US regulators have said they will have to decide whether to fine United Airlines for leaving passengers in idling planes for more than three hours.
The U.S. Department of Transportation, which has specified that passengers should not be held on delayed planes, has said the airline may be subject to fines of up to $27,500 per passenger.
Four U.S. flights were delayed by United in May, beyond the new limits.
Most of the delays were due to thunderstorm activity when take-offs and landings were halted at Denver International Airport.
United has said all customers were offered the opportunity to exit the plane and were provided snacks and water.
The planes took off after the weather had cleared.
Regulations in the US require airlines to provide passengers on long-delayed flights food, water and clean lavatories.
The airlines also are required to let passengers off planes stuck on the ground three hours or more.
Flights delayed for safety and security reasons are not usually subject to fines.