Aaron Karp, senior editor of aviation trade publication AirTransportWorld, tells PJ Media about the likelihood of a situation similar to what transpired over the final few minutes of the Germanwings flight occurring on a U.S. flight.
In the US, its standard airline policy to have two people in the cockpit at all times. So if one pilot leaves to use the bathroom or for any other reason, a flight attendant goes into the cockpit in case the remaining pilot passes out or becomes incapacitated, so he or she can alert the pilot who left and let him or her back into the cockpit.
Had Andreas Lubitz been flying an airline enforcing this policy, he would have needed to incapacitate this flight attendant prior to crashing the plane. This would not necessarily be an easier endeavor than incapacitating the captain. Waiting for the captain to exit and then going rogue is still possible, but would be a much more difficult feat. Two people in the cockpit is not an FAA requirement, but is generally standard procedure.
Read more from Karp on the Germanwings flight here: “Cockpit Doors: No Silver Bullet Solution“.