People walk past wreckage at the scene of the Ethiopian Airlines Flight 302 plane crash, near the town of Bishoftu, southeast of Addis Ababa, Ethi­o­pia, on March 10. (Tiksa Negeri/Reuters)

While authorities in Ethi­o­pia on Thursday described key similarities between the crash of a 737 Max in Addis Ababa and another in Indonesia, important questions about their causes remain unanswered, underscoring how difficult it will be for Boeing to rebuild trust and persuade newly emboldened international aviation safety regulators to allow the plane back in the air.

In both crashes, investigators say an automated anti-stalling feature known as the Maneuvering Characteristics Augmentation System, or MCAS, repeatedly pushed the planes’ noses downward, thwarting pilots who were struggling to regain control.

Ethio­pian Transport Minister Dagmawit Moges said Thursday that the crew of Ethio­pian Airlines Flight 302 “performed all the procedures, repeatedly, provided by the manufacturer but was not able to control the aircraft.” She cited “repetitive uncommanded aircraft nose-down conditions,” and called on Boeing to review “flight controllability” issues affecting the plane.

According to the preliminary crash report, which was released by Ethio­pian authorities, the plane’s two external “angle-of-attack” sensors gave dramatically different readings — 74.5 degrees on the left, and 15.3 degrees on the right. The sensors are supposed to measure the difference between the nose’s direction and the angle of the oncoming wind, key information to prevent stalling. But instead, one was going haywire.

At one point, both pilots called out “left alpha vane,” according to the cockpit voice recorder, using another name for the sensor.

But the crew was unable to regain control. Even after the plane’s auto­pilot was disengaged, the flight data recorder shows, the nose of the plane was forced down “four times without pilot’s input,” according to the preliminary report.


A Boeing 737 Max airplane sits on the production line at the company's manufacturing facility in Renton, Wash., on March 27. Boeing said it was agonizingly close to a software fix for its 737 Max jetliner when an Ethiopian Airlines jet plunged to the ground March 10, the second deadly crash in less than five months. (David Ryder/Bloomberg)

Among the core questions following Moges’s statement and the release of the preliminary report is how Ethio­pian authorities, on the one hand, and Boeing executives and the Federal Aviation Administration, on the other, could have presented such starkly different pictures of how pilots can handle such issues with the 737 Max.

Boeing acknowledged late Thursday that the system malfunctioned, and its chief executive apologized.

“We at Boeing are sorry for the lives lost in the recent 737 MAX accidents,” Boeing chief executive Dennis A. Muilenburg said. “These tragedies continue to weigh heavily on our hearts and minds, and we extend our sympathies to the loved ones of the passengers and crew on board Lion Air Flight 610 and Ethiopian Airlines Flight 302.

“The full details of what happened in the two accidents will be issued by the government authorities in the final reports, but, with the release of the preliminary report of the Ethiopian Airlines Flight 302 accident investigation, it’s apparent that in both flights the Maneuvering Characteristics Augmentation System, known as MCAS, activated in response to erroneous angle of attack information,” Muilenburg said.

But the company also acknowledged that during a review of the MCAS system, an additional software problem was discovered unrelated to the stall-prevention system. While Boeing described the additional problem as “relatively minor,” two officials familiar with the crash investigation said the issue is critical to flight safety. The FAA has ordered the problem fixed before the 737 Max is cleared to fly again.

It was a stunning turn of events for the company. Mike Sinnett, Boeing’s vice president of product strategy and development, had said last week that “pilots can always electrically or manually override the automatic system,” referring to MCAS and a related automated feature.

The FAA, in an emergency order nine days after the Indonesia crash, said bad data from an external “angle-of-attack” sensor could lead to “difficulty controlling the airplane” and “possible impact with terrain.” But the FAA said pilots could “disengage autopilot” and use other controls and adjust other switches to fly the plane in such a case.

Moges essentially said those procedures didn’t work. But why?

Kevin McAllister, president and chief executive for Boeing Commercial Airplanes, said the company will examine the preliminary report, adding: “Understanding the circumstances that contributed to this accident is critical to ensuring safe flight. We will . . . take any and all additional steps necessary to enhance the safety of our aircraft.”

In a statement, the FAA said: “We continue to work towards a full understanding of all aspects of this accident. As we learn more about the accident and findings become available, we will take appropriate action.”

Aviation experts also pointed to a host of additional ongoing questions about what led to the dual disasters that left 346 people dead in less than five months.

Among them: What was the precise sequence of failures — technological and human — at play?

Experts said erroneous information from the “angle-of-attack” sensors triggered MCAS in both disasters.

But how did such crucial data become flawed in the first place, given that Boeing says it has tens of millions of hours of previous experience with the sensors, which it considers highly reliable?

Amdeye Ayalew, who is heading the Ethio­pian investigation, said the sensor was neither damaged by a foreign object nor affected by a “structural design problem.”

According to the preliminary report, among the urgent issues in the cockpit was talk of icing warnings in connection with the sensor.

“The First Officer called out Master Caution Anti-Ice,” according to the black box recording. Four seconds later, a “heat parameter” for the left angle-of-attack sensor “changed state,” according to the report. Minutes later, the first officer again mentioned the anti-ice issue, seconds before referencing the left sensor.

