Investigation into the Crash of Ethiopian Airlines Boeing 737 MAX 8

Casualties unknown • Accident on 10 March 2019 in Ethiopia involving a Boeing 737 Max 8 aircraft from Ethiopian Airlines, SE

An investigation into the crash of ET-AVJ reveals how a single faulty sensor triggered repeated nose-down commands from the MCAS system.

What happened

On March 10, 2019, an Ethiopian Airlines Boeing 737 MAX 8, registration ET-AVJ, crashed during a flight following a series of intense aerodynamic disturbances. During the flight, the aircraft experienced unexpected nose-down movements of the horizontal stabilizer. The crew attempted to manage the aircraft's pitch, but the flight profile became increasingly difficult to control, ultimately leading to the loss of the aircraft.

The investigation

The investigation focused on the functionality of the aircraft's flight control systems, specifically the Maneuvering Characteristics Aug/Augmentation System (MCAS) and the Angle of Attack (AOA) sensors. Investigators examined the digital flight data recorder and cockpit voice recorder to reconstruct the sequence of events. The analysis centered on how the aircraft's sensors provided data to the flight control computers and how the flight control laws responded to erroneous inputs. The investigation also looked into the design of the Speed Trim System (STS) and the reliance on single-sensor inputs.

Findings

  • The primary cause of the unstable flight path was the activation of MCAS based on erroneous data from a single AOA sensor.
  • The original design of the MCAS allowed for repeated nose-down stabilizer trim commands if the pilot released the electric trim switch.
  • The system relied on a single AOA sensor input, creating a single point of failure that could trigger catastrophic trim movements.
  • The flight control laws allowed the system to re-activate every five seconds as long as the erroneous high AOA value persisted and the pilot interacted with the trim switch.

Safety action

Following the identification of these safety issues, several design and training modifications were implemented:

  • Redesign of MCAS: The updated software now requires inputs from both AOA sensors to activate MCAS, preventing activation from a single faulty sensor.
  • Flight Control Law Updates: New laws ensure that MCAS will not command repeated stabilizer movements during a single high-AOA event; the system now requires the aircraft to return to a low AOA state before it can re-activate.
  • Enhanced Monitoring: A new Cross-FCC Trim Monitor was introduced to detect and shut down erroneous stabilizer commands caused by computer failures.
  • Pilot Training: Updated training protocols were established to address runaway stabilizer conditions and manual trim techniques during critical flight phases like approach and go-around.

Probable cause

The accident was caused by the MCAS system receiving erroneous high angle-of-attack data from a single sensor, which triggered repeated, uncommanded nose-down stabilizer trim movements that the crew could not overcome.

Frequently asked questions

What happened in the 2019-03-10 aircraft accident near Accident on 10 March 2019 in Ethiopia involving a Boeing 737 Max 8 aircraft from Ethiopian Airlines, SE?

An investigation into the crash of ET-AVJ reveals how a single faulty sensor triggered repeated nose-down commands from the MCAS system.

What aircraft was involved and where did it happen?

The accident on 2019-03-10 involved a aircraft, at Accident on 10 March 2019 in Ethiopia involving a Boeing 737 Max 8 aircraft from Ethiopian Airlines, SE.

What was the probable cause of the accident?

The accident was caused by the MCAS system receiving erroneous high angle-of-attack data from a single sensor, which triggered repeated, uncommanded nose-down stabilizer trim movements that the crew could not overcome.

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