The CEO of Ethiopian Airlines says his airline might never fly the Boeing 737 Max again after a deadly crash in March, and if it does, it will wait until other carriers use the plane first.
Tewolde Gebremariam told NBC News that Ethiopian won't use the plane unless the airline, its pilots and passengers are convinced that it is safe.
"If we fly them again, we will be the last airline to fly them again," he said.
A Boeing spokesman, Charles Bickers, said the company "is working closely with pilots, airlines and global regulators to update the Max and help prevent this tragic loss of life from happening again."
Nearly 400 Max jets operated by dozens of airlines remain grounded around the world while Boeing works to fix flight-control software implicated in two accidents that killed 346 people, including 157 on Ethiopian Flight 302.
Boeing's changes are designed to reduce the software's ability to pitch the nose of the plane down in some circumstances, and to make it easier for pilots to control.
Boeing CEO Dennis Muilenburg said last month that the Chicago-based company had flown more than 130 test flights with an update to the software. Boeing expects the next step, a certification flight with Federal Aviation Administration people on board, "in the near term," he said.
However, that final test has not been scheduled yet, raising questions about how quickly Max jets can be retrieved from storage and put back in service.
In the U.S., Southwest Airlines and American Airlines have steadily pushed the return of the Max farther into the future. Both have removed it from their schedules into August, although executives suggested last month that the plane could be back before then. United Airlines, the only other U.S. carrier to operate the Max, took them out of the schedule until early July.
Boeing has not given Southwest a timeline for the plane's return, said Brandy King, a spokeswoman for the Dallas-based airline.
"It seems that there are too many variables at play," King said. She said it would take 30 to 60 days for Southwest to take the planes out of storage and get them ready to fly.
Also, extra training will be required for pilots, although the nature of that training is not settled. Boeing is pushing for computer-based tutorials that would be much quicker than training sessions in flight simulators.
American Airlines is working with Boeing, the FAA and its pilots, "and if necessary, will make additional adjustments to our schedule," said airline spokesman Ross Feinstein.
The FAA won't say how long its review of Boeing's changes will take. Spokesman Gregory Martin said FAA "will take whatever time is necessary to have the safety analysis we need to make an informed decision."
Industry experts believe that regulators in other countries will take longer than the FAA to let the Max fly again. Other regulators have indicated they want to conduct their own reviews, and they could impose additional conditions.
Canada's transport minister, for example, has said he wants pilot training to include time in flight simulators, which would delay the plane's return in that country.
The Justice Department is investigating Boeing. The FAA's 2017 decision to certify the Max has also come under scrutiny by the Transportation Department's inspector general and Congress. A House subcommittee plans to question acting FAA chief Daniel Elwell at a hearing Wednesday.
The FAA, as part of a plan to win international support for its handling of the situation, will meet with aviation-safety experts from other countries next week in Fort Worth, Texas.
Boeing has a backlog of about 4,600 orders for the Max and has avoided major cancellations despite the deaths of 189 people in an Oct. 29 crash of a Lion Air Max off the coast of Indonesia and the March 10 Ethiopian crash near Addis Ababa, which killed all 157 aboard.
Gebremariam, the Ethiopian Airlines CEO, said he couldn't say whether the airline will ever use the Max again.
"It takes a lot of effort to convince everybody that the airplane is safe," he said.