport Aviation magazine that the aircraft had been altered from its original design, with its wingspan shortened by 10 feet and its flight controls modified.
Asked by the magazine how fast the plane could fly, he said: "There are some things you never tell the competition, and that's one of them. But it's fast. Really fast."
In its statement, the NTSB suggested more safety training for pilots flying planes with substantial modifications, like the Galloping Ghost.
"Our investigation revealed that this pilot, in this airplane, had never flown at this speed, on this course," NTSB Chairman Deborah Hersman said. "We are issuing a safety recommendation to ensure that pilots and their modified airplanes are put through their paces prior to race day."
The agency issued seven safety recommendations overall to federal and local authorities in order to make the National Championship Air Races safer for spectators and pilots.
"We are not here to put a stop to air racing," she said. "We are here to make it safer."
The design of the Reno air race, in which pilots faced spectators in the box seats when making their final turn toward the finish, was part of the NTSB's focus.
A total of 30 people have been killed in the Reno Air Races since they began in 1964, though Reno Mayor Bob Cashell has said that the 2011 crash marked the first spectator deaths.
The next Reno Air show is scheduled to run September 12-16, according to the Reno Air Races Association. Pilot training begins in June.
(Writing and additional reporting by Mary Slosson; Editing by Steve Gorman and Xavier Briand)