A preliminary analysis of data from Ethiopian Airlines flight ET302’s flight data and cockpit voice recorders provides the strongest evidence yet linking the accident sequence to the October 2018 crash of Lion Air flight JT610.
“Our experts and US experts have proven the accuracy of the information,” Ethiopian Transport Minister Dagmawit Moges said in a statement March 17. “The Ethiopian government has absorbed the information. The cause of the crash was similar to that of Indonesia’s flight 610.”
A detailed analysis will be made public “in a month,” she added. ICAO protocol calls for an initial accident investigation update within 30 days.
While no specifics were provided on the data or voice recorder contents, downloaded by France’s Bureau d’Enquêtes & d’Analyses (BEA) over the last few days, Moges’s statement suggests that the two nearly new Boeing 737 MAX 8 accidents are linked to a flight control issue. Investigation into the JT610 accident is focused on the MAX’s maneuvering characteristics augmentation system (MCAS) flight-control law that assists pilots in certain manual, flaps-up flying scenarios, especially at slow airspeeds and high angles of attack (AOA). It automatically trims the horizontal stabilizers nose-down when it detects the AOA is too high.
Investigators believe faulty data from an AOA vane triggered MCAS during JT610’s flight, even though the aircraft’s nose was not too high. The flight crew responded with opposite nose-up commands, but MCAS is programmed to continue trimming nose-down based on the data it receives. With the AOA vane feeding erroneous data, MCAS kept attempting to push the 737 MAX 8’s nose down, and the pilots responded with nose-up commands. This back-and-forth continued for several minutes, causing the aircraft to lose and gain altitude, before it dove into the Java Sea, killing all 189 onboard.
A profile of ET302’s 6-min. flight based on satellite data provided to investigators by space-based ADS-B provider Aireon suggested similar flight control problems before it, too, dove to the ground. The transport minister’s statement solidifies the theory that the accidents are related.
Regulators and operators around the world grounded the MAX fleet in the days following ET302’s March 10 crash, which killed all 157 onboard. Canadian and US authorities were the last to issue MAX flight bans, on March 13. Both cited the Aireon data, provided hours before the grounding decisions, as the deciding factor.
Boeing is testing modifications to the MAX flight control system that will change how MCAS operates. FAA plans to mandate the upgrade as soon as it is validated.
“As part of our standard practice following any accident, we examine our aircraft design and operation, and when appropriate, institute product updates to further improve safety,” Boeing president and CEO Dennis Muilenburg said following Moges’s update on the recorder data. “While investigators continue to work to establish definitive conclusions, Boeing is finalizing its development of a previously announced software update and pilot training revision that will address the MCAS flight control law’s behavior in response to erroneous sensor inputs.”
Investigators’ ability to link MCAS to both accident sequences would further implicate the controversial system, which most pilots did not know existed prior to the JT610 accident and subsequent probe. But it also may expedite lifting the global MAX operations bans.
Boeing has been working on the flight control modifications for months, based on the early JT610 findings. If regulators determine the fixes and related training go far enough to reduce MCAS’s risk and improve pilots’ understanding, the ban could be lifted without awaiting further progress in the ET302 probe.
Meanwhile, Ethiopian Airlines issued a statement clarifying that ET302’s first officer had 350 total flying hrs., not 200 as the airline reported in a bulletin issued the day of the accident. The pilot-in-command had 8,100 hrs. and has been a 737 captain since November 2017. Ethiopian said the ET302 crew mix reflects its “effort to enhance safety” by ensuring its less-experienced first officers are paired with “highly experienced” captains.