Details about the 6 July crash of Asiana flight 214 at San Francisco suggest the pilot's uncertainty about the function of the aircraft's automated systems and his ability to perform a visual approach might have contributed to the accident.
US National Transportation Safety Board investigators have also looked at whether the pilot, who was training on the type, felt he had the authority to order a go-around while there was still time.
In an interview with NTSB investigators, a few days after the accident which left three people dead, the pilot said he lacked confidence in the operation of some of the 777-200’s systems.
“Asked how confident he felt about his knowledge of the Boeing 777 auto-flight system just prior to the accident, [the pilot] said he was not so confident because he felt he should study more,” states a summary of the interview released by the NTSB.
“He did not know if he had studied a proper amount; he had just followed the company programme.”
The pilot had logged nearly 10,000h in Boeing 737, 747 and Airbus A320 aircraft, but only 43h in 777s at the time of the accident.
He suspected the aircraft's autothrottle would maintain airspeed throughout the descent.
“He had learned that, on the Boeing 777, the autothrottle system was always working,” the NTSB document states.
“Sometimes, if the pilot wanted, they could disconnect [it] but to him it did not matter because in that situation, he could control the airplane manually...the autothrottle would maintain the speed always.”
The NTSB says a series of events caused the autothrottle to switch into "hold" mode, meaning it was not maintaining airspeed.
Shortly after the aircraft descended through 4,800ft the crew switched the autopilot to vertical-speed mode with a commanded descent rate of 1,000ft/min.
The pilots also switched the autothrottle to “speed” mode, with a selected airspeed of 172kt, while the altitude was set to 3,000ft in case of a go-around.
At 1,600ft the flight-level change switch was activated, causing the aircraft – already above the glideslope – to pitch up, in an effort to climb to the 3,000ft setting.
The pilot disconnected the autopilot and idled the throttle in response.
"In this configuration, the autothrottle would not be controlling speed," the NTSB said during a hearing into the accident held on 11 December. "The autothrottles transitioned to hold mode, with the thrust levers at the idle position due to the manual override."
Although the pilots set the airspeed at 137kt, they were apparently unaware that the aircraft’s system logic meant the autothrottle would not respond. The aircraft slowed to 103kt, nearly stalling, before slamming into the sea wall at the beginning of runway 28L.
"[The pilot] believed the autothrottle would come out of idle," NTSB board member Robert Sumwalt says. "That was clear…from his interview summaries."
The NTSB adds that none of the pilots in the cockpit seemed to notice the aircraft was rapidly decelerating: "There was no mention of the decaying speed on the recorders at this time.”
In addition to apparently lacking confidence in the operation of the autothrottles, the pilot told investigators he was "very concerned" about his ability to land the aircraft in visual conditions at San Francisco without the instrument landing system, which was temporarily inactive.
“It was very stressful, very difficult to perform a visual approach with a heavy airplane, always,” he said, according to the NTSB interview documents.
“From the planning phase it was very stressful because the glideslope was very, very helpful to making an approach.”
He had been sitting in the left-hand seat in the cockpit but monitored by an instructor pilot. Investigators asked whether he felt he had authority to commence a go-around in the final moments of the flight.
"That’s very hard because normally only in our Korean culture the one-step-higher level, the final decision people… he decide the going around thing," he responded. "But the instructor pilot got the authority. Even [if] I am on the left seat, that is very hard to explain, that is our culture."