According to an Airbus video, the cockpit door is locked by default when closed. But when a pilot wants to lock it to bar access to someone, he or she can move the toggle to a position marked "locked," which illuminates a red light on a numeric code pad outside. That disables the door, keypad and the door buzzer for five minutes.
While these functions are disabled, the video shows, the only way to contact the crew is via an intercom. The door can then be opened only if someone inside moves and holds the toggle switch to the "unlock" position.
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If someone outside the cockpit suspects the pilot is incapacitated, that person would normally first try to establish contact via the intercom or by activating a buzzer. If that was unsuccessful, the video shows, a crew member outside the cockpit would need to enter an emergency code on the keypad.
The code activates a loud buzzer and flashing light on the cockpit control panel, and it sets off a timer that unlocks the door 30 seconds later. The person outside has five seconds to enter before the door locks again.
Dominique Fouda, a spokesman for the European Aviation Safety Agency in Cologne, said that there was no requirement in Europe for a cabin crew member to be in the cockpit when one of the pilots leaves for "physiological reasons."
"Basically, unless they have a physiological need, they have to be in the cockpit," he said.
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Aviation safety experts said that the standards in the United States were similar, although Laura Brown, a spokeswoman for the Federal Aviation Administration in Washington, said she could not immediately confirm that.
As investigators continued to pore over the clues, and search crews scoured the rough French terrain for more, relatives of the victims began arriving Thursday near the site of the crash, where a makeshift chapel has been set up, and where psychologists are providing support. Lufthansa was to operate two special flights for family members from Barcelona and from Düsseldorf. A charter flight with 62 relatives and friends of victims landed in Marseille on Thursday after leaving Barcelona shortly after 10 a.m., Spanish television reported. Other relatives traveled overnight from Barcelona, by bus.