Washington (CNN) — Southwest Airlines’ newest jets are now flying with a barrier that can protect the cockpit when pilots need to open the door during flight, according to the airline.

The company’s first Boeing 737 equipped with the retractable barrier entered service in time for the Labor Day weekend, making it one of the first US air carriers to use it.

The “secondary barrier,” which looks more like a gate than a solid door, was originally required to be installed on all new aircraft starting this summer by a 2023 Federal Aviation Administration rule , however the airline agreed to push back the implementation of the rule by a year.

Aviation labor unions have long argued following the Sept. 11, 2001 attacks that secondary barriers were needed to protect the flight deck when pilo

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