diamondaviators.net

Hello all,

At the risk of publicizing my ignorance, I was asked today by the controller to say my direct to heading to destination. Clearly he had kind intentions. I made the mistake of looking at the bearing shown next to my destination on the flight plan window and responding back with that radial as my direct to heading. After sorting through and getting back to me with a direct to instruction, I navigated direct to destination only to see that the heading I told him was wrong by a few degrees. This is the second time I come up on such a question from a controller.

That's a reasonable way. Another way is to just select it and hit direct to see what the bearing would be. The G1000 prompt will tell you.

A few degrees (+/- 5) won't change much unless it is > 100 NM away. The controller is likely looking to figure out whether to give you direct given traffic down the road.

Don't sweat the discrepancy. Remember that heading is rarely equal to ground track.

I think the G1000 uses shortest distance (ie: great circle route) to the next point. The further away you are, the bigger the "heading" difference will be at the start of the "direct routing" to the end of the direct routing. This heading change is greatest when you are going directly east to west or west to east and the further you are from the equator. (check out wikipedia for a lot of info! here's an example https://en.wikipedia.org/wiki/Great_circle)

To see it "graphically" go to http://www.gpsvisualizer.com/calculators and enter KLAX to KATL (almost straight west to east), or any other airport (or fix) pairs you want to see. The "heading" from LAX to ATL starts about 8-10 degrees (eyeballing the heading!) "north" of the "final" heading when very close to ATL. (If you want the "exact number", use a plotter)

Enter "direct to" the point when the controller asks, and tell him/her that number. If it a "ways away", that number will change, over time, due to winds aloft and great circle!

You should arrange a tour of a local ATC facility, hopefully a low-altitude one. It is very illuminating. The controller was mostly asking "where are you going on my particular little radar screen." If you said 140 on the heading it wouldn't make a difference between that at 130 or 150. He's trying to see which quadrant you are going to be trouble for him in.

Likewise, I am often asked, as I fly up and down the California coast, what my "route of flight will be." I used to think they were auditing my flight planning and I'd rattle off all 12 waypoints that I had plugged in, worried that they were going to say, "Aren't you adding fifteen minutes to your flight instead of going direct?" But they just want to know what controllers they are going to be passing you off to, so now I say, "Just down over Paso Robles, Santa Barbara and along the coast." It's a general planning thing.

Over on BeechTalk there are a bunch of controllers in the mix, which is very helpful in the discussions of interaction.