There has been no information released publicly about any ongoing mechanical problems with the Ethio­pian Airlines plane, experts said.

“Lion Air had mechanical problems for days before the accident. Ethiopian did not. So there’s two completely different paths to get the erroneous data started,” said John Cox, a former pilot and an airline safety consultant who has been privately briefed on the evidence in Ethi­o­pia by people familiar with the investigation. “Once the data became erroneous, then you have very similar accidents. But you have very different accidents up to there.”

An FAA spokesman declined to say what the agency has learned about the source of the erroneous data in the crashes.

Shem Malmquist, a Boeing 777 captain and visiting professor at the Florida Institute of Technology, said a possible explanation for the bad data may be a malfunction in a device that filters critical flight data from the angle-of-attack sensors and other inputs and sends it to the pilots’ cockpit displays. Those devices are known as air data inertial reference units.

Similar problems on Boeing, Airbus and other planes were “triggered by a bad processing algorithm, or even just a momentary data spike,” he said.

He thinks a problem with the sensor itself is less likely.

“I know of only a couple of events directly related to the angle-of-attack vanes themselves. One of those involved frozen vanes due to water getting into them during some unusual circumstances,” Malmquist said.

Boeing has previously said it does not plan to make changes in the angle-of-attack sensors it uses on several models of its planes, the 737 Max among them.

Instead, Boeing last week detailed a software update, in the works for months, that will change the way the data from the sensors is processed. Once the fixes are made, the MCAS feature will use data from both sensors, rather than just one — and if there’s a disagreement of 5.5 degrees or more between the two, that automated feature won’t be used for the remainder of the flight.

Boeing said the software fixes will also include new limits on how often MCAS activates and how strongly it seeks to correct the plane’s flight path once it does. The feature was supposed to increase safety by preventing stalling, as the 737 Max’s designers had shifted the position of the plane’s engines as part of an effort to save fuel and compete with Airbus.

Civil aviation authorities around the world were among the first to ground the planes, and analysts said they will play a bigger role than before in decisions about returning them to the skies. Some say the FAA’s decision as the last of the major aviation safety authorities to ground the jets — along with its close ties to Boeing — may have cost it some credibility among its counterparts around the world.

Some U.S. officials also said they do not want the planes to resume flying until an independent third party analyzes the changes.

Rep. Peter A. DeFazio (D-Ore.), chairman of the House Transportation and Infrastructure Committee, and Rep. Rick Larsen (D-WA) said they want the FAA to bring in third-party experts to examine “any recommended technical modifications and any proposed new training requirements for pilots,” which would cover the changes connected with the MCAS feature.

To address those shared concerns, the FAA announced the formation of a team to conduct a technical review of the system. The team, chaired by former National Transportation Safety Board chairman Christopher Hart, will include representatives from the FAA, NASA and international aviation authorities. Representatives from civil aviation authorities in Ethiopia, Indonesia, the European Union, Brazil, Canada, China and Singapore have been invited to participate.

“Now it is not just up to FAA to certify and say, ‘It’s good,’ ” Malmquist said. “All of these different agencies around the world are now saying, ‘No, we really need to have this validated,’ and that is going to be a very interesting problem that I don’t think we have seen before.”

“China, the United Kingdom, Australia — all of them are going to be closely scrutinizing and needing to look at the changes and what’s being proposed, and everyone has to agree to it or you are going to have sections of the world where the airplane won’t be allowed to fly,” Malmquist said.

Officials in Canada said they will not lift flight restrictions on the 737 MAX until they are “fully satisfied that all concerns have been addressed by the manufacturer and the U.S. Federal Aviation Administration and adequate flight crew procedures and training are in place.”

In Australia, officials say they will conduct a thorough review, adding that it is still too early to know when the ban will be lifted. “Given there is not yet any formal advice from the Ethiopian accident investigation it is too early to make judgments on when and how the aircraft type can safely reenter service,” said Peter Gibson, a spokesman for Australia’s Civil Aviation Safety Authority (CASA).

At issue in the crashes are questions about the soundness of Boeing’s original design of the MCAS system, and why the company didn’t originally require training for pilots on how it works. It also renews questions about assurances offered by Boeing and the FAA that the planes were safe.

Cox, formerly the top safety official for the Air Line Pilots Association, pointed to differences in the run-up to the two crashes.

The Ethio­pian Airlines flight “had flown to Johannesburg and back without any maintenance issues,” Cox said.

By comparison, the Lion Air 737 MAZ had multiple failures starting Oct. 26, including during the four flights before the one that crashed on Oct. 29, according to the preliminary report on the crash. The plane’s maintenance log indicated that pilots reported problems with the speed and altitude displays and that mechanics were brought in to do troubleshooting.

Ultimately, investigators want to understand how much of the crash was due to the airplane itself and whether pilots might somehow have found a way to avoid disaster.

Boeing’s proposed changes also will include additional pilot training and updates to flight manuals.

Even as investigators examine the factors that contributed to the two crashes, the FAA continues to face accusations that it was too slow the ground the planes. There are also questions about the FAA’s relationship with Boeing and the process used to certify the plane was safe.

The Justice Department’s criminal division has launched a probe into the plane, and the Transportation Department’s inspector general is investigating the certification process, among other things.

Ashley Halsey III contributed to this report.