JAKARTA, Indonesia — The fatal crash of Lion Air Flight 610 last year was caused by systemic design flaws in the Boeing 737 Max that were compounded by flight crew lapses, Indonesian investigators told the relatives of victims on Wednesday.
Investigators from the Indonesian National Transportation Safety Committee briefed family members on their findings about the crash ahead of the release of the final accident report, which is expected within days.
A synopsis of the report, presented in slide-show format to victims’ relatives, put the bulk of the blame on Boeing for introducing an automated system in the Max without adequately briefing airlines and their crews about its existence or instructing them how to override the software should it malfunction. It also pointed to maintenance issues on the Lion Air plane.
All 189 people on board the Lion Air flight died when the plane plunged into the Java Sea on Oct. 29. Less than five months later, another 737 Max crashed in Ethiopia, killing 157 people.
The automated system, known as MCAS, played a role in both crashes. The system, which was designed to help prevent stalls, was triggered erroneously on faulty data from a sensor, sending both planes into irrecoverable nose-dives.
“We are common people, and we don’t understand the technical terms they are using,” said Epi Samsul Komar, the father of one of the passengers on the Lion Air flight. “But if you ask me, the blame is with the plane factory. The error was in the plane’s production.”
A Boeing spokesman, Gordon Johndroe, said it would be “premature” for the company to comment because the report had not been officially released.
The findings by the Indonesian authorities are the latest to challenge Boeing’s defense that pilots should have been able to deal with a malfunctioning system by following standard emergency procedures.
The United States National Transportation Safety Board said last month that the company was overly confident that pilots could easily recover the plane, in part because it underestimated the chaos within the cockpit. A review by a multiagency task force, including the Federal Aviation Administration, NASA and nine international regulators, faulted Boeing for not fully stress-testing the design of the system and not appropriately accounting for pilots’ reaction to a failure.
Boeing has struggled to contain the crisis surrounding the Max, as it faces scrutiny from lawmakers in the United States and authorities around the world. Messages from a Boeing pilot recently surfaced suggesting that he had raised concerns about the system before the plane was certified.
Boeing has faced multiple setbacks in its effort to return the Max to flight, after new problems emerged with the plane. This week, the company ousted one of its top executives, Kevin McAllister, the head of Boeing’s commercial airplanes division, who had been central to company’s response to the crashes.
On Wednesday, Boeing reported a sharp quarterly drops in sales and airplane deliveries and said earnings fell 43 percent to $1.26 billion.
In the presentation on Wednesday, Indonesian investigators also pointed to missteps by Lion Air and its crew. They said the flight crew, composed of an Indian pilot and an Indonesian co-pilot, was not able to effectively manage the multiple alerts and distractions caused by the malfunctioning system.
The day before the crash, the plane had experienced a similar problem with the anti-stall system on a flight from the island of Bali to the Indonesian capital, Jakarta. But the pilots were able to override the system in that case.
The Indonesian report also noted that Lion Air, a fast-growing budget airline with a long record of safety mishaps, did not properly service a sensor that was replaced before the flight.
The plane had experienced various data malfunctions during previous flights, including inaccurate readings of airspeed and the angle of the plane’s ascent, which is called the angle of attack, or A.O.A.
On the doomed flight, the miscalibrated A.O.A. sensor triggered the automated system and forced the plane’s nose down repeatedly to help prevent a stall that was not occurring. The plane ultimately crashed into the water with such force that some parts of the fuselage disintegrated into a powder.
In most cases, critical systems have backups to ensure that they are not susceptible to issues resulting from a single data point. But Boeing originally designed MCAS to rely on one sensor, even though two are mounted on each plane.
“MCAS was designed to rely on a single A.O.A. sensor, making it vulnerable to erroneous input from that sensor,” a slide presented to the victims’ family members said.
Nor were the Boeing 737 Max planes automatically equipped with alerts that warned the flight crew of a discrepancy between the angle of attack sensors, another slide said. There was a roughly 20 degree differential between the two sensors on Flight 610.
In Boeing’s fix to get the plane flying again, the Max will rely on data from both sensors. The warning light will also be standard.
“With the blindfolds that the pilots had on because Boeing didn’t include MCAS in pilot manuals, along with the flood of alerts, conflicting distractions in the cockpit and the now-understood relentless power of the old MCAS, even the most experienced and highly trained pilot would be challenged to recover the aircraft in the seconds required,” said Dennis Tajer, a spokesman for the union that represents American Airlines pilots. “If you design a critical flight safety system that counts solely on unrealistic pilot reaction times and do not take into account pilot human factors, then you’ve designed an system awaiting disaster.”
In their findings, Indonesian investigators also questioned whether the plane’s paperwork was in order, noting that there was a “lack of documentation in the aircraft flight and maintenance log” about the plane’s previous flights. As such, the last flight crew was not fully aware of the earlier problems.
Lion Air has been censured in the past for sloppy maintenance and record-keeping.
“Plane crashes are multifactor, never a single factor,” said Soerjanto Tjahjono, the head of the Indonesian National Transportation Safety Committee, in an interview.
The slide show on Wednesday included a dedication to the victims and their families, noting that “their sacrifice will not be in vain” and that the victims “will forever be missed and never forgotten.”
Mr. Epi, the father of one of the victims, said that Boeing has never reached out to him or to other victims’ families who he knows.
“We hope in the future for a revamping of the plane so a tragedy like this never happens again,” he said.
Muktita Suhartono reported from Jakarta, Indonesia, and Hannah Beech from Cesky Krumlov, Czech Republic. Natalie Kitroeff contributed reporting from New York.