That door combined with the airline allowing someone to be alone in the cockpit. I don't know if it's a regulatory requirement or just a practice but U.S. airli

That door combined with the airline allowing someone to be alone in the cockpit. I don't know if it's a regulatory requirement or just a practice but U.S. airlines don't leave a pilot alone. I think this practice predates the adoption of reinforced doors and is a sound one, whatever ability you might have to get through a non-reinforced door is going to be hard to follow if a suicidal pilot goes into a nose-dive.