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How Do Beacons Work?

Thu Jul 13, 2006 6:58 am

I know (well I think I know !) that A/C follow beacons en route to their destination A/P I have seen examples on several documentaries....Is it the beacons that send signals to the computer on the A/C and the Auto pilot changes course automatically towards the next beacon or have I got it wrong ? If the pilot decides to divert for any reason does this then alter the sequence ? If so, does this mean the plane has gone off course and therefore could pose a danger to other traffic ?

Again is this the right forum to ask ? (ps I am a newbie so my knowledge is basic, I'm learning though - thanks to you people !)


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RE: How Do Beacons Work?

Thu Jul 13, 2006 7:27 am

The beacons transmit radio signals. These can be interpreted by the equipment in the aircraft to give the direction to the beacon (NDB), how far off from a specific direction to the beacon the aircraft is (VOR) and/or the distance to the beacon (DME).

The aircraft systems use this information to determine the position of the aircraft and update the position the inertial navigation system is keeping track of, for increased accuracy.

The autopilot then knows where it is, and it is programmed with where you want it to go. Thus, it can calculate how to get there. The programming is done by the pilots in accordance with the flight plan and the clearances given by ATC, before the flight and then in flight as needed.

That's the basic principle. If you wanted the gory technical details, tell us and I or someone else will fill in with that part.

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RE: How Do Beacons Work?

Fri Jul 14, 2006 12:37 am

I'll add another question:

Modern airliners can navigate themselves to any specified lat-lon-- so when they're cleared direct to a VOR, do they always/sometimes/never actually tune in the VOR on their nav radio? Is the freq on the FMS somewhere or do they look it up on a paper chart?
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RE: How Do Beacons Work?

Fri Jul 14, 2006 2:11 am

If you add a VOR into the FMS flightplan it is its position that is the waypoint and it doesn't have to be transmitting to be used. So the crew would not have to tune the VOR as well. However, the FMS auto tunes navaids along the route to update its position, so it may well tune this VOR anyway. It would probably switch to another VOR as the aircraft approached as when nearly overhead the aircraft enters a "cone of confusion" and the VOR data is meaningless.

Pilots can manually tune specific navaids as required, using the FMS CDU to enter the navaid name.
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