Southwest Airlines

The cases will be determined on a case-by-case basis, depending on the company. Here's a look at where each of the major U.S. airlines stands.

Delta, United, Southwest, American and Alaska Airlines and other say masks are now optional for travelers on their aircraft.

Hundreds of Southwest Airlines flights are either canceled or delayed on Monday, after a weekend of disruptions that the carrier has blamed on air traffic control issues and weather conditions.

Southwest Airlines canceled 28% of its scheduled flights over the weekend, blaming the woes on air traffic control issues and weather.

Southwest Airlines flights were delayed across the country Monday night because of a problem with the company's weather data provider. Flights were again delayed on Tuesday.

Southwest said its decision is "based on continued uncertainty" about when the FAA will allow the Max planes to return to service. Boeing's 737 Max fleet has been grounded worldwide since March.

The pilots say they stand to lose $100 million in income. Southwest has canceled more than 30,000 flights since the FAA grounded the aircraft in March after two crashes killed 346 people.

The carrier says the Aircraft Mechanics Fraternal Association "has a history of work disruptions." The union says the airline is "scapegoating" its technicians.

The Federal Aviation Administration says CFM engines, which power many Boeing 737s, need to be ultrasonically inspected after a certain number of takeoffs and landings.

Seven other people were treated for injuries when the flight from New York, bound for Dallas, was forced to land in Philadelphia. Federal investigators say a fan blade separated from the engine.