DALLAS, Texas: Southwest Airlines has become the first U.S. carrier to fly a jet equipped with a secondary cockpit barrier, a safety feature designed to further secure the flight deck from intrusions.

Southwest said the new Boeing 737 MAX 8, delivered to the airline recently, took off on August 29 from Phoenix to Denver with the reinforced protection in place. The flight marks the start of a gradual rollout of secondary barriers across the U.S. commercial fleet.

The feature has been advocated for since the September 11, 2001, terrorist attacks exposed the risks of inadequate cockpit protection. Pilots' unions and aviation safety groups have long argued for an extra line of defense when cockpit doors are opened in flight, such as when crew members exit.

Boeing and Airbus confirmed they have begun delivering aircraft fitted with the new barriers under a Federal Aviation Administration (FAA) regulation announced in 2023, which formally took effect this week. The rule requires manufacturers to include secondary barriers on all new commercial passenger planes built for U.S. airlines.

In July, the FAA granted carriers until July 2026 to implement the feature on newly delivered jets. Most major airlines have indicated they will use the extra time before implementing the barriers. Southwest, however, chose to adopt the requirement immediately.

"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. The airline expects to receive about 25 additional Boeing planes with the barriers this year.

The FAA first mandated reinforced cockpit doors after 9/11. The new regulation goes a step further by adding a second physical barrier to block access when the cockpit door is open.

The requirement does not apply retroactively, meaning existing aircraft in service will not need to be retrofitted. Manufacturers that have not yet secured FAA certification for their barrier designs also do not need to comply until they are producing newly approved models.

Southwest's move signals that some airlines may use the rollout to highlight passenger safety, even as others delay compliance. Over time, the secondary barrier is expected to become a standard feature across the U.S. commercial fleet.