Every airline cockpit has a small crash ax mounted on the rear wall. The ax is there so they can chop their way out of the cockpit in the event of a crash. Since airline flights resumed after 911, some crews have moved the ax closer to their seats where it's in easy reach. Anyone who tried to come through their door is in for a rude surprise. Don't bring a box cutter to an ax fight.

All US airliners have reportedly had their cockpit doors reinforced to make it harder for anyone to break in. The pilots have sworn never to turn over control to hijackers again. It's very unlikely they would ever open the cockpit door to hijackers again regardless of what they may be doing in the passenger cabin.