American is extending cancellations through April 24 and and Southwest Airlines is making cancellations 5 days in advance.
American plans to cancel approx. 90 flights per day, not all of which are scheduled for a Boeing 737 Max. Plus, some flights scheduled with other models of planes may also be cancelled. American plans to contact those affected, directly.
United Airlines has 14 Boeing 737 Max planes, doesn't have flights scheduled with them through April 9, doesn't predict any future related cancellations, and is making up for the issue with other aircraft.
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