The Ethiopian Ministry of Transport’s interim report on the crash of Ethiopian Airlines flight 302 (ET302) focuses on the role played by a 737 MAX flight control law now redesigned involved in a previous MAX accident and inadequate pilot training.
Released on March 9, one day before the one-year anniversary of the ET302 incident, the 136-page report contains no analysis. “The analysis is ongoing and will be included in the final report,” he said.
Six recommendations highlight the problems already identified in other probes, including the final report on the first MAX fatal accident, the Lion Air Flight 610 (JT610) October 2018 accident and the work done by the US NTSB and several committees.
The ministry’s report recounts failures in designing the MAX, in particular its MCAS flight control software (maneuver characteristics increase system) which provides horizontal stabilizer inputs upside down in some flight profiles. In both MAX incidents, the MCAS was triggered by defective angle of attack (AOA) data, triggering a series of bottom-up inputs that confused and eventually overwhelmed the crews.
Designed to be transparent for pilots, the system was not included in the MAX training or flight manuals, so pilots had no idea that it existed. Boeing assumed that an MCAS-related failure would be recognized as a runaway stabilizer – a scenario he believed any pilot could handle in seconds. The alleged transparency of the function and the familiar failure modes led Boeing to conclude that the pilots did not need special training or information in this regard.
“The training provided by the manufacturer on the difference between B737NG and B737 MAX was judged inadequate,” said one of the results of the report.
The existence of the MCAS has become widely known following the JT610 accident. Boeing and FAA teamed up to issue an emergency airworthiness directive in early November 2018 that explained the system’s function and failure modes. Boeing followed the message of an aircraft operator referring specifically to the MCAS, presenting it for the most part to pilots for the first time.
While the first crash was enough to get Boeing to start working on changing the MCAS logic, the system wasn’t put in the spotlight until after the ET302 crash and subsequent MAX grounding. The FAA directive and the Boeing message are two fundamental differences between JT610 and ET302 accidents: the Lion Air crew had no idea that MCAS existed, but apparently the ET302 crew. The ministry report confirmed that the airline revised the MAX flight manuals on the day the directive was issued. The report does not provide details on whether additional information on MCAS has been provided to Ethiopian crews.
The reactions of the ET302 crew to MCAS activation included the use of electrical inputs to trim the trim to counteract nose movements downward and pull back on their control brackets. But they did not thwart all the inputs upside down and the bad AOA data continued to trigger the MCAS. After two MCAS activations, the crew activated the stabilizer trim clipping switches, interrupting the MCAS, but also left the pilots with only the manual trim wheel to move the stabilizer.
The aerodynamic forces created during a runaway stabilizer condition can make it difficult to move the trim wheel. If a rider is pulling back on the yoke, a natural reaction against uncommanded nose-down inputs, the force on the lift, part of the horizontal stabilizer, increases. This makes moving the stabilizer more difficult. Add in a speed increase that introduces a nose down attitude and the situation becomes more difficult. This is what the ET302 crew faced.
When the crew attempted to manually adjust the stabilizers, the misalignment was 2.5 degrees. The speed of the aircraft – its thrust levers set to 94% thrust at takeoff N1 and never adjusted – was 340 kt.
“At the moment the [first officer] tried to manually move the trim wheel, according to the aircraft manufacturer’s calculation, a force between 42 and 53 pounds was required. Investigators determined that the ET302 crew would need 40 wheel turns to correct the 2.5 degrees. incorrect adjustment. The simulator tests performed within the probe revealed that the difficulty of turning the wheel increased with increasing speed and the amount of misalignment. Investigators concluded that the trim wheel was not “mobile” at speeds above 220 kt. and incorrect values of 2.5 degrees. or more, says the report.
Unable to spin the trim wheel, the ET302 crew restarted the stabilizer motor, which allowed the MCAS, which still acts on defective AOA data, to reactivate. The ET302 was able to climb but was unable to thwart the nose-down inputs of the MCAS, leading to a fatal final dive.
The ministry report makes it clear that the defective AOA data started the fatal accident sequence because the MAX flight control software erroneously found that the nose of the aircraft was too high. This triggered the MCAS and the series of nose-down stabilizer movements. The failed AOA JT610 sensor was caused by an incorrect calibration by a supplier. The Ethiopian report does not shed light on why the ET302 blade failed, nor does it mention the part as among the recovered scrap. The report does not offer detailed information on flight crew actions not directly related to the management of MCAS activations.
The ET302 crash and the rapid link to an MCAS activation led to the global grounding of the MAX fleet – which is still ongoing – while Boeing faces several problems, including revisions to the MCAS logic. Recommendations made by the ministry report, including adding a second AOA data feed to the system, ensuring that an alert detailing AOA data changes is active on all MAXs, and adding simulation sessions to all MAX pilots are in the process of being processed or have been requested by Boeing.
Boeing’s changes to the MAX will also include training modules focused on manual scenarios of the trim wheels.