As the investigation into the downed Germanwings flight has revealed one pilot may have intentionally crashed the plane while his co-pilot was locked out of the flight deck, focus has shifted to how 27-year-old Andreas Lubitz was left alone at the controls.
Lufthansa's CEO Carsten Spohr said on Thursday that the airline added a cockpit lockout procedure meant to protect cockpit security after the 9/11 attacks. But regulations on when a pilot can be left solo in an aircraft cockpit vary from country.
Lufthansa's doors are accessible by coded entry door controls. At a press conference, Spohr detailed a procedure where a locked out flight crew member can enter a code that generates a special "ring" inside the cockpit. If it's not answered, the door opens automatically. However, this can be overriden by the pilot turning a switch from inside.
"Since September 11, we changed the procedures for entering cockpits. They have been reinforced to disallow any entry into the cockpit without pilots' approval," Spohr said. "It is proof against small handguns."
In the United States, when one pilot on a commercial flight like the Germanwings flight leaves their station, FAA regulations require that another member of the crew replace them in the cockpit and lock the door.
"For our members, all flights have two people in the cockpit at all times," said Melanie Hinton, a spokeswoman for Airlines for America, an industry trade group.
That's not always the case internationally.
Lufthansa earlier Thursday said it follows rules set out by German authorities that allow temporary absence from the flight deck.
"There is no rule that demands that always minimum two persons have to stay in the cockpit," said Holger Kasperski, a spokesman for the Federal Office for Civil Aviation of Germany. He said pilots are allowed to leave the cockpit, "for physical needs but not longer than necessary."
Regulations that cover all European Union airlines allow for flight crew members on duty in the cockpit to leave their station "for physiological needs" as long as "one suitably qualified pilot remains at the controls of the aircraft at all times." If the pilot who leaves the deck is the pilot in command, they have to formally hand over control to the pilot who remains behind in the cockpit.
As far as getting back in after leaving the cockpit, former pilots and aviation experts told NBC News that most planes have coded entry door controls, but these can be overridden with a double lock — a practice implemented industry-wide after the 9/11 attacks.
"The cockpit has the ultimate control of the door," said Captain John Cox, a former pilot and NBC News aviation analyst . "If it is placed in the override mode then no matter what is done with the code pad, the door will remain locked. The security people were very firm on the need for the flight deck to remain the ultimate authority."
Following the crash, discount European airline Norwegian Air Shuttle announced it would start requiring two people in the cockpit at all times.