DALLAS (CBSDFW.COM) – Some international airlines are grounding the new, most state-of-the-art passenger jets, but none are in the United States. Fear over the safety of Boeing’s 737 Max 8 model is spilling out on social media, after two tragedies involving the same aircraft within a span of five months. The crash of a Lion Air flight off the coast of Indonesia in October killed 189 people. The crash of an Ethiopian Airlines flight Sunday killed 157.

Southwest is the second largest carrier at Denver International Airport and American has dozens of flights in and out of Denver each day.

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“It seems unsafe to continue flying them,” one man wrote American Airlines on Twitter.

“I will straight up cancel my tickets if my flight is on a 737 Max,” a woman wrote Southwest Airlines.

The union representing American Airlines flight attendants, the APFA, expressed concern, as well, telling members in a letter, “If you feel unsafe working the 737 Max, you will not be forced to fly it.”

Southwest and American are the only US airlines flying the 737 Max 8, with 34 and 24 planes in their fleets respectively.

Both airlines released statements Monday expressing confidence in the safety of their planes.

“The fact that there’s been two crashes of this airplane in the past six months may or may not be relevant. It’s just hard to say,” said Denny Kelly, a retired airline captain and aviation safety consultant.

Kelly says he’s not worried about the US based airlines, where crews are experience and well-trained.

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“The problem is if you get outside the United States there are some airlines, foreign carriers that operate this airplane, that I wouldn’t ride on. Period,” he said.

The FAA said it is too early to know what caused Sunday’s crash or to know if there’s any connection between the two recent tragedies.

It announced, though, software enhancements would be implemented on the 737 Max planes as a result of its investigation into the October crash.

“We don’t have details, but we suspect it is what our pilots have been asking for,” said Dennis Tajer, a spokesperson for the Allied Pilots Association, which represents American Airlines pilots.

The union previously criticized Boeing for failing to inform pilots about a new safety feature in the 737 Max that may have played a role in the Lion Air Crash.

Tajer said Monday, he now feels confident the planes are safe, emphasizing pilots would refuse to fly them otherwise.

“We’re on the airplane with our passengers so we’re uniquely qualified to say this is good to go versus someone sitting at a desk,” he said.

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