U.S. flight regulations make it less likely that a single jetliner pilot could barricade himself or herself inside the cockpit as French prosecutors say the Germanwings copilot 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 copilot 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 [became] incapacitated and couldn't open the door," he told the Associated Press.

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

The Federal Aviation Administration requires that two qualified crew members remain in the cockpit of commercial flights at all times.

If a pilot wants to leave, a flight attendant or relief pilot must enter, lock the door, and stay until the pilot returns, the FAA said. In most parts of the world, including Europe, the practice remains discretionary. Worldwide, after the 9/11 terrorist attacks, 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 into that cockpit - and particularly not in the seven or eight minutes that apparently they had."

The captain of the Germanwings Airbus A320 had left the cockpit briefly and was locked out when he sought to return. Investigators say the copilot refused to open the door and crashed the plane.

Airlines in Canada and Europe said Thursday they would adopt the U.S. requirement that a commercial pilot cannot be left alone in the cockpit.

Air Canada said Thursday it would implement the change "without delay," while the U.K.'s EasyJet will adopt the standard Friday. Norwegian Air Shuttle ASA must await regulatory approval.

"It's a potential deterrent to extreme or unusual activity by the pilot remaining in the cockpit," Richard Healing, a former member of the National Transportation Safety Board, said Thursday. "I'm thinking Europe is going to implement that tomorrow."

Norwegian had been studying changes to its procedures "for a while," the airline said in a statement. "In light of the tragic Germanwings accident, we are speeding up the process so that two crew members always are present in the cockpit," according to the statement. "Our passengers' and crew's safety always comes first, which is why we have decided to change our procedures, in line with U.S. regulations."

An Air Berlin pilot took it upon himself to tell passengers that he would always have a second person in the cockpit during a German domestic flight from Stuttgart to Berlin, according to a Bloomberg reporter.

"I guess you all boarded with a queasy feeling today," the captain said. "I for myself decided to change procedures today. There will always be a second person in the cockpit."

Not all airlines have announced policy changes, with the Air France-KLM Group saying it was monitoring the situation. "Air France is following developments from the results of the judicial and technical investigations with keen attention," the company said Thursday, adding that it was in contact with the European Aviation Safety Agency about shaping future policy.

EASA rules say pilots must remain in the cockpit unless they must exit "in connection with the operation" or for "physiological" requirements, such as going to the lavatory, and at least one pilot must remain at the controls at all times.