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RNAV course

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Instrument Rating

the published final approach course to KLQK (RNAV RWY 23) is 228, but when flying the GPS instructs the course to be 231. Why the 3 degree difference?

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  1. John D Collins on Nov 13, 2021

    This is due to the fact that each airport uses a magnetic variation based on a specific epoch year, IOW when the airport runway number is determined, that is when the airport magnetic variation is specified and it does not change over time. Any instrument procedure that uses the airport will use that year’s magnetic variation. So with KLCQ, the epoch year was 1990 when the magnetic variation was 3W. From a current sectional, the magnetic variation is now 6W. The true heading for runway 10/28 is 097/277, Add the 3W to 277, and you get the charted value for the final approach course to be 280. But your GPS uses the current variation of 6W, so your magnetic track is 277+6 = 283. So the magnetic track has changed in the last 31 years, but the true track has remained the same. You will also find this effect with VOR radials, except the epoch year for many VOR may be back in the 1960’s and have a much greater difference over time. You set the VOR to the exact same radial to fly the airway of the approach procedure, but the current magnetic track can be quite different in 60+ years. Why is this done? It is very expensive to adjust the magnetic track on the charts or airways. In the case of VOR’s it means every airway, every fix, every approach chart or SID/STAR that uses the VOR radial must be recalculated, re-charted, and re-flight inspected, all for no real benefit. Most never know that the radials rarely line up with the current magnetic variation.

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