Photo from Boeing
The NTSB weighed in on an Ethiopian report made on the March 10, 2019 crash involving a Boeing 737-8 MAX airplane, registration ET-AVJ, in Ejere, Ethiopia. The plane took off from Addis Ababa Bole International Airport on its way to Nairobi, Kenya, Jomo Kenyatta International Airport. The plane had a normal take-off but then crashed shortly after, killing all 157 people on board.
Initial findings
Initial findings from the Ethiopian Accident Investigation Bureau said the aircraft had a valid certificate of airworthiness and was maintained in accordance with any applicable regulations. Take-off was found to be normal and the left and right recorded angle of attack (AOA) values deviated shortly after lift-off. The left AOA values reached 74.5 degrees while the right reached a max value of 15.3 degrees, with a difference of 59 degrees that remained until near the end of the recording.
The EAIB report said that five seconds after the crew’s manual electrical trim-up inputs ended, a third automatic node-down trim (MCAS) was triggered. There was found to be no corresponding motion of the stabilizer. Later, a fourth MCAS was triggered and the vertical speed decreased and became negative 3 s after MCAS was activated.
The report also stated that the difference in training from B737NG to B737 MAX provided by the manufacturer was inadequate. It said the AOA Disagree message did not appear as per the design described in the flight crew operation manual. The deviation in the AOA went undetected due to the MCAS design on single AOA inputs making it vulnerable to undesired activation.
The EAIB report found that because of the different failures that led to the uncommanded MCAS activation triggered other underlying failures.
According to Africanews, the head of Ethiopia’s investigation, Amdeye Ayalew, had blamed Boeing for the failure to disclose issues with the 737-MAX 8’s MCAS. They said the MCAS system was found to have caused the plane to pitch the nose down and subsequently led to the pilot losing control of the aircraft.
NTSB COMMENTS
The NTSB released comments on Tuesday that were provided to the Ethiopian government on the draft accident investigation report after receiving the EAIB final accident report. Comments were published after Ethiopia’s AIB did not include any of the NTSB comments on the final report into the investigation.
According to the NTSB, the revised report given to them failed to sufficiently address its comments.
The NTSB, FAA, Boeing and General Electric were involved in the on-scene investigation into the crash. Members of the US team conducted four visits between March 12 and Sep. 7, 2019. The NTSB said detailed technical reports pertaining to the AOA sensor, MCAS, other airplane systems and the US aircraft certification process were given. The team was not involved in any post-accident interviews or on-site data gathering in relation to the human aspects of the investigation.
A US-accredited representative kept the EAIB informed about the progress of any draft recommendations and gave the organization a copy of the final safety recommendation report. In NTSB comments, is said that the US team concurred with the EAIB investigation of the MCAS-related systems and the part it played in the crash.
The NTSB said the many operational and human performance issuers present in the accident were not fully developed in the EAIB investigation. The issues included flight crew performance, crew resource management, task management and human-machine interface.
The comments stated that it was important to provide a thorough look and discussion of all the issues, including human-related factors, in order to ensure all possible safety lessons can be learned. The NTSB listed all comments not included or explained thoroughly in the EAIB findings, breaking them down into three groups.
Draft probable cause
The NTSB agreed that the uncommanded nose-down inputs from the MCAS on the plane were a probable cause for the incident, but noted that the draft given indicated the MCAS alone caused the plane to be deemed “unrecoverable” and did not acknowledge other factors like crew management.
The comments said that proper crew management at the time of the event, following procedures at the time, would have allowed the crew to recover the plane even with the uncommanded nose-down inputs.
The new proposed probable cause in the final report is the nose-down inputs from the MCAS due to erroneous AOA values and the flight crew’s inadequate use of manual electric trim and management of thrust in order to maintain airplane control, in order to fully reflect the circumstances surrounding the crash.
The organization also concluded that the operator’s failure to ensure the flight crews were prepared to respond to the issues in the manner outlined in Boeing’s FCOM and the FAA’s emergency AD, which were both issued four months prior.
Airframe/systems aspects
The EAIB states that the AOA data resulted from an AOA sensor failure, but the NTSB said key findings about the root cause were omitted.
The NTSB said the damage was caused by an impact with a foreign object or bird. It was found the report missed the chance to address improvements in wildlife management at the flight departure location.
Collins Aerospace, the manufacturer of the AOA sensor was a technical advisor to the US team in April 2019 after a request from the EAIB. The NTSB found the report addressed the technical findings in Collins’ factual report but did not acknowledge Collins’ fault tree analysis. This analysis showed that recorded FDR data were inconsistent with the internal failure of the AOA sensor. This data was consistent with previous instances of AOA vane separation due to a bird strike.
Many possible factors were outlined in the analysis, including the potential of a bird weighing at least 0.5 pounds impacting the vane at 170 knots which would be sufficient enough impact to cause the vane to break at the hub and separate from the sensor.
Comments state that the EAIB did not take part in any in-person testing, despite invitations from Collins to witness the simulation testing or live demonstrations.
The draft report omits any information about findings, recommendations and hazards regarding birds at the Addis Ababa airport. The NTSB said the report “inappropriately” suggested the lack of bird remains or AOA vane indicates there was no impact by a foreign object.
A final report about an engine failure event on Nov. 26, 2018, involved a Steppe or Tawny eagle, which are common around the airport. The NTSB noted that the EAIB made recommendations about the bird hazards in the area in that report but not in this recent draft.
The NTSB said the EAIB draft report included many findings about the functionality of the manual electric trim system but did not provide facts to support these claims and much of the findings contradict the evidence from the investigation.
The draft also incorrectly stated that the design changes on the 737-8 MAX were not official or approved by the FAA. The changes in question were official in March 2016 and communicated to the FAA in July 2016 and in March 2017 Boeing applied and was granted the amended type certificate. It was also incorrectly stated that Boeing failed to comment on Ethiopian Airlines’ request for information about the MCAS after the Lion air accident, but Boeing provided all 737 MAX operators with information in Nov. 2018.
Operational and human factors aspects
The NTSB said that flight crew performance played a critical role in the accident, but it was not discussed in the draft report.
The NTSB said the draft report focused heavily on system design issues and ignored the flight crew’s performance, including crew resource management. It found the absence of flight crew performance information denies the opportunity to address broader and equally important safety issues.
Information on the crew’s performance was available in the CVR, FDR, airline manuals/procedures, crew training records and post-accident interviews.
“…Design mitigation must adequately account for expected human behavior to be successful, and a thorough understanding of the flight crew’s performance in this accident is required not only for robust design mitigations but also for operational and training safety improvements necessary to achieve multiple layers of safety barriers to trap human errors and prevent accidents,” the NTSB said in its comments.
The comments say that the draft report incorrectly stated that the MCAS made control of the plane impossible but did not state that if the crew had manually reduced thrust and appropriately used the manual electric trim, the aircraft would have been controllable despite the MCAS input. The draft report also stated that the crew followed procedure but the NTSB stated evidence from FDR data contradicts this.
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