Autothrottle Function Probed in Asiana 777 Crash
The pilots of the Asiana Boeing 777-200ER that crash landed at San Francisco International Airport on Saturday had armed the airplane’s autothrottles to maintain a 137-knot landing speed, leaving National Transportation Safety Board investigators questioning why the airplane slowed to 103 knots three seconds before it hit the sea wall at the threshold of Runway 28L.
By the time NTSB chairman Deborah Hersman began her third press briefing on the crash Tuesday afternoon, Safety Board investigators had finished interviewing the three pilots occupying the cockpit at the time of the crash. They hadn’t yet finished interviewing the fourth pilot—an alternate captain seated in the passenger cabin.
The instructor pilot who occupied the cockpit’s right seat at the time of the crash reported that he assumed the autothrottles were maintaining 137 knots when, at 500 feet, he saw three red precision approach path indicator (PAPI) lights and one white light, indicating a low approach. At that time, he told the pilot flying—a 9,700-hour captain in training who had accumulated roughly half of the needed initial operating experience in the 777—to “pull back.” As the airplane descended between 500 feet and 200 feet, they experienced a “lateral deviation” and continued to fly too low. By the time they reached 200 feet, the instructor pilot noticed four red PAPI lights, indicating a very low approach. [Two red lights and two white lights indicate a correct approach height.—Ed.] Finally recognizing that the autothrottles hadn’t maintained proper speed, the instructor pilot “established a go-around attitude,” and as he attempted to push the throttles forward, he realized that the pilot in the left seat had already done so. The airplane’s speed increased from 103 knots three seconds before impact to 106 knots at the time the tail section hit the sea wall. The airplane careened off the pavement, “ballooned” and spun nearly 360 degrees until it came to rest several hundred feet from the point of impact.
Hersman explained that “arming” the autothrottles simply means preparing them for engagement, and that investigators hadn’t yet determined their failure to function as expected represented a mechanical problem or an oversight by the crew. “Armed means they were available to be engaged, but depending on what mode is used…we really need to understand that a little bit better,” she said.
Neverthless, Hersman stressed that responsibility to “monitor” the airspeed rested with the pilots regardless of whether or not they engaged the autothrottles. “Let me be very clear. The crew is required to maintain a safe aircraft,” she said. “That means that they need to monitor. We have a flying pilot and we have two other pilots that are in the cockpit, and they have a monitoring function. One of the very critical things that needs to be monitored on approach to landing is speed. So we need to know what was going on in the cockpit and what was going on with the aircraft.”
Two of the 307 occupants died in the crash and rescuers sent 182 to area hospitals with injuries, some of which resulted in paralysis. Hersman also reported Tuesday that the two flight attendants seated in the rear of the airplane “were ejected” from the airplane upon impact. Both survived but suffered severe injuries.