On commercial aeroplanes, the pilot would be in charge of informing the Air Traffic Controllers of any issues with the plane. These come in the form of messages or MayDay calls. They also 'check-in' with different ATC's as they fly through different regions. The plane has to hit all of it's checkpoints along the route. If it misses one, it doesn't automatically mean that it would be in trouble - so no warning would be given to anyone other than the pilots. However, if the plane was subsequently identified as missing, they could trace its route based on which checkpoints it hit and which it missed.

Radar is constantly monitored by personnel and signals are constantly passed back and forth from the planes transponder to the radar receiver. These signals, on secondary radar, identify where the plane is both in the region and in relation to other planes in the area. It also shows vital info - such as altitude, direction, airspeed, heading, etc... Of course, if the plane crashes, the pilots wouldn't respond to any radio calls that the ATC would be directing at them AND the plane would disappear from the radar screens. Other pilots in the area may raise an alarm if they saw the plane crash, or the ATC may ask other planes in the area to look-out for the missing plane.

The only true beacon that emits a signal when a plane crashes is fitted to the 'black boxes': the Flight Data Recorder (FDR) and the Cockpit Voice Recorder (CVR). These beacons trigger automatically after they detect an impact and power is lost to them. Their built-in batteries keep it going for approximately 30 days.

--Mikey B--

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