The hijack code sent out by airliners' transponders has been around for a long time, based on the need for a flight crew to silently alert air traffic control that it is under duress.
The information has become very public, and the presumption even before the terror attacks of Sept. 11, 2001, became that a hijacker might well know about the 7500 code and be able to prevent the pilot from entering the signal.
Given that, it would be logical to come up with another means -- unknown to the public -- to trigger the 7500 code.
The statements by Virgin Atlantic officials today about a malfunction causing the transponder to send the hijacking code and the crew trying to solve the problem, the airline may have incorporated a feature such as a "one-touch" button or other device in the cockpit that, once activated, will switch the transponder to 7500.
Prior to Sept. 11, 2001, when terrorists hijacked four airliners and crashed three of them into buildings in New York and Washington, no transponder in Western aviation would automatically go to, or reset itself to, a hijacking code (7500) or even an emergency code (7700).
Normally, fixing the problem would require one quick click of the transponder's code wheels.
But once a hijack code is received, even a conversation with the crew won't resolve the suspicion that the plane has been hijacked, since one of two things are still possible: An intimidated crew might be required at gunpoint or under some other kind of personal duress to verbally confirm all is OK, or in a worst-case scenario, the plane might be in the hands of a renegade crew or pilot who knows precisely what to say to indicate everything is fine.
In any event, the aircraft must be diverted to a safe location, physically inspected and the crew interviewed to be absolutely sure.
Wherever in the world the code is sent, it has the same meaning and the response is the same.
The first step is for the controller to say something like: "I show you squawking code seven-five-zero-zero. Please confirm you are purposely doing so."
The question, which might be coded to mask the meaning from a hijacker who might be listening in, is worded so that it can elicit a single "roger" as confirmation of a crew under duress.
If there is confirmation or continued suspicion, there is a specific set of notification actions the controller takes, and it mushrooms from there.