(Fixes typo in paragraph three from detect to deck)

By David Shepardson

(Reuters) -Southwest Airlines began Friday flying its first jet with a secondary barrier to the flight deck designed to prevent intrusions.

The plane - a Boeing 737 MAX 8 which was delivered in recent days - took off Friday afternoon from Phoenix to Denver, the airline said.

Secondary barriers -- long sought after the Sept. 11, 2001 attacks that exposed the risks of inadequate flight deck protection -- are crucial to aviation safety, pilots unions have argued.

The flight is a milestone that marks the beginning of the roll out of the safety feature across the new U.S. commercial airline fleet in the coming years.

Boeing and Airbus told Reuters they have begun delivering airplanes with the barriers under the Federal Aviation Administration regulation announced in 2023 that took effect on Monday.

Last month, the FAA agreed to give airlines until late July 2026 to put the barriers in newly delivered planes into use. Most major carriers have told the FAA they plan to take advantage of the extension and not immediately begin using the barriers.

Southwest has opted to immediately begin complying as they take delivery of new airplanes and expects to take delivery of about 25 additional Boeing airplanes this year that will have the barriers.

"We felt like we could get it done and put it in production as soon as the aircraft was ready," said Justin Jones, Southwest's executive vice president for operations

After the hijacking of four U.S. airplanes on September 11, 2001, the FAA adopted standards for flight deck security to make them resistant to forcible intrusion and unauthorized entry.

The FAA rule requires aircraft manufacturers to install a second physical barrier on new planes used in commercial passenger service in the United States, but does not require existing planes to be retrofitted. Plane makers without FAA certification of barriers do not yet need to be in compliance.