Sylvia Wrigley, light aircraft pilot and author of Why Planes Crash, agrees it's unlikely anyone would be able to force their way in. "Even if the door was being broken down, they wouldn't be able to get in before there'd been a mayday call, unless the pilots were incapacitated," she says.
However, one former pilot, who did not wish to be named, has suggested there is theoretically a way to disable the lock and get into the flight deck.
But in any case, however secure the door, there are times when the door is open - when a member of the crew either visits the toilet or has to check on something in the cabin. It's always been pointed out that it would be possible to rush the cockpit when this is the case. Some airlines, including Israel's El Al, have double doors to guard against this scenario. Gratton says there's a procedure which requires a member of the cabin crew to guard the door when it's opened.
But even in the event of hijackers rushing the cockpit, it would be easy for either crew member to send a distress signal.
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