ADDIS ABABA, Ethiopia — Ethiopian investigators said on Thursday that the pilots of a doomed Ethiopian Airlines Boeing Max jet repeatedly followed procedures recommended by Boeing but were still unable to recover before the plane dove into the ground last month.
The crash of Ethiopian Airlines Flight 302 on March 10 followed the unrecoverable nose-dive almost five months earlier of another jet of the same model, a Boeing 737 Max 8, in Indonesia. Indonesian investigators have implicated a malfunctioning automated anti-stall program in that disaster, in which the plane’s computer system appeared to override pilot directions based on faulty data.
Ethiopia’s minister of transportation, Dagmawit Moges, explained investigators’ initial findings, based on analysis from 18 Ethiopian and international investigators and information from the jet’s flight data recorder and cockpit voice recorder, in a news conference in Addis Ababa.
The preliminary report was the first official investigation into the Ethiopian Airlines flight, and the news conference on its findings included few details, but it confirmed some initial suspicions about the crash.
Ms. Moges said that the flight crew repeatedly followed procedures recommended by the plane’s manufacturer “but was not able to control the aircraft.”
Ms. Moges recommended that the plane’s flight control system be reviewed by the manufacturer. And she suggested that the aviation authority ensure that a review of the aircraft flight control system be adequately addressed by the manufacturer before the aircraft is released for operations.
During the news conference, Ms. Moges did not specify who the manufacturer was or to which aviation authority she was referring. Nor did she mention the anti-stall system, known as MCAS, by name in the news conference.
After the conference, however, Ms. Moges told The New York Times that MCAS had been active during Flight 302’s brief journey. “The pilots have turned the MCAS on and off, but I can’t say how many times because we will find that out when we have the final report,” she said.
The initial findings are likely to heighten scrutiny of the Max, Boeing’s newest and top-selling generation of jets. Since the Ethiopian Airlines crash, airlines worldwide have grounded their Max fleets, amid concerns over the stall-prevention system’s apparent propensity to malfunction when fed erroneous data.
The crash of Ethiopian Airlines Flight 302, which killed all 157 people on board, came less than five months after Lion Air Flight 610 plunged into the sea off the island of Java, killing all 189 passengers and crew members.
Both 737 Max 8 jets crashed at high speed minutes after takeoff in clear weather, following roller-coaster trajectories that hinted at desperate struggles by pilots to control planes seemingly immune to their interventions.
Investigations into both crashes are continuing. A final report on the Lion Air accident is expected in August, at the earliest. Ethiopian officials said on Thursday that their final findings may take a year to be released.
Indonesian investigators have focused on whether the anti-stall system was triggered by incorrect data on the plane’s angle of attack, essentially a measure of an aircraft’s likelihood of stalling.
MCAS, the stall-prevention software, was introduced in all Boeing 737 Max jets, and it pushes a plane’s nose downward if data indicates that the aircraft could stall because it is angled upward too steeply.
After the Lion Air crash last October, pilots and airlines complained that they had not been adequately briefed on MCAS by Boeing. The Max manual had no specific mention of how to correct a malfunctioning MCAS. Some pilots reported that they hadn’t even known of the software’s existence.
In creating the Max jet, Boeing added bigger engines to the 737, which gave the airplane the fuel efficiency it needed to compete with a new model from its rival, Airbus. But the change also altered the jet’s aerodynamics: The larger engines had a tendency to push the airplane’s nose up in certain flight conditions.
To compensate, Boeing engineers created MCAS, which was meant to make the Max behave more like older versions of the 737. In order to receive certification to fly the Max, some pilots with prior 737 experience only had to complete a couple hours of online training.
Although Boeing has said that existing procedures were sufficient to address an MCAS malfunction, early data suggests that pilots may not have known how to disengage the system, or may have done so too late to save their flights.
While most airplane systems are built with backup redundancies to prevent a single data malfunction from altering a plane’s course, MCAS is triggered by data from just one angle of attack sensor, not two.
After the news conference, the United States Federal Aviation Administration released a statement saying it was still working with Ethiopian officials to investigate the crash. “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,” the F.A.A. said.
In the days leading up to the crash of Lion Air Flight 610, the Max plane experienced airspeed data problems that led a maintenance crew to replace an angle of attack sensor. That sensor was later deemed to be defective, according to Indonesian investigators who traveled to Minnesota to meet with employees of Rosemount Aerospace, the Boeing subcontractor that made the device.
Now, Indonesian investigators are looking at whether the replacement sensor, which was described last year as “serviceable” by the head of Indonesia’s National Transportation Safety Committee, may also have malfunctioned.
Indonesian officials have asked for their American counterparts to check in with XTRA Aerospace, the Florida-based company that serviced the replacement sensor, according to Nurcahyo Utomo, the head of the safety group’s air-accident subcommittee.
After the defective sensor was replaced, the plane recorded a 20 degree differential between the two angle of attack measurements — enough, in theory, to have triggered MCAS.
Indonesian investigators are also examining whether the computer unit that processes air data from various probes and sensors, including the angle of attack vane, may have had either a hardware or software malfunction. The Lion Air plane experienced days of data problems before the Oct. 29 crash.
Boeing created safety features to help pilots detect any erroneous readings from the angle of attack sensors — an angle of attack indicator that displays readings from both of the sensors that poke out from the plane’s nose, and a disagree light that activates if the two sensors are at odds with one another — but both were optional upgrades.
Lion Air did not purchase these optional features for its Max planes. Nor, it appears, did Ethiopian Airlines.
“When Boeing delivered the plane, it delivered everything that could help the plane fly safely,” said Tewolde GebreMariam, the chief executive of Ethiopian Airlines, in a news conference last month.
“Then it offered a lot of options for convenience,” he added, describing the angle of attack features as among these options.
Soon after the Ethiopian Airlines accident, as regulators around the world grounded the Max, the United States transportation secretary called for an investigation into the model’s approval process.
Questions have been raised about whether the F.A.A. allowed Boeing officials to aid in the certification of the Max.
Boeing says it is working on a software update to MCAS, new training guidelines for 737 Max pilots and a retrofitting of existing planes to provide them with the angle of attack features that had previously been optional.
The MCAS update was supposed to have been rolled out earlier this year, but it was delayed. Polana Pramesti, the head of Indonesia’s civil aviation authority, said on Thursday that she was told by the F.A.A. in a teleconference that the new software would be introduced “in a few weeks.”
But it will take airlines some time to test the new software and restore the trust of customers leery about the Max, given two fatal crashes within half a year.
In a statement released on Twitter after the news conference on Thursday, Ethiopian Airlines said pilots on the doomed flight had followed emergency procedures recommended by Boeing and approved by the F.A.A.
“It was very unfortunate that they could not recover the airplane from the persistence of nose diving,” the airline said.
In the news conference, however, Ms. Moges, the transport minister, cautioned against holding any party responsible for the plane’s fatal plunge.
“The major objective of this investigation is to make sure that there is safety in the aviation sector,” she said. “It is not to blame someone.”