Delta Air Lines plans to offer free Wi-Fi on select flights beginning May 13, the first step in a test to determine whether it will become the second major U.S. airline to offer free in-flight internet service.

The two-week test will feature free high-speed Wi-Fi on 55 daily flights. The flights, all within the United States, will change daily. Passengers will be notified via email, Delta's mobile app and on board. No streaming will be allowed.

Delta, like all U.S. airlines except JetBlue Airways, charges a fee to go online during a flight. Delta's fees vary by flight and how long passengers want to be online, and it also offers an all-day pass for $16 on flights in North America.

Many passengers opt out given the fees and historically spotty service. Reliability and speed has improved as Delta and other carriers move to satellite-based Wi-Fi service.

Delta would not disclose how many passengers pay for in-flight internet service, saying it varies by flight. An industry consultant told the Wall Street Journal last year that fewer than 10 percent of passengers on most flights pay to stay connected. The figure increases to 40 percent when Wi-Fi is free, Valour Consultancy told the Journal.

Delta's move toward free Wi-Fi is not a surprise. Ed Bastian, the airline's CEO, said at a travel industry conference last fall that free Wi-Fi was in Delta's future.

“I don’t know of anywhere else besides in an airplane that you can’t get free Wi-Fi,” he said.

Ekrem Dimbiloglu, Delta's director of onboard product, said the airline plans to eventually offer free Wi-Fi to all passengers but does not know when or what the free service will look like, including whether streaming will be allowed. The tests will determine that, he said.

The airline will monitor the usage rates on the flights with free Wi-Fi and will survey passengers extensively to see what they used it for, how the service was and whether they see value in it because free Wi-Fi would be a big investment for Delta, he said.

"It's just an evolution of us continuing to lead ... the industry when it comes to connectivity and entertainment,'' he said. "We want to be No. 1 in customers' minds.''

Delta offers free in-flight movies and TV shows and introduced free instant messaging in 2017. Travelers can text in-flight using apps including iMessage and WhatsApp.