What happened
On March 10, 2019, an Ethiopian Airlines Boeing 737-8(MAX), registration ET-AVJ, departed from Addis Ababa Bole International Airport for a scheduled flight to Nairobi, Kenya. Shortly after takeoff, the aircraft experienced a malfunction involving its left Angle of Attack (AOA) sensor, which began providing erroneous data. This triggered the activation of the stick shaker and caused significant discrepancies between the left and right AOA readings.
As the flight progressed, the aircraft's automated systems initiated several uncommanded nose-down stabilizer trim movements. The crew attempted to counteract these inputs using manual electric trim and physical force on the control column, but the nose-down commands continued to occur. At one point, the captain notified air traffic control of a flight control problem and requested to return to the departure airport. However, following a subsequent automatic trim activation, the aircraft entered a steep dive with a descent rate exceeding 33,000 feet per minute. The aircraft impacted terrain approximately 28 nautical miles southeast of Addis Ababa near Ejere village. All 157 individuals on board, including the flight crew, cabin crew, and passengers, were killed in the high-energy impact.
Findings
The investigation determined that the primary cause was the repetitive and uncommanded nose-down inputs from the MCAS system, triggered by erroneous AOA sensor data. The system's activation became unrecoverable, leading to the terminal dive. Key contributing factors included:
- The design of the MCAS relied on a single AOA sensor, creating a single point of failure.
- The MCAS design failed to account for the potential for uncommanded activation and its impact on pilot workload.
- Inadequate pilot training regarding the MCAS system and its operation during emergencies.
- The absence of an AOA DISAGREE warning flag on the primary flight displays.
- Failure by the manufacturer to provide necessary procedures or address safety-critical concerns raised by the airline.