Indonesian investigators have determined that design flaws in the Boeing 737 Max combined with insufficient oversight by U.S. regulators, failures in crew resource management (CRM), and maintenance lapses all contributed to the October 2018 crash of Lion Air Flight 610 off the coast of Java, killing all 189 on board. A final report into the accident released Friday by Indonesia’s National Transportation Safety Committee (KNKT) gives nine main contributing factors that led to the crash, including the fact that a faulty reading in only one angle of attack sensor caused the airplane’s maneuvering characteristics augmentation system (MCAS) to pitch the nose of the airplane downward, sending the airplane into a steep dive from which the crew failed to recover.
“During the design and certification of the Boeing 737-8 (Max), assumptions were made about flight crew response to malfunctions which, even though consistent with current industry guidelines, turned out to be incorrect,” said the report. “Based on the incorrect assumptions about flight crew response and an incomplete review of associated multiple flight deck effects, MCAS’s reliance on a single sensor was deemed appropriate and met all certification requirements.”
Other factors include a lack of documentation about the existence and function of the MCAS in flight manuals or training programs and the U.S. Federal Aviation Administration’s over-reliance on Boeing’s own employees to oversee the system’s certification.
“The absence of guidance on MCAS or more detailed use of trim in the flight manuals and in flight crew training, made it more difficult for flight crews to properly respond to uncommanded MCAS,” said the report.
The report also named the absence of an angle of attack disagree alert—a system that came standard on the 737NG—as a contributing factor.
Still, the KNKT did not place all blame on the airplane design, noting that maintenance crewmembers failed to detect a miscalibration of an AOA sensor they replaced on the Max before its fateful flight. However, said the report, the investigation could not determine whether or not the mechanics properly performed the installation test of the AOA sensor, only that the miscalibration “was not detected.”
Meanwhile, the KNKT attributed a lack of detection by maintenance and the flight crew’s failure to recognize the reasons for the uncontrolled dive at least partly to a lack of documentation related to MCAS.
Finally, for their part, the flight crew did not “effectively manage” the multiple alerts, MCAS activations, and distractions resulting from numerous ATC communications due to a failure of CRM. “This was caused by the difficulty of the situation and performance in manual handling, [non-normal checklist] execution, and flight crew communication, leading to ineffective CRM application and workload management,” concluded investigators. “These performances had previously been identified during training and reappeared during the accident flight.”
In reaction to the report, Boeing issued a statement on Friday that commended the investigative committee for its efforts.
“We are addressing the KNKT’s safety recommendations, and taking actions to enhance the safety of the 737 Max to prevent the flight control conditions that occurred in this accident from ever happening again,” said Boeing.
“Over the past several months Boeing has been making changes to the 737 Max,” it added. “Most significantly, Boeing has redesigned the way angle of attack (AOA) sensors work with a feature of the flight control software known as [the] maneuvering characteristics augmentation system (MCAS). Going forward, MCAS will compare information from both AOA sensors before activating, adding a new layer of protection."
In addition, MCAS will now only turn on if both AOA sensors agree, will only activate once in response to erroneous AOA, and will always be subject to a maximum limit that can be overridden with the control column.
“These software changes will prevent the flight control conditions that occurred in this accident from ever happening again. In addition, Boeing is updating crew manuals and pilot training, designed to ensure every pilot has all of the information they need to fly the 737 Max safely,” the company concluded.