Boeing executives said they expect the Boeing 737 Max jet to return to service in mid-2020, but announcements of schedule changes by the three major U.S. carriers that operate the jets are raising new questions about that timeline.

American, Southwest and United airlines all announced they are revising their schedules for when the jet will return to commercial service. American announced Friday that it was pulling the jet from its schedule through Aug. 17. American had previously expected to return the jet to its schedule June 4.

Just one day before, Southwest said it would pull the plane from its schedule through Aug. 10.

“By proactively removing the MAX from scheduled service, we can reduce last-minute flight cancellations and unexpected disruptions to our Customers’ travel plans,” Southwest said in a news release announcing the schedule change. “The limited number of customers who have already booked their travel and will be affected by our amended schedule will be notified of their reaccommodated travel according to our flexible accommodation procedures.”

Southwest said the change would remove roughly 371 weekday flights from its “total peak-day schedule” of more than 4,000 daily flights.

United said it would cancel all Boeing Max flights through Sept. 4.

The airlines said the decision was based on the latest guidance it had received from the Federal Aviation Administration, the Department of Transportation and Boeing.

The jetliner has been grounded worldwide for almost a year after two fatal crashes in less than five killed 346 people. The first in October 2018, involved a Lion Air jet, which crashed shortly after takeoff into the Java Sea, killing all aboard. Then on March 10, an Ethiopian Airlines jet, carrying 157 passengers and crew crashed after taking off from Addis Ababa Bole International Airport. In all, 346 people died in the crashes.

Boeing has been working for months on a software fix designed to address issues with an automated flight control system known as the Maneuvering Characteristics Augmentation System (MCAS), which has been implicated in the crashes. Investigators believe the system triggered after receiving erroneous information from a single angle-of-attack sensor. Pilots were unaware MCAS had been added to the 737 Max and investigators think that complicated their ability to respond when the system malfunctioned, pushing the planes’ noses down repeatedly.

The software fix and new requirements for pilot training must be approved by the FAA before the plane is allowed to resume flying. Speaking to reporters in Singapore this week, FAA Administrator Steve Dickson said there is no set timeline for returning the plane to service, Reuters reported.

A spokesman for Boeing said the company continues to work through the recertification process.

“With safety as our highest priority, we are working with regulators to appropriately address all certification requirements and safely return the [plane] to service,” said Peter P. Pedraza, a Boeing spokesman.