WASHINGTON — Asiana Flight 214’s pilots caused the crash last year of their airliner carrying 307 people by bungling a landing in San Francisco, including inadvertently deactivating the plane’s key control for airspeed, the National Transportation Safety Board concluded Tuesday.
The board also said the complexity of the Boeing 777’s autothrottle and auto flight director — two key systems for controlling flight — contributed to the accident. Materials provided to airlines by Boeing that fail to make clear under what conditions the autothrottle doesn’t automatically maintain speed were also faulted.
The 777, in service 18 years, is one of the most popular wide-bodied airliners. Until the accident, it had not been involved in a single fatal crash.
The board’s acting chairman, Chris Hart, warned the accident underscores a problem that has long troubled regulators: Increasingly complicated automated controls designed to improve safety also create opportunities for error. Asiana’s crew ‘‘over-relied on automated systems that they did not fully understand.’’
Three teens were killed in the July 6 crash. Nearly 200 people were hurt, 49 seriously.
Asiana said it has already implemented the NTSB’s training recommendations, and it agrees with the finding that one factor was the complexity of the autothrottle and autopilot systems, as well as training manual descriptions.
Boeing rejected the notion that automated systems contributed to the crash: ‘‘The auto-flight system has been used successfully for over 200 million flight hours across several airplane models, and for more than 55 million safe landings. The evidence . . . demonstrates that all of the airplane’s systems performed as designed.’’
The board did not say the autothrottle failed to perform as designed, but that its design could lead to confusion as to whether it was controlling speed or in an inactive state.
Investigators said the pilots made 20 to 30 errors, including failing to call out notifications about altitude, speed, and actions they were taking. Nor did they closely monitor airspeed. Instead, they assumed the autothrottle was maintaining the required speed.
The captain, Lee Kang Kuk, inadvertently prevented the autothrottle from controlling the speed. Kuk put the throttle in idle after the plane had climbed too high. He assumed the throttle would resume controlling speed. But because he turned off autopilot at the same time, autothrottle remained in idle.
One pilot noticed the plane was descending too fast but didn’t speak up right away. By the time pilots called for a ‘‘go-around’’ it was too late.