Pilot Locked Out Of Cockpit By Co-Pilot In Germanwings Plane Crash

BOSTON (CBS) - With the horrifying imagery of a pilot locked out of the cockpit while his co-pilot apparently sent the plane into a dive resulting in more than 150 deaths, the question becomes: Why couldn't the pilot get into the cockpit? To answer that you have to look at the plane's instrument panel and the switches that control the cockpit door.

Investigators say the co-pilot flipped that switch to "lock," which means the pilot, who had left the cockpit, was stuck outside, pounding on the door. But on the Airbus there's also an override panel outside of the cockpit. But that didn't work because of what was happening inside.

Here's how the door controls are supposed to work. If all is well the control switch is in the normal mode and the cockpit door is locked. If one of the pilots leaves, the other can unlock the door from the inside to let him back in.

"Most cockpit doors are hardened so they're not easy to get through. Almost all of them are locked," says John Hansman, Ph.D, the director of MIT's International Center for Air Transportation.

In another scenario, where the pilots are incapacitated, he says the flight crew can use the override keypad located outside the cockpit.

"If you know the code, it'll unlock the door," Hansman says.

But to prevent a passenger from taking over the plane, the Airbus even has an override for that override.

"If the cockpit is under attack you can put it into lockdown or lock mode, and then the keypad doesn't work," says Hansman. And that's what investigators say the co-pilot did, intentionally. That kept the pilot out.

Investigators say the co-pilot then sent the plane into its deadly dive.

Dr. Hansman says it was the September 11 attacks that caused airlines to strengthen cockpit security.

"By making the cockpits effectively fortresses that you can't get into, you do create some vulnerabilities of this type where if somebody is in there and wants to do the wrong thing, there's not much you can do about it," Hansman says.

U.S. carriers use a different system to secure cockpits by requiring 2 people to be in them at all times. So, if one of the pilots leaves, a flight attendant must have access to the cockpit.