American Airlines plans to join United in filling flights to capacity as the number of travelers choosing to fly continues to rise.
However, air travel is still down almost 80% compared to this time last year, and airlines expect a long, sluggish recovery. Industry experts have expressed concern over capping seat capacity.
"In the long term, that's a business model that cannot be sustained," Nick Calio, head of a lobbying group that represents the major U.S. airlines, said at a hearing in early May. "If it costs more to fly people from point A to point B, it's a total money-losing proposition, which then means it's a job-loss proposition."
Here's what airlines are doing now:
Starting July 1, American is lifting its seating cap of 85%, allowing all middle seats to be filled.
American said it will continue to notify customers through the end of September if planes are filling up and "allow them to move to more open flights when available" without paying a change fee. Additionally, passengers can ask to move to another seat within their ticketed cabin once boarding is complete if there are no weight or balance restrictions.
An American spokesperson pointed to its notification to customers, enhanced cleaning procedures, HEPA filters, and mandatory mask-wearing policy as reasons for deciding to fill flights.
United, which originally said it was limiting advanced seat selections, now said it never put a cap on capacity or blocked middle seats, according to a company spokesperson.
United said customers are contacted 24 hours before departure if their flights approach full capacity "so they can decide whether to adjust their plans before they arrive at the airport." If a traveler gets to the gate and more than 70% of customers on that flight have checked in, customers will be allowed to adjust their plans.
DELTA AIR LINES
Delta said it will cap cabin seating at 60% in the main cabin, at 50% in first class and block middle seats through the end of September.
JetBlue said it would have empty middle seats through the end of July.
Southwest said middle seats will remain open through the end of September.
Customers still can pick their own seats, and the airline will not block or direct seating so families may sit together.
ABC News' Sam Sweeney contributed to this report.