It wouldn't happen in exactly the same way, with one member of the flight crew left alone in the cockpit, because on U.S. airlines a flight attendant or other member of the cabin crew is always supposed to temporarily take a pilot's place in the cockpit if a pilot leaves for the bathroom or for other reasons. Obviously the flight attendant is not expected to fly the airplane, but he or she could help if there were a medical emergency—letting the other pilot back in if one pilot passed out, etc.—and, by mere presence, presumably deter any weird solo activity by someone alone at the controls.

But obviously this step is not foolproof. Most pilots are men; most flight attendants are women; and, regardless of any strength imbalance a pilot planning to attack or overwhelm a flight attendant would have the advantage of surprise.

So, the two-person rule would prevent the exact Germanwings scenario. But there is no foolproof way to prevent a pilot intent on crashing from carrying out that plan.

3) Does this mean that the fortified-cockpit rules put in place after 9/11 are fundamentally flawed? Based on what we know now, I don't think so. Here's why:

After any disaster of this sort, it's natural to think: Oh, if only we had Different Rule X, or Different Piece of Equipment Y, then none of this would have happened. And after the 9/11 attacks, that impulse led to the quite important step of fortifying cockpit doors. This was intended to ensure that no attacks like 9/11's could ever happen again: Even if hijackers got weapons onto the plane, even if they surprised and overwhelmed the cabin crew, they wouldn't be able to get to the controls and turn the plane into a flying bomb.

But there is an unavoidable dilemma with these cockpit doors. They have to be impregnable against normal threats. But there has to be some way to override them. Otherwise you could think of nightmare scenarios involving doors no one could unlock. For instance: a cabin fire or emergency decompression while one pilot and a flight attendant are inside the cockpit, behind a locked door. They pass out; meanwhile the other pilot, who had been in the bathroom, stands desperately outside the locked door, unable to get at the controls, put on an oxygen mask, and save the plane.

The Airbus instructional video below is today's most popular YouTube clip, because it shows how an override system works. The details of these systems may vary: some with passwords, some with numerical codes, some with keys or secret access points.

But I think spending too much time on details of the doors is pointless. In the end, any of these systems will finally rest on the judgment and trustworthiness of the people using them. Precisely because you have to allow override measures in case of emergency, you necessarily will leave a system vulnerable to abuse by someone in a position of trust. Something similar is true of the use of autopilots and automated flight-management systems. Because you have to give pilots the ability to override automated controls that go wrong, you necessarily leave the system vulnerable to someone intent on harm. This is an insoluble dilemma.