Aviation experts at Embry-Riddle Aeronautical University said it's possible the airline disaster over the French Alps which claimed the lives of 150 people could have been avoided if the co-pilot had not been left alone in the cockpit.
"It’s very unusual for a pilot to be locked out of a cockpit," said Embry-Riddle Associate Professor Les Westbrooks, a former airline pilot.
But evidence from the black box voice recorder shows that chief pilot, the captain of the Germanwings flight, was trying to get back inside the cockpit.
Just moments before, you can hear the captain calmly ask the co-pilot to take over as he stepped out of the cockpit.
But then, the co-pilot can be heard breathing while the captain was banging on the cockpit door trying to get back in. That's when the plane crashed into the French Alps, killing everyone onboard.
Rules governing how many people must be inside the cockpit vary from country to country, and from airline to airline. Under United States rules, two people must be inside the cockpit, at all times, even if the one person is a flight attendant.
In the wake of the crash, many foreign airlines have immediately changed their policies to require two people in the cockpit at all times.
Former NTSB investigator and now professor of Aviation accidents forensics, Anthony Brickhouse, calls the U.S. regulation the "one in, one out rule," never leaving a pilot alone.
The rule is in place so a second person is there in case one pilot is incapacitated.
"That's something that they'll be looking into, why one person was allowed to be in the cockpit alone," said Brickhouse.
"They would've let somebody in. The other person would've allowed somebody to get back in," said Westbrooks.
If the pilot was intent on crashing the plane, the black box voice recorder would have recorded that struggle as well. That would have given investigators more insight as to why the Airbus 320 crashed into a mountain, killing 144 passengers and six crew members.