FORT WORTH — After an all-day meeting with global aviation regulators, the chief of the Federal Aviation Administration sounded more upbeat than ever about prospects for clearing the troubled Boeing 737 Max to fly again.

Aviation officials from more 30 countries met with the FAA to hear the U.S. regulator’s approach to reviewing changes that Boeing is making after two crashes that killed 346 people.

“We are going through an incredibly intensive and robust process to make the safety case to unground the Max,” acting FAA Administrator Daniel Elwell told reporters when the closed-door meeting was over. He added that the agency won’t let the plane fly “until we have made that safety case.”

Boeing has not yet submitted a final, formal application for its update to a flight-control system that has been implicated in crashes in Indonesia and Ethiopia. That submission will be followed by test flights to demonstrate the changes to FAA experts.

Elwell declined to put a timetable on the plane’s return or comment on reports that FAA officials told a separate meeting of airline officials in Montreal that the plane could be cleared for flights in the U.S. as soon as late June.

The meeting Thursday in Fort Worth, Texas, was crucial to the U.S. agency’s hopes of convincing other regulators around the world to lift their bans on the plane soon after the FAA does.

Among those scheduled to attend were regulators from China, Europe and Canada, as well as officials from Indonesia and Ethiopia, sites of the two crashes that occurred before the Max was grounded worldwide in March.

Boeing is fixing flight-control software that in each accident pushed the plane’s nose down based on faulty readings from a single sensor. It will tie the system to more than one sensor and make it less powerful _ pilots for Lion Air and Ethiopian Airlines were unable to counter the system’s automatic nose-down pitch.

Elwell has said he hopes other regulators will lift their bans on the plane soon after the FAA does.

However, regulators in China, the European Union and Canada have said they plan to conduct their own reviews of Boeing’s software changes and have stressed the need for additional pilot training.

Once airlines get the green light, they will have to remove their Max jets from long-term storage and prepare them for flying. That will take anywhere from two days to a week, said Ali Bahrami, the FAA’s associate director of aviation safety.

A far more significant delay in the plane’s return to service could occur if regulators decide that pilots should train in flight simulators first. Boeing is pushing for computer-based training only. Elwell said the FAA has not made a final decision, and that during Thursday’s meeting no other country said it would insist on simulator training.

The FAA did not allow reporters to attend or watch the meeting, and it kept them away from international aviation officials who attended the all-day session at a gated FAAoffice.

It is unclear whether the event will do much to convince travellers that the Max is safe.

Barclays said that its survey this month of 1,765 travellers in North America and Europe found that nearly half plan to avoid flying on the Max for a year or longer. About half said they would pick a non-Max flight if given the choice.

Airlines are making plans for a campaign to reassure nervous customers. They know it won’t be easy.

American Airlines CEO Doug Parker told NBC that because of all the news coverage of the crashes and their aftermath, no amount of marketing will sway worried passengers.

“There may be some period of time” before customers are comfortable flying on the plane, Parker said, “but we will work through that.” He suggested that if passengers see U.S. pilots getting on board, they will follow.

United Airlines CEO Oscar Munoz said this week that he will be on the carrier’s next Max flight. He added, however, that United would let passengers who don’t want to fly on the plane rebook without the customary ticket-change fee.

The Flight Safety Foundation, a non-profit group based in the Washington suburbs, is urging regulators to co-ordinate recertification of the Max. The group’s CEO, Hassan Shahidi, said that would lift public confidence and be less disruptive than a fragmented, country-by-country return of the plane.