Should commercial airline pilots working in tandem—a captain sitting beside a first office—ever be left alone on the flight deck? For many airlines, there's no rule to stop a routine and temporary absence of one pilot during a flight.

But the crash of a Germanwings plane into the French Alps, an apparent act of deliberate destruction by the co-pilot at time when his senior counterpart was outside the cockpit, is likely to prompt a reexamination of airline crew policies in the age of fortified cockpit doors.

Reinforced doors spread to major airlines around the world in the wake of the Sept. 11, 2001, terrorist attacks, but a parallel requirement for at least two crew members to be present inside the cockpit at all times hasn't been adopted everywhere. All U.S. carriers must have two on the flight deck at all times.

Lufthansa, the corporate parent of the low-cost Germanwings, does not have a rule against a lone pilot. In a sign of the changes prompted by Tuesday's crash, Norwegian Air announced on Thursday that it would begin requiring two crew members in its cockpits at all times, according to report by state-owned broadcaster NRK. 

The captain left the cockpit during Tuesday's Germanwings flight. The co-pilot—identified today as Andreas Lubitz, a 28- year-old German citizen—apparently used a manual override to prevent the captain from returning before the crash, which killed 150 people. The cockpit door of the Airbus A320 single- aisle aircraft is controlled by an outside keypad. At times when pilots inside wish to prevent the door from opening, there is a locked position on the door settings cannot be overridden outside, according to an Airbus instructional video.

After the 2001 terrorist attacks involving four U.S. commercial flights, flight-deck doors were turned into impenetrable barriers. Those measures have greatly improved protection from potential hijackers—and the Airbus video shows that the defensive measure have made it equally difficult for crew to access the cockpit in the event of an emergency or aberrant action by a sole pilot. That scenario is one reason U.S. pilots do not command the flight deck solo.

European Aviation Safety Agency rules say that pilots must remain in the cockpit unless they have to be absent “in connection with the operation” or for physiological requirements, such as the need to go to the lavatory, and at least one pilot must remain at the controls at all times. On the flight’s cockpit voice recorder, French investigators heard what they described as increasingly frantic banging on the cockpit door as the captain or other crew apparently tried to get into the cockpit, the New York Times reported citing a military official it did not identify.

Under U.S. regulations, by contrast, a pilot leaving the flight deck must be replaced by a flight attendant before the door is relocked. During these times, some U.S. carriers instruct flight attendants to position a galley cart in front of the door to prevent anyone from approaching.

The hijackers responsible for the Sept. 11th attacks succeeded in entering the cockpits, and in response U.S. carriers rushed to install security measures. The doors on 4,000 airplanes received “short-term fixes” in only 32 days, according to the Federal Aviation Administration. After the attacks, Congress appropriated $100 million to help carriers install new, more fortified cockpit doors, which cost as much as $17,000 each depending on aircraft type. The standards proposed after the 2001 terrorist attacks also included video monitoring of the cockpit door, allowing flight crew to see the area outside the door.

Nearly all commercial airplanes around the world now have the fortified doors—and it remains to be seen if the Germanwings crash will have similarly pervasive impact on cockpit access procedures.