Pilots of an Asiana Airlines IAnc plane that crash-landed in San Francisco last summer were aware the plane was traveling too slowly and tried to correct it in the final seconds before impact, according to documents released on Wednesday by U.S. aviation safety investigators.
The crash on July 6 killed three people and injured more than 180, and was the first fatal commercial airplane crash in the United States since February 2009. The co-pilot, who was training the pilot flying the plane, thought the autothrottle on the Boeing 777 jet might not have been operating, the documents showed.
The pilot flying the plane said he wasn't sure whether autothrottle, used to set engine thrust power, was maintaining speed, according to the documents.
The low, slow landing caused the plane's tail to hit a seawall short of the runway. The plane spun 330 degrees as it broke apart and caught fire, strewing wreckage along the runway.
The National Transportation Safety Board released dozens of detailed documents related to the crash as it opened an investigative hearing into its causes.
The hearing, which began on Wednesday, focused on whether an over-reliance on autopilot systems in modern aircraft has led to degraded human flying skills and increased the risk of accidents.
It also is looking at the design of cockpit equipment on Boeing's 777 plane, an aircraft that, until the crash, had not been involved in a fatal accident since it entered service in 1995.
The Asiana pilots said in interviews with the NTSB that they left their "flight director" system, which includes the autopilot, partially on. In that mode, the system prevented a "wake-up" from hold mode from taking place, Capt. John Cashman, a retired Boeing 777 test pilot, testified at the hearing on Wednesday. The lack of wake-up meant the autothrottle was not going to prevent the plane's speed from slowing, he said.
The design is consistent with Boeing's philosophy of leaving the pilot in charge of the controls. "We try not to put in design elements that override the pilot," Cashman said at the hearing. "We try to let him be the decider."
Cashman said the design principles had been consistent for 31 years, across Boeing's 747, 757, 767, 777 and 787 models and had logged nearly 210 million flight hours and 55 million landings.
Documents released on Wednesday showed the "wake up" issue had been raised before, including during certification of Boeing's latest jet, the 787 Dreamliner, which has a similar autothrottle system and was certified in 2011.
In an interview following the Asiana accident, Eugene Arnold, a test pilot for the Federal Aviation Administration, said he raised concern that the Boeing autothrottle would not "wake up" while in two of its modes.
Arnold noticed this while flying a Boeing 787 Dreamliner during its test phase. The 787, a next-generation jet that was introduced in 2011, had a similar autothrottle to the 777.
According to NTSB documents, Arnold "felt that even though the system had been certified previously and had met the requirements of FARs (Federal Aviation Regulations), it was a less-than-desirable feature and it could be improved upon."
In response, Boeing said the autothrottle had been certified and had no problems in service, and "met the requirements for what was intended for the system," according to Arnold.
The FAA later informed foreign regulators, including the European Aviation Safety Agency (EASA), about the issue.
The FAA and EASA both said the autothrottle function in flight change mode "was a less-than-desirable feature that could be improved upon," Arnold said, according to the report.
Later, EASA noted that autothrottle would not wake up in two modes of the system, and suggested that eliminating the exceptions would improve the system.
"Although the certification team accepts that this autothrottle wakeup feature is not required per certification requirements, these two (mode) exceptions look from a pilot's perspective as an inconsistency in the automation behavior of the airplane," EASA wrote in a report to the FAA, cited by Arnold.
"Inconsistency in automation behavior has been in the past a strong contributor to aviation accidents," the EASA continued. "The manufacturer would enhance the safety of the product by avoiding exceptions in the autothrottle wake-up mode condition."
According to Arnold, Boeing noted in the 787 operating manual that in certain flight modes, the autothrottle "will not wake up even during large deviations from target speed and does not support stall protection."
The matter was also noted in the 777 operation manual, Arnold said, according to the report, but in more abbreviated form.
"Airlines could elect to go ahead and put the (flight manual) note in their own" operating manuals for pilots, "but he doubted that a lot of airlines had done so," according to the report.