Pilot's tip of the week

Tracking with GPS

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Subscriber question:

"Even with GPS, l struggle to find a heading that keeps me on course throughout the flight. I have friends who stay perfectly centered from takeoff to touchdown. I'm too embarrassed to ask them how they do it, so I'm asking you." — Cal S.

Jeff:

“The two elements to staying on course are knowing what heading will cancel the wind drift and then holding that heading with level wings. Presuming you’re doing a good job holding a specific heading, let’s look at finding the right heading to fly.

When you plan your flight (or program your GPS) you’ll get the course you want to fly. Let’s say that’s 270 degrees. That can also be called your desired track over the ground. If there was no wind, you’d fly a heading of 270 and your actual track over the ground would be 270. Your actual track would match your desired track and you’d stay on course.

Ah, but today there’s a strong wind from the north that will blow you off course. In the pre-GPS days, we would estimate the wind correction heading before takeoff. Maybe we would launch and fly 280. We would see if we passed north or south of checkpoints and make adjustments. We could even compare two objects (towns, lakes, whatever) and watch how they shifted relative to each other for more real-time adjustments. Of course, the correction would change as we went because the wind is never exactly the same throughout the flight.

With GPS, we can simply turn to a heading of 270 and then reference the screen. The easiest method is with a moving map that’s track-up. Our desired track is the magenta line. Our actual track due to the wind is always the top center of the map. So turn left or right as needed to keep the magenta line pointed straight up. Note the heading that accomplishes that task and keep flying that heading. Periodically check the map to make sure you’re still on the magenta line and that it’s pointed straight up.

If you need more precision, such as under IFR, or you like a north-up map, you can do this with raw numbers. Most GPS units abbreviate the desired track as DTK, which is 270 in our example. They can also show your current track, usually abbreviated TRK. Let’s say that with the north wind, your track is 260. Even though you might be centered on course right now, your current heading will have you drift south of course at a 10-degree angle.

Fix the issue by turning 10 degrees right. Check your track. If it’s 270, the new heading will keep you on course. If your track is now 272, 10 degrees was too much. Turn back to the left two degrees. Keep doing this until track and desired track match. Continue making corrections as you fly and you’ll stay perfectly on course to your destination.

There are other GPS data fields such as track angle error (TKE) and cross-track error (XTK) that can come in really handy, especially under IFR. Some digital HSIs show a track indicator right on the compass rose. Some apps show a suggested heading that takes into account forecast winds. However, those are just a starting point, not a final heading.

Also, remember the GPS is supplemental if you can look outside. That’s where your eyes should be most of the time if you’re not in the clouds.

And if you can look outside, consider traveling in something other than a straight line from A to B. It can be a lot more fun.”

Do you think pilots rely too much on GPS?

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