Updated to include details about Boeing's accident report, also released by the NTSB on 31 March.
A report from Asiana says failure by its pilots to monitor and maintain airspeed led to the crash of one of its Boeing 777-200ER aircraft at San Francisco in July 2013.
But the report, released on 31 March by the US National Transportation Safety Board (NTSB), says that the poor design of the 777’s autothrottle and airspeed warning systems and other factors contributed to the accident.
A separate report from Boeing places blame on the pilots.
“The crew, composed of three highly-trained and experienced pilots, did not ensure a minimum safe airspeed during the final approach,” says the report, dated 17 March. “The record shows that there were complex and interrelated causes of this accident.”
The accident occurred on 6 July when Asiana flight 214 from Seoul slammed into a seawall at the end of San Francisco’s Runway 28 Left, leading to the deaths of three passengers.
Because an instrument landing system was inoperative that day, the pilots conducted a visual approach.
The NTSB, which is still investigating the accident, has said it found no mechanical problems with the aircraft.
But in the days following the accident, the NTSB revealed that the trainee pilot flying the approach, Lee Kang Kuk, told them he was unaware that the aircraft’s autothrottle had not been maintaining speed.
The autothrottle stopped maintaining speed when one of the pilots retarded the throttles while the aircraft’s autopilot was in flight-level change (FLCH) mode, the NTSB says.
Asiana’s report describes the autothrottle as having a “FLCH trap” that can lead crews to wrongly believe the aircraft is maintaining airspeed.
“There were inconsistencies in the aircraft’s automation logic that led to the unexpected disabling of airspeed protection without adequate warning,” the airline says.
The report notes that in 2010, a test pilot flying a Boeing 787, which has a similar autothrottle, was caught “by surprise” when the autothrottle did not engage during descent in FLCH mode.
In response, the US Federal Aviation Administration (FAA) recommended that Boeing change the software so that the autothrottle will “wake up during large excursions from target speed,” the report says.
Asiana adds that the 777’s low airspeed alert, which activated less than 11 seconds prior to impact, did not provide enough warning to prevent the accident.
The pilots could have avoided the crash had the warning activated 12 second before impact, Asiana says.
The airline also responds to statements made by pilot Kuk to the NTSB that suggested he was concerned about his ability to fly the visual approach.
“It was very stressful, very difficult to perform a visual approach with a heavy airplane, always,” investigators said the Kuk told them.
Asiana says Kuk misused the word “stressful”, believing it meant being “alert and attentive”, not “anxious or worried”.
“Elsewhere in the interview, the [Kuk] reported that he felt well-prepared to perform the SFO approach and that it was ‘nothing special,’” says Asiana.
The NTSB also released documents on 31 March from Boeing in which the company notes the aircraft did not meet industry standards for a stable approach when it passed 500ft.
Below 500ft, there were “numerous cues — visual and tactile — provided to the flight crew that showed the aircraft’s speed was decaying, the aircraft’s thrust setting was incorrect and the aircraft was increasingly below the glide path,” Boeing says.
The company adds that engaging FLCH mode had been contrary to its guidance.
The accident was caused by the flight crew’s failure to monitor and control airspeed, thrust and the glide path, the company concludes.