Those protocols have steadily evolved over the years, but it was one big event almost 15 years ago that changed everything – the 9/11 attacks.
Within weeks, the U.S. Federal Aviation Administration (FAA) had mandated new standards to strengthen cockpit doors to protect pilots from intrusion and small arms fire, even grenades. Most airlines, including Germanwings, reinforced their cockpit doors.
But fortifying those doors was just one part of the security upgrade. The FAA also required that cockpit doors remain locked during flight.
"The door will be designed to prevent passengers from opening it without the pilot’s permission," a 2002 FAA release said. "An internal locking device will be designed so that it can only be unlocked from inside the cockpit."
That's where things appear to have gone terribly wrong on board the Germanwings flight.
Airbus cockpit doors lock by default when they are closed. An Airbus training video shows that the A320 cockpit has procedural safeguards in place in the event that one pilot inside the cockpit becomes incapacitated while the other is outside, or if both pilots inside are incapacitated.
When one of the pilots leaves the cockpit, they can get back in by communicating by intercom or buzzer with the person inside. The inside person can peer through a peephole or use a camera feed to decide whether to allow that person to enter. If there is no answer to a request for access, the person outside can enter an emergency keycode to get access. If there is still no response, the door opens automatically 30 seconds later.
"There was always the opportunity to open the door from the inside and if that is not possible, then you can open the door from the outside via a code," German Transport Minister Alexander Dobrindt confirmed Thursday.
But the Airbus video says the person on the inside of the cockpit has the ability to specifically deny the emergency access request by pressing an override switch that causes the door to do into lockdown mode for five minutes. Lufthansa's CEO acknowledged that may be what the co-pilot did in this case.
"There is another code you can actually operate which will also lead to a bell ringing, and if nobody actually reacts, the door will open electrically and automatically, and this can be impeded by those in the cockpit by actually pressing a lever which says 'lock' and the doors will be closed for five minutes," Lufthansa CEO Carsten Spohr told a news conference.
"It seems to be true that the colleague who remained in the cockpit, the co-pilot, denied him access back into the cockpit in order to start the fatal descent in the French Alps," he said.
"The most plausible, realistic interpretation as far as we are concerned is that the co-pilot – through voluntary abstention, through voluntary abstention – refused to open the door of the cabin to the captain, and pressed the button which caused the aircraft to lose altitude," French prosecutor Brice Robin said.
Canadian airlines make changes
Many U.S. airlines have a "rule of two" that requires that if one pilot leaves the cockpit, another crew member, such as a flight attendant, must replace them in the cockpit. But this is not an FAA requirement.
Air Canada and Air Transat both said Thursday they would now ensure that there are always two people in the cockpit at all times on their flights.
"Following initial reports on the Germanwings accident, we are implementing without delay a policy change to ensure that all flights have two people in the cockpit at all times," Air Canada said in a statement to CBC News.
Air Transat's move to a "rule of two" in its cockpits will be in effect as of Friday, spokeswoman Debbie Cabana told AFP.
Lufthansa, owner of the downed Germanwings plane, does not have such a rule, nor is it required to follow such a practice.
Dominic Fouda, a spokesman for the European Aviation Safety Agency, told CBC News that it does not require two people in the cockpit at all times.
But as a matter of airline policy, several European airlines, including Finnair, do require two people to be in the cockpit whenever a flight is airborne. Norwegian Air Shuttle, Europe's third largest budget airline, announced Thursday it would also adopt the rule requiring two crew members to always be present in the cockpit.
Norwegian spokeswoman Charlotte Holmbergh-Jacobsson said the new rules will be in place "as soon as possible" on all commercial flights globally.