Radials

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flyguy
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Radials

Post by flyguy » 17 Jun 2009, 12:06

I noticed that almost every SID or STAR involves the use of radials but my problem is I don't really know what they are or how to fly with them. Just one example of a dep procedure that uses radials-http://www.nats-uk.ead-it.com/aip/curre ... 6-1_en.pdf I read the article on Vatsim about them but it was a bit too complex for me :? Any help on this subject would be very much appreciated :D
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RatherBFlyin
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Re: Radials

Post by RatherBFlyin » 17 Jun 2009, 13:39

Radials are electronic signals radiating outwards in a circle from a VOR. Theoretically, there are an infinite number of radials for each VOR, but for all practical purposes they are limited to 360, one for each degree around the VOR to complete a full circle. To get really technical on you, VOR's actually send out two signals. One is a directional signal that sweeps around the VOR at a very specific rate, and the other is an omnidirectional signal that is sent out every time the rotating signal reaches a certain point (I believe it is north, but I could be wrong). The instrument in the cockpit times the difference between the signals to determine what radial you are on.

Now for the tricky part. In your example, after takeoff you are instructed to intercept the 261 radial from the DET VOR. 261 is the direction you are in relation to the VOR (you are west of the VOR). However, you need to be tracking towards the VOR, so you will actually be following a course of 081 (course, not heading). In navigational terms, this means you are navigating on the 261 radial to the VOR. That is where a lot of new IFR pilots get tripped up, though.

If you set up your VOR indicator (called an Omni-Bearing Selector, or OBS) in the cockpit correctly, it is a very intuitive tool to use. If you are drifting off course to the left, the bar will move to the right telling you that you need to fly to the right a little to get back on course. However, if you were to dial in a course of 261 on the OBS and attempted to fly this procedure, you would find yourself quickly becoming confused. The reason for this is because the OBS assumes you are flying away from the VOR and will provide you with course corrections based on that assumption. What that means is that if you are on course and begin drifting to the left (north of course in this instance), the instrument (thinking you are flying to the west) will move the guidance bar to the left telling you to adjust your heading slightly to the left (south) to regain the course. Since you are actually flying east though, turning to the left will only take you further off course to the north.

So how do you avoid the confusion? Always make sure you dial in the course you want to actually fly, not the radial you will be following. If you are flying away from a VOR it's easy, just dial in the radial you want. If you are flying towards a VOR, then dial in the reciprocal radial (add or subtract 180 from the radial you will be using).

I hope that helped a little. Just remember that a radial is just a radio signal telling you what direction you are in relation to a fixed point on the ground (the VOR). The trick is outsmarting the OBS in the cockpit so that it always shows you what you are expecting to see.

Oh, and one more thing. When navigating by VOR's in FS, always make sure you have the autopilot in NAV mode, and not GPS. :oops:

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flyguy
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Re: Radials

Post by flyguy » 17 Jun 2009, 15:23

Thanks for the very helpful explaination :) with constant reference to your reply i managed to fly the procedure with out much difficulty. I only had one question, after DET D10 I'm supposed to intercept the Lambourne R159, do I put the frequency for the lambourne VOR in the standby NAV1 radio or in the NAV2 radio box? Also do I tune the OBS1 to 339 degrees after D10 or do I tune the OBS2 to 339? Sorry if this is confusing :wink:
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RatherBFlyin
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Re: Radials

Post by RatherBFlyin » 18 Jun 2009, 13:52

You want to always use NAV1 and OBS1. Some FS models let you select which nav radio to use for flight information, but to avoid confusion I have found that it is best to always use NAV1.

So I would have the LAM frequency dialed into the standby frequency on NAV1, start a turn towards LAM when reaching 10 DME from DET, then move LAM's frequency over to the active window and dial the OBS to 339.

Now there's nothing wrong with using your NAV2 radio and OBS as a reference. I will do that from time to time, especially if a waypoint is marked on a chart with a radial from a nearby VOR.

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