A set of interconnected problems, ranging from design and certification flaws to insufficient information from the manufacturer about the plane system, contributed to the crash of a Lion Air Boeing 737 Max aircraft last October, Indonesian investigators said yesterday.
The National Transportation Safety Committee, or KNKT, listed nine factors that caused the ill-fated aircraft to plunge into the Java Sea, killing all 189 passengers and crew on board.
It said a critical failure was centred on an anti-stall system called the Manoeuvring Characteristics Augmentation System (MCAS).
"Incorrect assumptions" were made about the ways the MCAS functioned and how pilots would respond to it, Mr Nurcahyo Utomo, head of air safety investigation at KNKT, told a news conference yesterday.
The system's reliance on a single angle-of-attack sensor had been seen as "compliant (with) certification criteria", he said. That angle is a key flight parameter that must remain narrow enough to preserve lift and avoid an aerodynamic stall.
"We see that by depending on this single sensor, the system is more vulnerable to disturbances because the sensor can be damaged, or experience other problems," Mr Nurcahyo added.
The Lion Air plane, which was bound for the tin-mining town of Pangkal Pinang in the Bangka Belitung Islands province, crashed into the Java Sea just a few minutes after taking off from Jakarta.
Less than five months later, in March this year, an Ethiopian Airlines Boeing 737 Max crashed just six minutes after take-off, killing all 157 passengers and crew on board.
The two incidents prompted a worldwide grounding of the aircraft, which was a best-selling plane for Boeing.
United States-based Boeing, the world's largest plane manufacturer, which has since acknowledged that the MCAS and a faulty sensor played a role in the crashes, has said that it has made improvements to the aircraft to allow it to return to the skies.
The grounding of the plane is estimated to have cost the company US$9.2 billion (S$12.5 billion).
The KNKT's report also revealed that Boeing did not provide guidelines about the MCAS, both during training on the new aircraft and in the manual provided, and this in turn led to pilots not being familiar with the system.
The other factors which the Indonesian investigators said contributed to the Lion Air crash included the inability of the pilots to manage multiple alerts, repetitive MCAS activation and distractions in the cockpit.
The lack of communication between the pilot and co-pilot was also to be blamed, said the investigators. Both were preoccupied "with their own tasks" while searching for information about troubleshooting and controlling the plane operation.
KNKT chief Soerjanto Tjahjono, who was at the news conference yesterday, emphasised that the report cannot be used for legal claims in court as well as for compensation by families of the victims. "KNKT investigation does not seek who is at fault... Our explanation cannot be used in court," he said.