Getting My GPS Fix

As I searched my bookshelves for a book I needed to prepare for my PPC, I realized that I have been incredibly remiss in not devoting a blog entry to my favourite GPS book, Max Trescott's GPS and WAAS Instrument Flying Handbook. It is dog-eared and filled with notes on the multiple ideas for future blog entries it inspired. Max There are two reasons this entry never has never been written. One is that I had this crazy idea that I was going to be able to fully digest all the information in this book, and then write the mother of all blog entries on GPS, and the second reason is that when I pick it up it's usually because I have to learn something, and get carried away with that.

Today is one of the latter occasions, and I've given myself a time deadline to study this, so I'm just going to transcribe some of the marginal notes I made reading through the opening chapters. "Excellent and now obvious," "Max knows what he is talking about, doesn't just parrot, which explains why I am not understanding things I didn't before." "Explains from zero knowledge without insulting intelligence."

There's lots of information on GPS on the Internet, but my time is worth something in sifting the wheat from the chaff. The Internet seems to be at odds over whether the unit can be programmed to fly a hold. Given that none of the Internet people I saw say it could be done gave useful step-by-step directions on how to do it, I was believing the naysayers, but Max says, "The Garmin GNS 430W and 530W send commands that let an autopilot automatically fly a procedure turn or a course reversal in a holding pattern. On the moving map page, it adjusts the size and shape of holding patterns based on an aircraft's speed and the winds aloft. Once established in a hold, it can command the autopilot to fly the holding pattern as long as you'd like." And now I see that the unit will only do this for published holds. Max does give some tips, some credited to a colleague, Doug Stewart, on flying unpublished holds. I'll reproduce them here to enhance my understanding and remembering.

The typical hold clearance (a "hold" is the way an air traffic control asks a pilot in an aircraft that requires speed in order to stay aloft to "go wait over there for a bit") instructs the pilot to fly directly from her present position to a specified point (called a "fix"). The fix can be any defined point in space, like "thirteen miles from the ABC VOR on a bearing of 298 degrees" but usually it's a VOR or an NDB. The clearance then includes a vague direction (north, east, southwest) from the fix to hold and an exact track on which to be inbound to the fix each time she circles around. The goal is to fly a little racetrack pattern, with each inbound leg taking exactly one minute and being right on the specified track. The outbound leg isn't necessarily parallel or one minute, because winds mess us up. What also messes me up is trying to learn a procedure I use once a year when I renew my qualifications. But if I master this I can probably also use it to fly shuttle climbs and descents, which I do use in real life.

So here is what I should do: Once assigned the holding fix I set the course direct to it, with the autopilot in NAV mode to track to it. Once the autopilot has figured out the heading that will hold me on that course, I should bug that heading and fly in HDG mode on the autopilot, while I set up the hold. I hit direct to again on the autopilot and enter the inbound course as the desired track. That makes it show a pink line to the fix along that track. I should put it in OBS mode so it doesn't automatically go on to the next waypoint. Reaching the fix, I turn the heading bug to my calculated outbound heading. When I have flown outbound for the necessary time I should use the heading bug to turn inbound, but when I am within 45 degrees of the desired inbound track, I turn the autopilot back to NAV and it will intercept and track inbound. In a direct entry, I can use the ETE feature of the GPS to figure out my timing, by looking at the distance when it hits one minute, and I can look at the wind correction to figure out my outbound correction angle. Curiously he suggests triple the inbound correction, while I use double.

I see Max Trescott also sells a CD ROM course for $100. Given that that's the cost of about twenty minutes of dual instruction in a light twin, I suspect it would be worth one's while.