Getting Boeing's top-selling 737 MAX back in the skies faces a critical test this week as the company and US regulators each seek to restore their reputations after two deadly crashes.
The US Federal Aviation Administration convened a summit of global aviation regulators on Thursday to walk through the steps taken to address concerns with the MAX following criticism the agency dragged its feet on the decision to ground the jets.
Most agencies around the world have said little or nothing about the situation since the 737 MAX was grounded following the March 10 Ethiopian Airlines crash, which together with a Lion Air crash in October, claimed 346 lives.
So the gathering in Fort Worth, Texas is expected to provide clues as to whether the aviation safety authorities will be willing to set aside any skepticism about the FAA, which has not yet given the green light for the 737 MAX to fly again.
Regulators "are going to want a lot of explanation," said Richard Aboulafia of the Teal Group. "We're going to learn a lot."
The FAA would prefer to have other agencies quickly follow its lead -- which previously would have been likely -- but several aviation experts think that is improbable.
Europe and Canada could follow the US by weeks in allowing the MAX jets to return to the skies.
China is a wildcard. It was the first country to order the planes grounded in March, and has been sparring with the US for months over trade policy.
- Still the 'gold standard'? -
The FAA said 57 agencies from 33 countries will attend the summit, including China, France, Germany, Britain, India, Indonesia and Ethiopia, as well as the European Union Aviation Safety Agency (EASA) and the United Nation's International Civil Aviation Organization.
The meeting comes after Boeing announced on May 16 that it completed a software update to address a problem with the flight handling program known as the Maneuvering Characteristics Augmentation System.
In both of the MAX crashes, the MCAS pointed the plane sharply downward based on a faulty sensor reading, hindering the pilot's effort to control the aircraft after takeoff, according to preliminary crash investigations.
Boeing CEO Dennis Muilenburg has said repeatedly that there was no design flaw in the 737 MAX, and has described changes to the MCAS system as an "update" rather than a fix.
Still the company's reputation has taken a hit amid reports it knew of the issue before the Ethiopia Airlines crash, and received complaints from US pilots.
Boeing said it is providing additional information to the FAA in anticipation of a certification test flight, a key step in winning regulatory approval.
Long considered an "gold standard" internationally, the FAA's reputation has suffered, amid scrutiny of the oversight process and reports it allowed Boeing to effectively self-certify some features of the MAX.
US lawmakers last week once again castigated acting FAA Administrator Daniel Elwell for lagging behind virtually all other regulators in grounding the planes, acting only after seeing data linking the two crashes.
"So the opposite of data is common sense," bellowed Representative Steve Cohen, a Tennessee Democrat, bellowed during a hearing.
"Data is fine but it's something that's right before your eyes," he said, noting that non-US regulators reacted with appropriate urgency.
Elwell told the panel the agency would permit the 737 MAX to resume flights "only when the FAA's analysis of the facts and technical data indicate that it is safe to do so."
- Lag after US? -
Aviation experts expect the US agency to clear the way for the 737 MAX in time for major American airlines to resume flights in mid-to-late summer, as they have publicly announced.
Elwell said he hoped other regulators "make their un-grounding as close to ours as possible."
Thursday's gathering will offer clues as to whether that is likely.
"We'll have a much better idea of the timeline for the return of the 737 MAX," said Michel Merluzeau of aerospace consultancy AIR.
The process would be slowed if other regulators break with precedent and insist on conducting their own test flights with the upgraded 737 MAX rather than deferring to the FAA's judgment.
Another key question is whether other regulators have different expectations on pilot training than the FAA, which has said simulator training is not needed for all pilots.
Scott Hamilton, founder of Leeham Company, an aviation consultancy, said the FAA's job has been made more difficult with a "drip drip" of negative media reports.
And over the weekend, Boeing acknowledged that it had to correct flaws in the flight simulator software for the MAX.
"How has Boeing assured the FAA there are no more surprises?" Hamilton asked in an email.