The National Transportation Safety Board on Tuesday published a second round of comments detailing concerns with several findings in the Ethiopian Aircraft Accident Investigation Bureau's (EAIB) final report on the March 10, 2019, crash of an Ethiopian Airlines Boeing 737 Max 8. The comments come less than a month after the NTSB protested what it called the investigation’s insufficient attention to the human performance aspects of the accident.
Although the NTSB said it agrees with the finding in the EAIB report related to the role the Maneuvering Characteristics Augmentation System (MCAS) and related systems played in the accident, the report also contains findings not supported by evidence, according to the Safety Board. For example, the NTSB specifically cites the EAIB’s conclusion that aircraft electrical problems caused erroneous angle-of-attack (AOA) output.
In its final report, the EAIB wrote electrical anomalies that existed since the time of the accident airplane’s production caused the AOA sensor heater to fail. As a result, the Ethiopian authority added, the AOA sensor provided erroneous values that caused the MCAS to repeatedly pitch the nose of the airplane downward until it struck the ground.
The NTSB, however, found that impact with a foreign object, most likely a bird, resulted in the separation of the AOA sensor vane, causing erroneous readings. The NTSB added that it provided the EAIB with evidence that supports the finding of a foreign object strike during the accident investigation, but the EAIB failed to include it in the final report.
According to the EAIB, an open circuit, wire fatigue, multiple arcing events, unexplained electrical/electronic anomalies, and the loss of heater power led to the failure of the originally installed and tested Boeing AOA sensor. The U.S. team believes that an electrical failure affecting the left AOA sensor did not occur before the left AOA vane’s impact with a foreign object, said the NTSB.
“The conditions present at the time of the accident were above freezing temperatures with no moisture present (that is, ice could not form regardless of the heater’s operational status),” the NTSB commented. “Thus, a loss of electrical current through the vane heater at any time during the accident flight would not explain the event because the loss of electrical current would have had no effect on the AOA sensor output. Another factor that occurred simultaneously with the loss of the AOA sensor vane heater’s electrical current would have been required to cause the observed change in the AOA output signal.”
Additionally, the NTSB said the AOA sensor vane heater and the two internal AOA resolvers operate on different electrical circuits, so a loss of electrical current in the AOA sensor vane heater does not indicate an electrical failure of the two internal AOA resolvers. The accident FDR data showed no indication of an electrical issue with the resolvers, it added.
Finally, the maker of the AOA sensors, Collins Aerospace, performed a fault-tree analysis to evaluate possible AOA vane failure scenarios caused by internal and external electrical faults. This analysis also considered every short circuit path to the AOA connector pins. The analysis found no electrical failure mode consistent with the circumstances of the accident, according to the NTSB.
The U.S. authority also called misleading the EAIB’s finding about the lack of MCAS documentation for flight crews because Boeing had provided the information to all 737 Max operators four months before the Ethiopian Airlines crash.
Although the NTSB acknowledges that information about the flap position required for MCAS to activate did not appear in Boeing’s flight crew operating manual (FCOM) bulletin and the FAA’s airworthiness directive in response to the crash of a Lion Air Max 8 on Oct. 29, 2018, Boeing provided the information in a multi-operator message (MOM-MOM-18-0664-01B) sent to all 737NG and Max customers on Nov. 10, 2018.
Although the EAIB appended Boeing’s multi-operator message to the final report, the EAIB failed to mention that the flaps information appeared in that document—thus rendering the finding misleading, concluded the NTSB.
According to the NTSB’s most recent statement, the EAIB issued its final report without giving the U.S. regulator the chance to review and comment on new information incorporated since its last review, as stipulated by the International Civil Aviation Organization’s Annex 13.