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Ethiopian ET302 final report released, NTSB and BEA issue comments

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Just before the end of the year, Ethiopian investigators published their final report on the crash of Ethiopian Airlines flight ET302. Flight 302 crashed shortly after takeoff from Addis Ababa Bole Airport on 10 March 2019. Operated by Boeing 737-8 MAX ET-AVJ, it was the second crash of a 737 MAX in just over four months and led to a worldwide 20 month grounding of the MAX while Boeing and regulators made changes to the aircraft.

Following the publication of the final report, both the US National Transportation Safety Board (NTSB) and French Bureau d’Enquêtes et d’Analyses (BEA) took the highly unusual step of publishing separate comments on the report.

Ethiopian investigators’ probable cause: MCAS

In their final report, Ethiopian investigators solely focus on the activation of the Maneuvering Characteristics Augmentation System as the probable cause of the accident.

“Repetitive and uncommanded airplane-nose-down inputs from the MCAS due to erroneous AOA input, and its unrecoverable activation system which made the airplane dive with the rate of -33,000 ft/min close to the ground was the most probable cause of the accident.”

Section 3.2, ET302 final report

MCAS is the software system Boeing introduced on the 737 MAX to make the aircraft’s behavior in certain phases of flight the same as the previous generation 737NG. This was done to to minimize the training necessary for pilots moving between the two generations of aircraft. When activated at high angles of attack, MCAS pushes the nose of the aircraft downward.

At the time of the accident, MCAS was activated based on data from a single angle of attack sensor, which in the case of ET302 was damaged and providing erroneous data. This caused the aircraft to believe its AOA was much higher than it actually was.

The Ethiopian investigators further list a series of contributing factors to the accident that focus on the design of MCAS, the design and certification of the 737 MAX generally, and Boeing’s training and operational materials. The report does not address any issues related to human factors or crew actions.

ET302 accident investigation report process

In most accident investigations, the final report is a product of the collaborative effort of all agencies that are party to the investigation speaking in a singular voice. To this end, the state conducting the investigation sends a copy of the final report to all parties before publication, inviting their “significant and substantial comments” [ICAO Annex 13, Chapter 6.3] on the draft report.

Any comments received within 60 days are to either be incorporated into the draft report or, “if desired by the State that provided comments, append the comments to the Final Report.” [ICAO Annex 13, Chapter 6.3] In the case of the Ethiopian ET302 final report, the Ethiopian Aircraft Accident Investigation Bureau neither incorporated the comments of the US NTSB and French BEA nor did it append their full comments to the final report as requested. The electronic version of the Ethiopian final report includes only links to the “earlier and now outdated version of the NTSB’s comments” and according to the BEA, “the EAIB report contains a link to a BEA document which does not contain the comments that the BEA had finally requested to be appended [to the final report].”

NTSB and BEA release their comments

Given this unusual situation, both the NTSB and BEA separately released the full comments each agency wished to see appended to the final report. Both the NTSB and BEA agree with the EAIB regarding MCAS’ contribution to the accident and both agencies’ comments relate to the human factors and crew actions that are not discussed in the EAIB report.

“The BEA shares the analysis and conclusions of the EAIB report regarding the contribution of the MCAS system of the 737 Max to the accident. BEA's comments are mainly related to the analysis of the crew's performance and its contribution to the accident scenario, in particular during the first part of the flight (between the rupture of the angle of attack vane and the activation of the MCAS system). The BEA considers that this analysis would make it possible to draw safety lessons beyond those related to the MCAS system.”

BEA press release accompanying comments on the ET302 final report

For its part, the NTSB also says the “final report included significant changes from the last draft the EAIB provided the NTSB” and it is in the process of reviewing those changes to see if further comment is necessary.

More to the probable cause than just MCAS

The NTSB and BEA’s comments both focus on the additional contribution the crew’s actions had on the accident, areas on which the EAIB spends little to no time in the final report. The NTSB comments propose two additional factors be added to the probable cause:

  • the operator’s failure to ensure that its flight crews were prepared to properly respond to uncommanded stabilizer trim movement in the manner outlined in Boeing’s flight crew operating manual (FCOM) bulletin and the FAA’s emergency airworthiness directive (AD) (both issued 4 months before the accident) and
  • the airplane’s impact with a foreign object, which damaged the AOA sensor and caused the erroneous AOA

The BEA focuses most closely on the crew’s actions during the period of time after the AOA vane was damaged, but before the first activation of MCAS. Regarding the accident’s probable cause, the French agency says it “believes that the crew’s inadequate actions and the insufficient Cockpit Resource Management (CRM) played a role in the chain of events that led to the accident, in particular during the first phase of the flight, before the first MCAS activation.”

NTSB releases additional comments

The NTSB released a second set of comments regarding the Ethiopian investigators’ final report on 24 January 2023. The NTSB says that the final report released to the public included substantive changes made after the NTSB reviewed the report, in contravention to ICAO annex 13 stipulations. The NTSB says further that the report includes findings that are “unsupported by evidence — for example, that aircraft electrical problems caused erroneous angle-of-attack (AOA) output.”

Ethiopian 302 ADS-B Data

See the ADS-B data received by Flightradar24 from Ethiopian 302 and our discussion of the data from the time of the crash.

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