(Adds Asiana comment in 13th paragraph and flight number in 5th paragraph)
By Alwyn Scott and Annika McGinnis
WASHINGTON, June 24 (Reuters) - U.S. investigators on Tuesday said Boeing Co should consider modifying flight controls on the 777 jetliner in response to an Asiana Airlines crash in San Francisco last July that killed three people and injured more than 180.
The National Transportation Safety Board accepted 30 findings following an 11-month investigation into the July 6, 2013 crash, and made more than two dozen recommendations to the Federal Aviation Administration, the Seoul-based airline, Boeing, firefighters and San Francisco city and county.
The NTSB said its probe did not find any failures in the auto-throttle system or any other flight control or warning system. The pilots committed 20 or 30 errors in the final 14 miles of approach, the NTSB said, and it cited "mismanagement" by the pilots as the probable cause of the crash.
The pilots, though experienced, didn't understand exactly how the auto throttle functioned and that it would not maintain minimum air speed in all circumstances.
That complexity, and flight training manuals that did not clearly describe how the controls would operate, contributed to the crash of Asiana Airlines Flight 214, the NTSB investigation found.
"The automation performed as designed, but the pilots did not fully understand what the automation would and would not do," NTSB Acting Chairman Christopher Hart said. "It was not a design issue by itself, it was the intersection between the design and the pilots' understanding of how the design worked."
The airline's training may not have adequately prepared the co-pilot, who was supervising the captain making his first landing at the airport in a Boeing 777. Crew fatigue also played a role, the NTSB investigators said.
The board recommended that Boeing develop and evaluate changes to the control systems to ensure the plane's "energy state" - a combination of speed, altitude, engine thrust and other factors - "remains at or above the minimum desired ... for any portion of the flight."
It also said Boeing and Asiana should revise flight training manuals to better explain the auto-throttle functions.
And it called on the FAA to convene a special certification review of how the 777 automatically controls air speed and use that to make existing and future controls more intuitive.
The 777 auto throttle protects against low speed, and will even "wake up" from being off to correct speed. But in "hold" mode, the system requires a pilot to control speed, and will not prevent speed from slipping below the minimum needed to stay aloft, the NTSB said. The Asiana flight's system was in "hold" mode and the pilots did not realize the risk, the NTSB said.
Had "wake up" been designed to occur in "hold mode," the auto throttle "would likely have increased 20 seconds before impact, which may have prevented the accident," said Roger Cox, a senior NTSB air safety investigator.
Asiana said the NTSB had "properly recognized the multiple factors that contributed to the accident, including the complexities of the auto-throttle and autopilot systems, which the agency found were inadequately described by Boeing in its training and operational manuals. The recommendations made by the agency can help ensure such an incident does not happen again."
The airline also said it had already implemented the four training recommendations the NTSB made for its own operations and had further strengthened training and safety.
Boeing said it "respectfully disagrees" with the NTSB's finding that the automated system on the 777 contributed to the accident. "We do not believe (it) is supported by the evidence," spokesman Doug Alder said in a statement. "We note that the 777 has an extraordinary record of safety - a record established over decades of safe operation."
The Boeing 777, a wide-body, long-range aircraft, had no prior fatal accidents since its introduction in 1995.
The 777 auto-flight system has flown more than 200 million hours on several models and made more than 55 million safe landings, Boeing said.
Boeing will review the board's recommendations carefully but said any potential changes to systems must be weighed "with due consideration for the potential unintended consequences."
The NTSB also raised concern that firefighters lacked training. Airplane fires require specialized skills, and city firefighting officers who took over at the scene had no previous experience working at an airport, the NTSB said.
Emergency responders transported more than 300 people within 90 minutes, including 192 people to local hospitals.
But responders did not adequately verify their belief that one passenger - 16-year-old Ye Meng Yuan - had died. She was run over by two firefighting vehicles about a half an hour after the crash, the NTSB said. (Reporting by Alwyn Scott in Seattle and Annika McGinnis in Washington; Editing by James Dalgleish, Sofina Mirza-Reid and Jonathan Oatis)
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