A lot of instrument aproach charts (VOR or ILS) have a circle around the navaid. It can be 10/20 NM or in km, like the 20km circle in the image. What does this circle mean?
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2$\begingroup$ You may want to ask what it's used for rather than what it means, as I think you know it shows the distance that is 10 NM from the centre. $\endgroup$– Notts90Commented Sep 10, 2017 at 21:33
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$\begingroup$ Yeap. Because, as I understood from the answer below (as Annex 4 says), it shows only radial scale. I thought, it has some additional function. $\endgroup$– dovganutsraCommented Sep 11, 2017 at 15:41
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2 Answers
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It can be 20 km, or 10 NM.
Since you've shown an ICAO-style plate, ICAO Annex 4 Aeronautical Charts says:
11.3.3.1 Except where this is not practicable, a distance circle with a radius of 20 km (10 NM) centred on a DME located on or close to the aerodrome, or on the aerodrome reference point where no suitable DME is available, shall be shown; its radius shall be indicated on the circumference.
So whenever possible, this circle must be shown.
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7$\begingroup$ But why is it required? To quickly estimate the relative distance? $\endgroup$– EtheryteCommented Sep 10, 2017 at 20:40
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https://www.flightliteracy.com/instrument-approach-procedure-charts-part-two/
Normally, approach features within the plan view are shown to scale; however, only the data within the reference circle is always drawn to scale.
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$\begingroup$ Wow, after 3 years! Congrats. Not sure that this is the fundamental reason for the reference circle, but-- definitely a good insight there, $\endgroup$ Commented Nov 15, 2020 at 7:43