The investigation into the final minutes of Ethiopian Airlines Flight 302 turned on Tuesday to the secrets in the cockpit voice recorder as Boeing and a shaken global aviation industry hung on the outcome.
The voices of Captain Yared Getachew and First Officer Ahmednur Mohammed could reveal what led to the March 10 crash of the Boeing 737 MAX that has worrying parallels with another disaster involving the same model off Indonesia in October.
The twin disasters killed 346 people.
Black box data was downloaded in France but only Ethiopian experts leading the probe have heard the dialogue between Getachew, 29, and Mohammed, 25. The data was back in Addis Ababa on Tuesday, sources familiar with the probe told Reuters.
Experts believe a new automated system in Boeing's flagship MAX fleet - intended to stop stalling by dipping the plane's nose - may have played a role in both crashes, with pilots unable to override it as their jets plunged downwards.
Both came down just minutes after take-off after erratic flight patterns and loss of control reported by the pilots. However, every accident is a unique chain of human and technical factors, experts say.
The prestige of Ethiopian Airlines, one of Africa's most successful companies, and Boeing, the world's biggest planemaker and a massive U.S. exporter, is at stake in the inquiry.
AWKWARD QUESTIONS FOR INDUSTRY
Lawmakers and safety experts are questioning how thoroughly regulators vetted the MAX model and how well pilots were trained on new features. For now, regulators have grounded the existing fleet of more than 300 MAX aircraft and deliveries of nearly 5,000 more - worth well over $500 billion - are on hold.
Pressure on the Chicago-headquartered company has grown with news that federal prosecutors and the U.S. Department of Transportation are scrutinising how carefully the MAX model was developed, two people briefed on the matter said.
The U.S. Justice Department was looking at the Federal Aviation Administration's (FAA) oversight of Boeing, one of the people said. And a federal grand jury last week issued at least one subpoena to an entity involved in the plane's development.
In the hope of getting its MAX line back into the air soon, Boeing said it will roll out a software update and revise pilot training. In the case of the Lion Air crash in Indonesia, it has raised questions about whether crew used the correct procedures.
"Lives depend on the work we do," acknowledged Boeing boss Dennis Muilenburg, facing the biggest crisis of his tenure.
The MAX, which offers cost savings of about 15 percent on fuel, was developed for service from 2017 after the successful launch by its main rival of the Airbus A320neo.
After Ethiopia, France and the United States all noted parallels with the Indonesia crash, one person familiar with the probe said black box data showed the Ethiopian Airlines jet's "angle of attack" was "very similar" to the Lion Air plane.
The angle of attack is a fundamental parameter of flight, measuring the degrees between the air flow and the wing. If it is too high, it can throw the plane into an aerodynamic stall.
In the hot seat over its certification of the MAX without demanding additional training and its closeness to Boeing, the FAA has said it is "absolutely" confident in its vetting.
But given the U.S. probe, Canada said it would re-examine its acceptance of the FAA validation and do its own independent certification.
The crisis has put the airline world in a spin.
One company, Norwegian Airlines, has already said it will seek compensation after grounding its MAX aircraft.
Various firms are reconsidering Boeing orders, and some airlines are revising profit forecasts given they now cannot count on maintenance and fuel savings factored in from the MAX.
Beyond the corporate ramifications, anguished relatives are still waiting to find out what happened.
Many have been visiting the crash site in a charred field to seek some closure, but there is anger at the slow pace of information and all they have been given for funerals is earth.
Abdulmajid Shariff, a Yemeni who lost his brother-in-law, was heading home on Tuesday. "I'm just so terribly sad. I had to leave here without the body of my dead brother. But I have to praise almighty God, there is nothing more to do."