U.S. safety investigators will focus on how pilots misunderstood an automated speed-control system and its design in a hearing starting tomorrow into the July 6 crash of an Asiana Airlines Inc. plane in San Francisco.
The Boeing Co. 777 wide-body aircraft struck a seawall short of the runway, had its tail torn off and skidded down the runway, killing three teenage girls. It was the first U.S. airline accident to claim a passenger’s life since 2009.
While information released so far by the U.S. National Transportation Safety Board has focused on why the pilots allowed the plane to get almost 40 miles (64 kilometers) an hour slower than their target approach speed, the hearing also explore the design of Boeing’s auto-throttle, according to the agency’s agenda.
“There are a variety of ways things can go wrong that people can’t anticipate,” David Woods, an engineering professor at Ohio State University who specializes in how humans interact with technology. Woods hasn’t participated in the investigation.
Woods helped author a U.S. Federal Aviation Administration study released last month that found the growing reliance on automation in the cockpit has led to occasional confusion and new safety risks.
The NTSB’s two-day hearing, designed to probe broad safety issues raised in the accident without determining its cause, begins tomorrow in Washington.
The pilots believed they had set the auto-throttle system to hold their desired speed, NTSB Chairman Deborah Hersman said in briefings after the accident. While Hersman said the system was “armed,” she hasn’t said which of several available modes the pilots had set it to.
Ki Won Suh, an Asiana spokesman, declined to comment about the hearing in an e-mail. The airline has increased the hours of flight-simulation training its pilots receive and taken other steps to make a “fundamental improvement” in safety, Akiyoshi Yamamura, senior executive vice president of safety and security management, said Dec. 3.
A Boeing spokesman, Marc Birtel, said in an e-mail that the company wouldn’t comment before the hearing.
Three former 777 pilots said in interviews that the plane’s auto-throttle, which normally sets its speed, can at times become dormant, leaving a poorly trained or distracted pilot in trouble if it isn’t noticed soon enough.
In one mode, known as Flight Level Change, the auto-throttle may leave engines in idle even as a plane becomes so slow its wings can’t keep it aloft, Kenneth Musser, of Roswell, Georgia, said. Musser flew the 777 for Asiana and other airlines.
“It was something that could bite you,” said Musser, who called the plane the best-designed aircraft he has flown. “It’s great as long as you use it properly.”
It’s important in accidents such as Asiana that investigators don’t stop with blaming the pilots, Woods said.
“It is neither purely equipment or purely human,” he said. “You should respond in both directions.”
Auto pilots, automatic throttles and computerized navigation systems have helped improve safety in recent decades, the FAA study concluded. The price for that is occasional confusion because the systems, which sometimes interact with each other, may be improperly set or act in ways crews don’t anticipate, it said.
Pilots accustomed to having automation handle mundane flying tasks may also lose basic manual flying skills, the report said.
Musser and another Asiana pilot, Vic Hooper, said Asiana pilots they worked with weren’t as confident hand-flying their planes as U.S. pilots.
Planes landing on that runway in San Francisco on the day of the accident required a manual approach because the airport’s instrument landing system was switched off during a construction project, according to the NTSB.
The interaction between pilots and cockpit automation has come up in several previous accident investigations involving planes made by different manufacturers.
A Boeing 737 also lost lift and almost crashed while on approach to Hampshire, England, on Sept. 23, 2007, after its auto-throttle disconnected for undetermined reasons, the U.K.’s Air Accidents Investigation Branch concluded. The pilots recovered and none of the 137 people aboard were hurt.
Investigators weren’t able to determine what caused the auto-throttle to switch off, allowing the plane to lose speed, according to the report.
Such auto-throttle failures were “not an unusual event,” the investigation concluded. The AAIB found another case on a 737-300 in June 2007 in which the auto-throttle shut off while the plane was approaching Belfast, Northern Ireland. The pilots lost 300 feet of altitude before recovering, it said.
In a survey of data from more than 2,300 flights, the AAIB found that in 2.5 percent of them pilots had gotten an auto-throttle warning for more than 9 seconds, an indication that pilots hadn’t noticed it.
After the AAIB recommended further study, the FAA issued a mandatory directive to improve the 737’s warning system when the auto-throttle disconnects, according to AAIB’s 2013 annual report.
Automation-related accidents have also occurred in Airbus SAS planes, according to crash reports. On Feb. 14, 1990, an A320 operated by Indian Airlines crashed short of the runway in Bangalore, India, killing 92 of the 146 aboard, according to AviationSafetyNetwork, an accident-information website.
The plane descended too rapidly because it was in an automation mode that kept the throttles at idle, the investigation concluded, according to AviationSafetyNetwork. Investigators couldn’t determine why the pilots selected that mode, it said.
The NTSB hearing will also examine how Asiana’s pilots were trained, the airport’s emergency response to the crash and how well seat belts and other cabin safety features performed, according to the safety board.