A design flaw, inadequate pilot training and poor flight crew performance contributed to a Boeing jet crashing in Indonesia last year, killing all 189 people on board, investigators said Friday.
The Lion Air disaster was followed months later by a second crash -- involving the same model of aircraft -- when an Ethiopian Airlines plane went down with 157 people aboard, leading to the global grounding of Boeing's entire 737 MAX fleet.
The crashes had thrown a spotlight on the MAX model's maneuvering characteristics augmentation system (MCAS), an anti-stall mechanism, that pilots in both planes had struggled to control as the jets careered downwards.
On Friday, Indonesia's National Transportation Safety Committee said there were flaws in Boeing's design of the anti-stall system and of its certification by US regulators.
"The design and certification of this feature was inadequate," a summary of the report said, referring the MCAS.
The MCAS was vulnerable to a sole sensor that it relied on for inputs, and 737 MAX pilots were not properly briefed on how to handle a malfunction, it said.
"The aircraft flight manual and flight crew training did not include information about MCAS," the report said.
A sensor on the doomed jet's system was "miscalibrated" and the problem was not caught by Lion Air maintenance crews, it said, after the plane's previous flight also experienced loss-of-control problems.
The report also said the emergency was not "effectively managed" by the crew, who had previous performance issues.
- 'Heartfelt condolences' -
An earlier report released by international regulators said the US Federal Aviation Administration (FAA) lacked the manpower and expertise to fully evaluate the jet's MCAS when it certified the plane.
Friday's report comes after Boeing -- facing scores of lawsuits -- replaced the chief of its commercial plane division this week, the most significant executive departure since the 737 MAX grounding plunged the US-based company into crisis seven months ago.
Boeing has faced fresh scrutiny following the revelation of text messages from 2016 in which a test pilot described the MCAS during a simulation as "running rampant" and behaving in an "egregious" manner.
On Friday, following the release of the Indonesian report, Boeing expressed its "heartfelt condolences" to victims' families, and said it had since fixed the flight-control system's software.
"These software changes will prevent the flight control conditions that occurred in this accident from ever happening again," Boeing CEO Dennis Muilenburg said in a statement.
"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."
Design problems trumped other factors, said one expert.
"Pilots can make mistakes -- they're human, but the root of the problem is the design," said Jakarta-based aviation analyst Gerry Soejatman.
"The MCAS was created to be able to react based on a single input only, via one computer, (and) there was no cross-checking system to confirm whether or not the input was accurate. That's the issue."
- 'My only son' -
Lion Air, Southeast Asia's biggest carrier by fleet size, called the crash an "unthinkable tragedy".
"It is essential to determine the root cause and contributing factors to the accident and take immediate corrective actions to ensure that an accident like this one never happens again," said Lion spokesman Danang Mandala Prihantoro.
The FAA said it would monitor Boeing's changes to the single-aisle jet, adding that it would "return to service only after the FAA determines it is safe".
After getting a briefing on the report this week ahead of its public release, some of the victims' relatives expressed disappointment.
"However, we've got no choice but to accept it," said Epi Syamsul Qomar, who lost his son, after families met with investigators in Jakarta this week.
"People keep telling me to let go, to stay strong, but how do I do that? It's not that easy. He was my only son and I miss him every day."