The pilots of the Asiana Boeing 777-200ER that crashed 6 July in San Francisco made a number of changes to the aircraft’s autopilot system in the moments prior to the crash, according to the US National Transportation Safety Board (NTSB).
In opening remarks held during a hearing in Washington, DC, the board adds that investigators have found no evidence of mechanical problems with the aircraft, which operated flight 214 from Incheon International Airport near Seoul.
The crash, which led to three fatalities, was the first fatal 777 accident.
“The investigation to date has not identified any anomalies with the airplane prior to impact, although airplane systems testing and performance evaluation is on-going,” says the NTSB in opening remarks.
The board says that 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 per minute.
They also switched the autothrottle system to “speed mode” with a selected speed of 172kt.
But the rate of descent was not fast enough to remain on the normal glidepath, causing the aircraft to diverge above the normal angle, the board says.
When the aircraft was 5.2nm from the runway and descending through 2,400ft at 175kt, the altitude in the aircraft’s “mode control panel” was set to 3,000ft to prepare for a possible go-around, a normal action, the board says.
Then, at an altitude of about 1,600ft and 3nm from the runway, data indicates the flight level change switch, an autopilot mode normally used to climb or descend to a selected altitude, was activated.
That caused the autopilot to pitch the aircraft up and increase power in an effort to climb to 3,000ft, says the board.
The pilot responded by disconnecting the autopilot and manually pulling the throttles to idle, which moved the autothrottle switch into “hold” mode, meaning it no longer controlled the aircraft’s speed, says the board.
“When the flight level change mode was set... [the aircraft] wanted to climb... When [the pilot] overrode that mode by disconnecting the autopilot, he was telling the system... that he wanted to fly manually,” witness John Cashman, a retired Boeing 777 chief test pilot, explained to the board.
“He is flying away from the requested commands of the system, which would normally be controlling the speed,” Cashman says.
The pilots then changed the selected airspeed to 137kt, but the aircraft slowed below that target as it descended through the glidepath and continued “rapidly decelerating”, says the board.
At the same time, the pilot applied back pressure to the control column, increasing the aircraft’s pitch attitude.
“But there was no mention of the decaying speed on the recorders at this time. The thrust levers remained at idle, [the] airplane continued to lose airspeed and sink below the glidepath,” says the board.