The NTSB has published on its website a series of comments that Ethiopian authorities failed to include in their final report on the March 10, 2019, crash of an Ethiopian Airlines Boeing 737 Max 8. The report from Ethiopia’s Aircraft Accident Investigation Bureau (EAIB), which the NTSB received on Tuesday, omitted comments—largely related to human factors—the Safety Board provided after reviewing a draft report last year.
After the EAIB reviewed the comments, it provided the NTSB with a revised draft report for its review. The Safety Board determined the revised report failed to sufficiently address its remarks. As outlined by the ICAO Annex 13 process, the NTSB provided the EAIB with more expansive and detailed statements. Instead of incorporating the most recent and expanded remarks into their report or appending them, however, the EAIB included a hyperlink in its final report to an earlier and now outdated version of the NTSB’s comments, according to the U.S. authority.
The NTSB also noted that the final report included significant changes from the last draft the EAIB provided. As a result, it has begun carefully reviewing the final EAIB report to determine if it needs any further commentary.
The Safety Board said that, overall, it concurs with the EAIB’s conclusion that the probable cause of the accident should include uncommanded nose-down inputs from the Max 8’s Maneuvering Characteristics Augmentation System (MCAS). However, the probable cause cited in the draft indicates that the MCAS alone rendered the airplane “unrecoverable,” a conclusion the U.S. authority disputes.
Proper CRM Lacking
According to the NTSB, the probable cause also needs to acknowledge that appropriate crew management of the event under the procedures that existed at the time would have allowed the crew to recover the airplane even when faced with the uncommanded nose-down inputs. It added that although flight crew performance played a critical role in the accident sequence, the EAIB did not sufficiently discuss the pilots’ performance in its draft report, which the Board said continued to focus heavily on system design issues. The absence of flight crew performance information limits the opportunity to address broader and equally important safety issues, it noted.
For example, according to the NTSB, the EAIB draft report inappropriately states that the IAS (indicated airspeed) DISAGREE and ALT (altitude) DISAGREE messages did not appear during the flight, and that the Ethiopian authorities used that incorrect assumption as a basis for its assessment of the crew’s performance. The Safety Board notes that the crew’s lack of conversation or action in response to the annunciations “should be explored” in the context of the flight deck environment, workload, crew experience, and training. The report’s assumption that those messages did not appear severely limits the opportunity for recognizing and addressing potential crew training and experience improvements, the NTSB concluded.
The EAIB draft report also states that no flight crew reference document explained that erroneous AOA inputs could affect autothrottle thrust commands. Even if such a reference document did not exist, the flight crew should have received training on 737 Max 8 non-normal procedures, according to the NTSB. “Because crew response to in-flight anomalies is time critical, these in-flight reference documents are not intended to provide flight crews with an in-depth understanding of a system before responding to an anomaly,” said the Safety Board. “Rather, non-normal procedures are designed to provide flight crews with information to diagnose and respond to a system-related issue in a timely manner based on observable flight deck effects.”
Non-normal procedures related to erroneous AOA inputs instruct the crew to disengage both the autopilot and autothrottle, thereby preventing the erroneous AOA inputs from affecting flight control and throttle movements, it added, noting that the observable flight deck effects associated with erroneous AOA inputs include the activation of the stick shaker and the annunciation of the IAS DISAGREE and ALT DISAGREE messages.
Among the most notable assertions by the NTSB centers on incorrect statements “in several locations” of the draft report that the MCAS made control of the airplane “impossible” but neglects to state that, if the crew had manually reduced thrust and appropriately used the manual electric trim, the airplane would have remained controllable despite uncommanded MCAS input. According to the U.S. authority, the flight crew’s failure to reduce thrust manually and the excessive airspeed that resulted played a significant role in the sequence of events that led to the accident.
The NTSB asserts that appropriately countering uncommanded nose-down inputs with manual electric trim nose-up inputs—as expected under the crew procedure described in Boeing’s flight crew operating manual (FCOM) bulletin and an FAA emergency airworthiness directive (AD)—would have resulted in six control column forces remaining in a controllable regime during the flight, including when the stabilizer trim cutout switches were in the CUTOUT position. The Safety Board notes that an evaluation of the crew’s failure to apply manual electric trim nose-up inputs would strengthen the report. Meanwhile, it said, the draft report does not examine the flight crew’s understanding of the effect of airspeed on the control forces required to move the control column and trim wheel.
The EAIB report also inaccurately states that the crew performed actions “per the procedure,” asserted the NTSB, which added that evidence shows that the crew did not appropriately perform non-normal procedures after receiving annunciations relating to unreliable airspeed, stall warning, and runaway stabilizer. The crew also did not respond as expected to the overspeed warning by disconnecting the autothrottle and reducing power, the Board noted.
Flight Crew's 'Inadequate' Use of Manual Trim Cited
In summary, the NTSB proposes that the probable cause in the final report present as causal factors uncommanded airplane-nose-down inputs from the MCAS due to erroneous angle-of-attack (AOA) values and the flight crew’s inadequate use of manual electric trim and management of thrust to maintain airplane control. It also said contributing factors should include the operator’s failure to prepare flight crews to properly respond to uncommanded stabilizer trim movement as outlined in the FCOM bulletin and emergency AD issued four months before the accident and the airplane’s impact with a foreign object, which damaged the AOA sensor and caused the erroneous AOA values.
The EAIB draft report states that the erroneous AOA data resulted from an AOA sensor failure yet omits key findings about the root cause: damage from impact with a foreign object, likely a bird in this case. Therefore, the report misses the opportunity to address improvements in wildlife management at the flight’s departure location—Bole International Airport in Addis Ababa, Ethiopia.
The EAIB draft report provides some details regarding a runway area search after the accident but inappropriately suggests that the lack of bird remains or AOA vane remnants indicates that a foreign object did not hit the airplane, according to the NTSB. The EAIB report also fails to state that the search occurred eight days after the accident and that the search did not include the area surrounding taxiway D, even though FDR data indicated that the airplane would have been positioned above the taxiway when the left AOA sensor data failed.
Separately, the EAIB draft report includes multiple findings that question the functionality of the manual electric trim system but presents no facts to support them, according to the NTSB, which added that the findings contradict evidence from the investigation indicating that the system worked as intended.
The NTSB also asserts that draft report incorrectly states that the FAA did not officially approve design changes to the 737 Max 8. In fact, the Safety Board said, Boeing’s changes to the MCAS design became official in March 2016 and “were communicated” to the FAA in July 2016, as described in the NTSB System Safety and Certification Specialist’s Report. Boeing earned an amended type certificate for the 737 Max 8 in March 2017.
Meanwhile, according to the NTSB, the EAIB draft report incorrectly states that Boeing did not respond or failed to respond appropriately to Ethiopian Airlines’ request for more information about the MCAS after the October 2018 crash of a Lion Air Max 8 in Indonesia. In fact, Boeing provided information to all 737 Max operators in November 2018 to address uncommanded MCAS inputs including an operations manual bulletin, a multi-operator message, dedicated meetings, and email messages, said the Board. Boeing’s response to Ethiopian Airlines’ request for more information about the MCAS, dated December 3, 2018, provided specific guidance about the OMB and checklist prioritization, it concluded.