A twelve hour hearing by the National Transportation and Safety Board was held last week regarding the Asiana Airlines crash last summer which killed three teenage passengers. While the NTSB is a long way from issuing a final report determining the probable cause of the aviation accident, what seems clear from this hearing is that the pilots were relying on automated flight systems to keep the plane operating at proper flight speed, even though the systems weren’t doing that.
The Boeing 777 is equipped with autothrottles, which the pilots believed would keep the airplane at the proper speed as it approached the airport. While there are a number of computer modes in which that would be the case, the Asiana pilots selected an inappropriate flight mode for the autopilot and autothrottle causing the plane’s autothrottle to stop maintaining speed. The pilots then failed to take corrective manual action to control the flight speed until it was too late.
While it appears that poor piloting and poor pilot training were likely contributing causes of this accident, what is also evident is that the 777’s autothrottle design and operating manuals may have also contributed to the crash. The 777 has an autothrottle “wake-up” feature which prevents the aircraft’s airspeed from decreasing to dangerously low levels. However, this “wake-up” feature is not functional across all the 777’s modes of automation. The pilots had apparently selected a mode in which the “wake-up” feature doesn’t function, but continued to rely on its operation. The combination of autopilot and autothrottle modes which the pilots utilized was neither recommended or prohibited in the Flight Crew Operating Manual nor was there any Boeing simulator or operational training regarding that, according to Asiana officials.
The European Aviation Safety Agency had notified Boeing by letter several years before this accident that this inconsistency in automation behavior has been a strong contributor to aviation accidents – and recommended Boeing remove the exceptions to the autothrottle wake-up feature. A FAA test pilot for the newer 787, which uses the same autothrottle system, reported that the ability to select flight modes in which the autothrottle does not wake-up is “less than desirable.”
While the final report is yet to be issued, it appears that at least one of the contributing causes of this this tragedy may have been forwarned.