A group of major international airlines said Thursday it will now require two crew members to be in the cockpit at all times, a rule U.S. carriers adopted after the Sept. 11, 2001, terrorist attacks.

The change comes after details emerged that the co-pilot of Germanwings Flight 9525 had apparently locked himself in the flight deck before deliberately crashing the Airbus A320 jet into the French Alps earlier this week. All 150 aboard died.

Former Federal Aviation Administration safety inspector Richard Wyeroski, who teaches airplane maintenance and flight instruction on Long Island, said requiring two crew members in the cockpit at all times is the best way to prevent tragedies like Tuesday's crash. But Wyeroski added, "There's always something we can't plan for."

Every U.S. airline has procedures designed to ensure there is never a situation where a pilot is left alone in the cockpit, according to the Air Line Pilots Association, International.

On domestic flights, two crew members must be in the cockpit at all times, FAA spokesman Jim Peters said Thursday. A qualified crew member could be a flight attendant or a relief pilot serving as part of the crew, the FAA said.

Massachusetts Institute of Technology expert John Hansman said if the pilot or co-pilot of a U.S. carrier leaves the flight deck for any reason, a flight attendant goes in. "The reason for that is in case the remaining pilot becomes incapacitated and couldn't open the door," he told The Associated Press.

Whenever the cockpit door is open, flight attendants create a barrier between the cockpit and passengers. Typically, that is done with a beverage cart but some jets are outfitted with a mesh wire barricade.

Leading European budget airlines Norwegian Air Shuttle and EasyJet, along with Air Canada, say they will now require a minimum of two crewmen in the cockpit while a plane is in the air. A group representing Germany's biggest airlines, including Lufthansa and Air Berlin, say it plans similar rules.

After 9/11, the FAA also began requiring airline operators to install reinforced cockpit doors strong enough to resist intrusion by a person using physical force, small-arms fire or even a small grenade, according to its website.

The Airbus A320 is designed with safeguards to allow emergency entry into the cockpit if a pilot inside is unresponsive. But the override code known to the crew does not go into effect -- and indeed goes into a lockdown -- if the person inside the cockpit specifically denies entry.

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An Airbus training video shows that the cockpit door of the A320 has safeguards in case one pilot becomes incapacitated inside while the other remains outside, or if both pilots inside lose consciousness.

If there is no response, a member of the flight crew can tap in an emergency code, and if there is still no response, the door opens automatically.

If a person has been denied access, which one person could do by switching to the "lock" position, the door remains locked for five minutes, according to the training video.

With Candice Ruud