Both American Airlines and Southwest Airlines are pushing back the planned return of their Boeing 737 MAX jets until early September.
Over the weekend, Fort Worth-based American removed its 24 MAX jets from the flight schedule through Sept. 3.
On Thursday, Dallas-based Southwest followed by removing its MAX aircraft from service through Sept. 2, CNBC reported.
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The plane has been grounded around the world since mid-March, after the second of two crashes that killed a combined 346 people in Indonesia and Ethiopia.
Boeing is updating anti-stall software implicated in the crashes. The aircraft maker hopes to conduct a test flight in the coming weeks to demonstrate the changes for the Federal Aviation Administration.
American is canceling about 115 flights a day because of the grounding of its MAX jets.
Chicago-based United Airlines dropped its MAX jets from schedules into early August.