CAMBRIDGE, Mass. (AP) -- U.S. flight regulations make it unlikely that a single jetliner pilot could barricade himself or herself inside the cockpit like French prosecutors say the Germanwings co-pilot did before crashing his airliner into the French Alps, a Massachusetts Institute of Technology aviation analyst said Thursday.

MIT expert John Hansman said U.S. 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 told The Associated Press.

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.

Lufthansa CEO Carsten Spohr holds a press conference in Cologne, Germany, Thursday. The airline did not require two crew members in the cockpit at all times. (AP Photo/Frank Augstein)
Lufthansa CEO Carsten Spohr holds a press conference in Cologne, Germany, Thursday. The airline did not require two crew members in the cockpit at all times. (AP Photo/Frank Augstein)
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Airlines in Europe are not required to have two people in the cockpit at all times, unlike the standard U.S. 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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