Systems that lock a cockpit door have existed since the 1980s and strict procedures became standard after September 11, 2001 to prevent attackers from taking control of civilian aircraft.
"The cockpit is equipped with an armoured door," confirmed a spokesperson for Germanwings, the budget carrier whose Airbus A320 plane crashed on Tuesday in the French Alps killing 150 passengers and crew.
"There is a video surveillance system that allows someone who wants to enter the cockpit to be identified. Only a pilot inside can unlock the door," the spokesperson added.
The system was widely adopted after planes were used in the September 11 attacks that killed almost 3,000 people in New York and Washington.
The European air safety agency EASA and US counterpart the FAA told the airline industry to find ways to prevent the takeover of passenger planes, even under the threat of deadly force.
"The systems differ according to each plane and airline to avoid a standard and prevent would-be terrorists from knowing how they work from one carrier or plane to another," said a sector specialist who asked not to be identified.
Family members of people involved in the Germanwings jetliner that crashed on Tuesday in the French Alps arrive for a gathering in Le Vernet, France. Source: AAP
Germanwings planes require an access code to open the door but the airline, which is owned by Lufthansa, did not wish to give details "for security reasons" nor say whether a crow-bar or similar device was available to force the door open.
Some carriers, but not all, have placed tools such as axes in planes for use in such cases.
The sector specialist said that a separate code sets off a signal to ask that the door be opened.
If no one inside the cockpit responds, the door is unlocked automatically a minute later.
[img:https://s.yimg.com/dh/ap/default/150325/A320wreckage638Reuters.jpg|caption=Debris from crashed Germanwings Airbus A320 are seen in the mountains, near Seyne-les-Alpes. Photo: Reuters|size=O]]
Cockpit access can nonetheless still be prevented in such cases if a pilot determines that protecting the plane's controls is the top priority.
The video camera allows the pilot or co-pilot to see what is happening on the other side of the door, and they have a switch that can block access "with the aim of preventing an illicit act," Boeing 737 pilot Daphne Desrosiers told a French radio.
Yet another secret code triggers a stronger signal but if the person inside the cockpit is determined to prevent access, it is not possible to enter, the sector specialist said.
"The systems are variable and can be personalised" by airlines and for now, "no one is able to explain the chain of events" that led to the Alps crash, the specialist added.
A Lufthansa spokesperson confirmed the cockpit door could only be unlocked "from inside the cockpit by pressing a button."