Investigators suspect that MCAS was triggered by erroneous angle of attack ( AoA ) inputs , as if the plane had pitched up excessively . On both flights , shortly after takeoff , MCAS repeatedly actuated the horizontal stabilizer trim motor to push down the airplane nose . Boeing emphasized in the service bulletin issued November 6 , 2018 , that this could happen when the plane is under manual control , along with a number of indications and effects . Satellite data for the flights , ET 302 and JT 610 , showed that the planes struggled to gain altitude . Pilots reported difficulty controlling the airplane and asked to return to the airport . In the months between the accidents , the FAA Aviation Safety Reporting System received numerous U.S. pilot complaints of the aircraft 's unexpected behaviors , and how the crew manual lacked any description of the system . On April 4 , 2019 Boeing publicly acknowledged that MCAS played a role in both accidents . Most air regulatory agencies , including the FAA , Transport Canada and EASA , did n't require specific training on MCAS . Brazil 's national civil aviation agency " was one of the only civil aviation authorities to require specific training for the operation of the 737-8 Max "
ET 302
Ethiopian Airlines Flight 302
Boeing 737 MAX groundings