Sunday, August 12, 2007

Arcs through the Sky

For something that is known for being able to get you from point A to point B using a "straight line", flying sure has a lot of circles in it. To begin with, because of the earth's curvature, a straight line isn't really the best way to think about long cross-country flights. In fact, flight planning software usually calls point-to-point travel something like the great circle route. Navigation aids are typically circular in their coverage area. As a new student pilot, you learn to do circles over the ground in ground reference maneuvers. Compasses and heading indications indicate degrees (a circular concept); and of course, some of the favorite aerobatic maneuvers are circles (loops, barrel rolls, etc.). Oh-- and airports are typically shown as circles on the sectional! :)

Friday night, in our second instrument flight lesson, we practiced yet another kind of aeronautical circle: DME arcs. They were pretty cool. They remind me of these paper circles that you use in cooking (this is why I started thinking about aviation circles!). In addition to my love of flying, I also love to cook and bake. In some situations in the kitchen (like with braising), you need a kind of cover for a pot to keep the moisture inside and the heat nice and even-- but you don't want to use (or don't have) an actual pot lid. In these cases, you take a piece of parchment and fold it in half, then half again, and then start folding it into triangles-- keeping a folded point at one end, and the open edges at the other. When you get it nice and narrow, then you estimate the radius of the pot you want to cover, measure that off on your triangle, and use your knife to cut the open end at that point. When you unfold the paper, then you have something that resembles a circle-- though you used straight lines to make it.

That's kind of like DME arcs. You use straight lines to create the arc-- flying from one VOR radial to the next, keeping the same distance (measured through DME) away from the VOR station. In other words, say you start at 10 miles out on the 360 radial of a VOR. Your plane is turned perpendicular to the radial-- so that the VOR CDI shows horizontal on your HSI. You fly until the CDI comes together in one line, and then you twist the course knob to the next 10 degree radial for the VOR. At the same time, you turn 10 degrees in the same direction. And you just keep repeating, correcting for wind as you go. It's really quite elegant-- much more so than my description!

I was worried that I might have a hard time catching all of the technical nuances of instrument flying… but I’m finding that I really love the precision of it!


Rebecca said...

Jane wants to be like her Aunt Christa. This morning she was playing, "pilot." She told me Ron was playing pilot too, but he was playing music pilot. I'm not quite sure what a music pilot is, but Ron was content with the role.

DC Flyboy said...


I've enjoyed reading your posts. It looks like, from an earlier post, you are doing a lease-back. I've been considering it. How's it working out?