Southwest Airlines on Friday began flying its first jetliner built with a secondary flight deck barrier designed to prevent intrusions in a design change that's been in the works since the Sept. 11, 2001, terror attacks.

The airline took delivery of a Boeing 737 Max 8 in recent days that flew from Phoenix to Denver and the flight serves as a milestone for the beginning of the new safety feature being deployed across new commercial airliners entering U.S. service in the coming years.

Secondary barriers have been sought after since the Sept. 11 attacks occurred after terrorists were able to force their way into cockpits and hijack the four airliners used in what became the deadliest terror attack in history. Aviation safety experts and pilots' unions have argued the secondary barriers wi

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