Indonesian investigators released a preliminary crash report today (Nov. 28) that described a battle between the pilots of Lion Air flight JT610 and an automated anti-stall system on the Boeing 737 Max 8 aircraft that continually forced the plane downward in reaction to incorrect flight data. Less than 15 minutes after the flight took off from Jakarta on Oct. 29, the plane crashed in the Java Sea, killing all 189 people on board.

The anti-stall system is known as the Maneuvering Characteristics Augmentation System (MCAS), and is a new addition on the 737 Max 8 and 9 models that can pitch the plane’s nose down without pilot input when sensor data indicates the possibility of a stall.

The 78-page report from Indonesia’s National Transportation Safety Committee, known by the acronym KNKT, described in detail problems experienced with the plane in flights before its crash, and shared information from the flight’s data recorder—the cockpit voice recorder hasn’t been recovered—without drawing conclusions. “When it comes to faulting, I don’t know, our job isn’t to find faults,” Nurcahyo Utomo, who heads KNKT’s aviation-accident subcommittee, said at a press conference (paywall).

In addition to Indonesian investigators, the crash probe involved the US National Transportation Safety Board and investigators from Australia and Singapore.

The report made few safety recommendations other than calling on Lion Air to improve its safety culture. It said that in the penultimate flight, pilots continued flying even though the stick shaker—which generates a noisy warning before a stall—continued to vibrate, instead of opting to land at the nearest airport.

On the final flight, according to the Satcom Guru website run by Peter Lemme, a former Boeing engineer, the flight data recorder information for the JT610 showed “there are 26 occurrences of MCAS trim down, pilot trim up”—with trim referring to the efforts to redirect the plane.

A chart included in the report shows a series of orange lines that designate the automated system’s efforts, matched by a series of blue ones in the opposite direction, indicating the pilots’ efforts.

Indonesia National Transportation Safety Committee Significant parameters from the flight.

The Oct. 29 crash was the first such incident with the 737 Max variant, which Lion Air was the first carrier to begin using in 2017. This particular aircraft had only been put into operation in August—the flight recorder included 69 hours of operation for 19 flights, including the final one.

The report detailed steps taken by Lion Air, Boeing, and the US Federal Aviation Administration (FAA) in the wake of the crash, which drew attention to the automated feature of the new 737 Max 8 and 9.

A spotlight on the 737 Max’s anti-stall system

A little over a week after the crash, Boeing put out a bulletin advising airline operators on how to deal with erroneous sensor information that would lead to “uncommanded nose down” maneuvers, while the FAA ordered flight manuals to be updated with the process to follow in such a situation. Boeing has said that the aircraft is safe, and that it is working with regulators and investigators to understand the factors leading up to the crash.

The directives prompted several of the biggest US pilots’ unions to say this was the first time they were hearing of the new anti-stall system. “Before the crash we were not provided any information on the MCAS or even its existence,” captain Dennis Tajer, spokesman for the Allied Pilots Association, the union for American Airlines pilots, told Quartz.

He added that bulletins after the crash provided clarity on differences between the override process for this 737 variant compared with the older 737NG model, which the Max succeeds. “We have those differences… [and] are asking further questions to better understand our airplane’s automated flight control systems,” said Tajer.

Tajer added that the directives and bulletins describe a “fairly complex emergency situation,” involving a system that can engage soon after takeoff, when the plane is still at a low altitude, and a number of alerts that could prove confusing or distracting.

In a statement released today, Boeing said that passengers “have our assurance that the 737 MAX is as safe as any airplane that has ever flown the skies.” It also noted that the day before it crashed, the aircraft experienced incorrect air speed data and the activation of the anti-stall system. In that case, the pilot used the recommended procedure, which Boeing’s manual prescribes as a “memory item”—something that can be performed quickly without using a checklist.

According to the crash report, on that flight the shaker indicating an impending stall activated at about 400 feet. The flight system carried out three maneuvers to adjust the nose downward, but the commanding pilot initiated steps to override it a little over eight minutes into the flight. The flight landed safely in Jakarta about an hour and a half after takeoff.

It’s not clear why pilots of the Oct. 29 flight weren’t able to override the system or what procedures they tried to use. According to today’s report, the 31-year-old Indian commanding pilot of JT610 had more than 6,000 hours of flying experience, while the second-in-command, a 41-year-old Indonesian national, had over 5,000 hours of flying experience.

The crash has turned attention to Lion Air’s safety record and lapses, including a plane’s wing clipping an electricity pole in the days after the crash.

What’s next?

As Indonesia continues to investigate the crash, three lawsuits have also been brought against Boeing over the 737 Max’s design.

Boeing may be working with the FAA on a software update for the system. It’s also addressing pilot questions about how the system is designed, said captain Tajer, noting that many are now keen to know more about the software. He says pilots are trying to understand why the system appears to be set up to engage based on a single data source. They’re also asking why, after engaging once automatically, the system keeps triggering repeatedly without in some way seeking confirmation of a stall, perhaps via another sensor or pilot input.

Globally, Boeing has received nearly 4,800 orders for the Max 737 through October. In 2011, Lion Air, one of the biggest customers for the model, ordered more than 200 737 Max 8 and 9 aircraft. The carrier is set to meet with Boeing Friday (Nov. 30) to talk about its order status.

After the crash, other airlines have continued to announce orders of the 737 Max, seen by many as a game-changer for short-haul flights, because of its improved fuel efficiency and higher passenger capacity, and carriers are also putting it into operation on new routes.

This story was updated with additional details from the report and comments from captain Dennis Tajer.