Airlines and policy makers rushed to mandate two crew members be in the cockpit at all times following revelations that the co-pilot of the doomed Germanwings flight deliberately crashed the plane when left alone at the controls.
British low-cost carrier easyJet was the largest company to announce a change in policy on Thursday after French prosecutors said co-pilot Andreas Lubitz locked out the pilot before slamming the Airbus A320 into a mountain on Tuesday.
Similar announcements in the wake of the French Alps crash that killed all 150 people on board came from the Canadian government, Icelandair and Norwegian Air Shuttle.
Germany's aviation association BDL said it too wanted to introduce a two-person cockpit rule among its members, while Lufthansa said the measure would be discussed at an industry meeting on Friday.
The second person could be a flight attendant if the pilot or co-pilot has to exit the cockpit in flight.
It emerged on Friday that the captain locked out of the cockpit of the doomed Germanwings plane that crashed in the French Alps used an axe to try and force his way back in, German daily Bild said Friday, citing security sources.
This could not be immediately confirmed, but a spokesman for Germanwings confirmed to AFP that an axe was on board the aircraft.
Such a tool is "part of the safety equipment of an A320," the spokesman told Bild.
Thomas Hesthammer, head of flight operations at Norwegian Air Shuttle, Europe's third-largest low-cost carrier, said the Alps disaster was the trigger for his company's change of procedure.
"We have been discussing this for a long time but this episode speeded things up," he said.
Icelandair said it too had been spurred to act by the shock revelations about the final minutes of Germanwings Flight 4U 9525 from Barcelona to Düsseldorf.
Canada also ordered all its airlines to always have two people in the cockpit, in an emergency directive the government said was mandatory and effective immediately.
"If you're carrying passengers, this is going to apply to you," Canada's Transportation Minister Lisa Raitt said. "You have to have two crew members in the flight deck at all times."
Canada's flagship carrier Air Canada, Westjet and charter airline Air Transat had already said they were putting the policy in place in response to the Germanwings crash.
In Europe, any other airlines that follow suit will do so voluntarily because European air safety regulations -- unlike those in the United States and now Canada -- are currently silent on the subject.
"The European Aviation Safety Agency rules don't require that the pilot be replaced by a crew member when he leaves the cockpit," said an agency spokesman.
Pilots, however, are expected to stay at the controls, except for trips to the bathroom or a break during long-haul flights.
"Nothing prevents a company from putting in place its own procedure that is tougher than the regulations," said Frode Lenning, an official at Norway's civil aviation authority.
American regulators at the Federal Aviation Administration already require a crew member sit in the locked cockpit if one of the plane's pilots needs to go the toilet or take care of another "physiological" need.
Investigators suspect the Germanwings pilot left the flight deck to relieve himself, putting 28-year-old Lubitz in charge.
A two-person cockpit policy is relatively rare in Europe, with easyJet's Irish rival Ryanair, Finland's Finnair and Spanish carrier Iberia among the few who said they already adhered to it.
Carsten Spohr, who heads Germanwings parent company Lufthansa, claimed the policy was not just unusual in Europe. "In the world there are few companies that do that."
Experts point out that even for airlines who impose a strict "rule of two", there are no guarantees against the erratic actions of a crew member causing a tragedy.
In the aftermath of the 9/11 attacks airlines had been particularly focused on the terrorist threat, with many companies adopting systems that lock a cockpit door to prevent attackers from taking control of civilian aircraft.
Lubitz is not believed to have been part of a terrorist plot, German and French authorities said, but the young first officer's motives remain a mystery.
Neighbours and fellow flying club members described him as a "friendly" guy-next-door type who enjoyed jogging with his girlfriend.
The Germanwings crash was the deadliest on the French mainland since 1974 when a Turkish Airlines plane went down, killing 346 people.