MIT expert John Hansman on Thursday said US safety procedures require two people in an airliner's cockpit at all times. If the pilot or co-pilot of an American carrier leaves the flight deck for any reason, a flight attendant goes in, he said.

"The reason for that is in case the remaining pilot becomes incapacitated and couldn't open the door," he said.

Hansman spoke after French prosecutors said Germanwings co-pilot Andreas Lubitz barricaded himself inside the cockpit of an Airbus jetliner on Tuesday and deliberately crashed it into a mountainside in France, killing all 150 people aboard.

Airlines in Europe are not required to have two people in the cockpit at all times, unlike the standard US operating procedure, which was changed after the 9/11 attacks to require a flight attendant to take the spot of a briefly departing pilot.

Worldwide, many airlines reinforced cockpit doors with steel plates and made them bulletproof to thwart hijackers. But the added protections can make it difficult if not impossible for the crew or passengers to neutralize a threat from someone already inside, Hansman said.

"The problem is once we've created a fortress in the cockpit, if there is a problem inside the cockpit, there's nothing anybody on the outside can do, really, to prevent it," he said.

"With the normal things that you have in an airline cabin, there's no way you can break in into that cockpit and particularly not in the seven or eight minutes that apparently they had."

The Germanwings flight was "doomed the moment the co-pilot decided to crash it," Hansman said.

"I think it was doomed when the co-pilot decided that he was going to crash it because at that point, it didn't appear there's any way anybody could prevent it," he said.


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