The FAA has issued a final rule requiring all new aircraft flown by U.S. commercial airlines to have a secondary cockpit door.
The rule will be in effect for all planes manufactured for delivery to a U.S. airline beginning two years from its upcoming publication in the Federal Register.
The second door will enable a pilot to close the door to the cockpit before opening a door to the airplane cabin, preventing a hijacker from rushing the cockpit when a pilot steps out to use the lavatory.
The regulation has been a priority for the Air Line Pilots Association (ALPA) since 9/11.
The FAA estimates that compliance with the rule will cost as much as $296.5 million over the next 25 years and up to $505 million over the next 50 years.
The FAA had long been out of compliance with a congressional mandate to address the matter. The 2018 FAA Reauthorization Act directed the FAA to issue an order requiring secondary cockpit barriers by October 2019.
ALPA issued a statement Wednesday praising the finalization of the rule, but calling for a redoubling of efforts to achieve secondary cockpit door requirements for existing planes, as well.
"Ensuring that no terrorist -- domestic or international -- breaches another aircraft flight deck door again should be one of this nation's highest security priorities," ALPA president Jason Ambrosi said.