July 09, 2013
Credit: Justin Sullivan
The instructor pilot in command of Asiana Airlines Flight 214 arriving into San Francisco on July 6 said he had assumed the aircraft’s autothrottle system would keep the aircraft flying at 137 kt. as he and the “pilot-flying” in the left seat guided the 777-200ER to Runway 28L in visual conditions.
Too low and slow on the approach, the aircraft clipped the sea wall leading to the runway threshold with its main landing gear and tail as the pilots attempted to abort the landing. The left-seat pilot was in the process of getting qualified to fly the 777 for the Seoul, South Korea-based airline.
During interviews with the U.S. National Transportation Safety Board, Korean investigators and the airline on July 8 and 9, the instructor pilot told officials that the aircraft was “slightly high” when it descended through 4,000 ft. on the approach. He then set the aircraft’s vertical speed mode for a 1,500 ft./min. descent rate, NTSB chairman Deborah Hersman said at a July 9 report on the investigation. During a July 8 update, Hersman said the pilots disconnected the aircraft’s autopilot at 1,600 ft., presumably to hand-fly the approach.
The instructor pilot told investigators that at 500 ft. altitude, he realized the aircraft was below the visual glideslope provided by the precision approach path indicator (PAPI) lights on the airport, since the system showed three red and one white light. When on the proper glideslope, pilots see two white and two red lights. All red indicates a position significantly below the glideslope while all white lights indicate being well above the reference glideslope. The electronic glideslope generally used as part of an instrument landing system was not operating due to runway construction at the airport this summer.
“He told [the left-seat] pilot to pull back [on the control wheel],” says Hersman of the interview. “He had set the speed at 137 kt. and assumed the autothrottles were maintaining the speed.” Autothrottles, if armed and turned on, will automatically increase or decrease engine thrust to maintain a preset speed, in this case 137 kt., the reference landing speed for the 777-200ER that day. The NTSB is investigating why the autothrottle did not work as the instructor had expected, an issue that could include mode confusion related to the interaction of various auto-flight modes.
Hersman says investigators documenting switch positions is the cockpit after the crash noted that the autothrottles were armed. In that state, the system will automatically activate when speeds are low regardless of whether pilots have the autothrottle system turned on or off.
Preliminary data from the aircraft’s flight data recorder shows that the speed had already decayed to 134 kt. as the aircraft passed through 500 ft., and would ultimately drop as low as 103 kt. at 3 sec. before impact.
The 777 continued to slow as the pilots attempted to “correct a lateral deviation” as it descended from 500 ft. to 200 ft. “At 200 ft., the four PAPIs were red and the airspeed was in the hatched area,” says Hersman. The “hatched” markings on an airspeed tape warn pilots of an impending stall. The instructor pilot at that point recognized that the autothrottle was not maintaining speed and established a nose-high go-around attitude. He attempted to push the throttles forward for more power, but says the pilot-flying had already done so.
Information from the flight data recorder showed that the pilots first increased engine power from flight idle at 125 ft. altitude, reaching 50% thrust 3 sec. before impact.