The Chart vs. Your Head......

[Nelson L.]

Been meaning to ask someone this for a while, but since I keep forgetting...

On a SID/STAR/Airway/etc..., if the published course is different from the reciprocal of the new radial, do you follow the published course or the "actual" course. Seeing as that probably made 0 sense, an example would be the KAYOH5 arrival (can be found for Ontario, John Wayne, Long Beach, and maybe 1 or 2 more). From Hector to POM, the published course is 211. The leg from Hector to POM is 45 miles long, the changeover at 44 miles (ignore the distance, pretend it's in the middle of an airway). The published radial to fly to POM is 030. The reciprocal of 030 is 210. It's not a big difference (especially on this arrival, where you turn to Homeland before the COP...), but it would be nice to know.


Disclaimer - Numbers may be off, I did the KAYOH off memory. Concept shouldn't be affected.

[wmburns]

I think I can help with this one. What might be missing is that the PDZ R-030 segment is not intended to be flown. It is there to identify the DAWNA fix for the slant alpha users.

The key is there isn't an altitude on the arrow. Further the arrow is printed in a light weight font. The flyable segments are in a heavier weight font.

Regardless, there are tons of VOR's that don't match up. It's because of magnetic drift. Also because the charts haven't been re-worked for magnetic drift in a long time. For that reason one should fly the radial as published for the VOR segment in question.

[Peter Grey]

This depends on what nav source you are using to fly the arrival. It also relates to a someone common topic on the forum, declination vs variation. So I'm going to quote a previous post I've made on the topic which should answer your question (and give the reasoning behind the difference).

Welcome to the world of "declination", which is sort of like "variation" and "deviation" that most pilots are familiar with.

As you most likely know true north and magnetic north aren't in the same place, for various technical reasons aviation uses magnetic north for most navigation purposes.

What you may or may not know is that the magnetic north pole moves at a slow rate, this means that over time the course between 2 points will change as the magnetic north pole changes.

The government remaps the lines of variation every 5 years or so and updates IFR/VFR charts. However, they do NOT reprogram the magnetic north of VORs (way too much time and money to do so). Additionally, your GPS uses current (in some cases more current then the government data) magnetic variation data to determine courses between points.

So this means the VOR and your GPS are using different "magnetic north pole" points, which causes these small course errors. The "assumed variation" of a VOR is called "declination".

Using your specific example:

The true course from OCN to KELPS (first point on V23) is 316. The declination of the OCN VOR is 15E (determined in 1965...), making for a mag course of 301, hence the use of the 301 radial from V23.

Now let's look at the modern day GPS. The actual current variation near the OCN VOR is 12E (determined from the section chart which uses 2010 data). If we take our true course of 316 (which doesn't change) and take out 12, we get 304, or what your GPS is using.

So that's what's going on.

A couple of questions people have on flying this.

Using a VOR what course do I dial in? 301 (the VOR Radial)
Using a GPS what course do I dial in? 304 (what the GPS says)
What heading do I fly (assuming no wind, or to base wind correction angles off of)? 304

[Nelson L.]

Thanks for the answers guys. Peter's post remains as confusing as when I first read it... took me a few attempts to get the idea.



PS - Random question - http://peaware.pilotedge.net/flight.cfm?id=108422

Have noticed on all PEAware logs that the "charted" speed and the "graphed" speed don't match up (they're always faster by 10kts on the chart). For example, 13:41 shows 21kts ground speed on the chart, but 11kts ground speed on the graph. Not a big deal, but I was wondering why that is?