Friday, March 27, 2015

"What's it doing now?", or GPS turn anticipation-gone-wild...

I was on a recent flight with an instrument student in a very well-equipped Bonanza that provided a very instructive example of a few things:

1. Know your avionics equipment.
2. Know your autopilot.
3. When flying instruments, slowing down is your friend!

We were headed from Wichita, KS (ICT) to the Stillwater, OK VOR (SWO) in more or less a direct routing as part of the required “long IFR cross country”. The intent was to fly the KSWO VOR RWY 17 with the procedure turn and everything for training purposes. Kansas City Center provided us with “direct SWO VOR” and “maintain 4000 until established”.

Our approximate course:

The procedure for reference:

Now, this is in an area where Center’s radar coverage does not go all the way to the ground – that’s why the clearance was only down to 4000. You may also notice that there is a feeder route from the PER VOR to SWO VOR published at 3000. Though we were close, we weren’t actually on the PER-SWO route, so we had to maintain 4000 as assigned. In addition, I wanted us to start at 4000 - it would set up a great scenario for the “slowing down and going down” dilemma faced by faster, slipperier airplanes – you can descend OR slow down, but it’s hard to do both at the same time. Being at 4000 once we started the outbound procedure turn, then down to 2600, then down to 2100 once inbound could mean a lot of juggling and planning of power settings and configuration changes (as we all know, CFI's love to inflict this kind of torture…).

We descended to 4000, but hadn’t slowed down yet – we still had a ways to go, after all. Eventually the GTN 750 showed about 10 miles to go to the VOR, and we were doing around 155 kts GS (and airspeed too, it was pretty calm). The GTN's CDI output was in “GPS” mode – appropriate for this phase of the flight, and the autopilot was in GPSS mode, following the GPS course exactly.

Note - the following screen captures are from Garmin's GTN 750 simulator - so they're not from the real flight. However, they're representative of what was going on and pretty accurately depict what was happening.

At about 10 miles out, the pilot told me he’s going to start slowing down. Okay. Shortly thereafter, the GTN then shows us the following course:

Holy turn anticipation, Batman! The GTN plotted a course that would turn before the VOR (as expected) to intercept the procedure turn outbound course. However, due to our ground speed and the angle of turn, it had to lead the turn by several miles. (If you're interested, the turn radius of a standard-rate turn at 155 KTAS is about 5000 feet, so twice that to make essentially a 180-degree turn). This several-mile lead turn would make us roll out on the procedure turn outbound PAST where the GPS had also calculated we should have finished the procedure turn and been back inbound (dashed white line). Notice the "miles to go" in the bottom right corner (7.4nm) is still showing the distance to the VOR. How far until the turn starts is not depicted.

At this point the pilot realized he'd sure better get slowed down. The Bonanza is pretty slippery, of course, and we were only able to drop a few knots by the time the turn started. We elected to leave the autopilot on to "see what it's going to do" now - something I wouldn't have recommended in actual IMC, but a possibly informative moment in training.

The GPS started around the turn as expected, and rolled out on the PT outbound course. The GPS auto-sequenced to now highlight the PT course. Notice that we have not yet started the PT yet, and are at the end of it - we should be pointed the opposite way. Also, our TAS (GS) is still pretty high (105-110 is normal in the Bonanza) because of the previously-discussed need to descend and slow down simultaneously:

Now I was really intrigued - how is the GPS going to get out of this? Keeping in mind that GPS-steering essentially tries to correct left/right deviations from course - and at this point we are well left of the intended course, which is over a mile southeast of us at this time. So it should correct to the right, right?

And it did!

At this point the programming of the GPS apparently decided we must have already completed the procedure turn and therefore should be inbound, as it did two things - one, it highlighted the inbound course as our current leg, and two, it kept us turning around to the right to intercept, the "opposite" way that a PT is normally flown:

Finally, having intercepted the final approach course, the GPS and autopilot did line us up nicely on final:

Back on course, the pilot switched from GPS to VLOC mode and the mean instructor made him turn the autopilot off and hand-fly the rest.

I love educational moments like this! There were several lessons to be learned:

- SLOW DOWN! There's never such a thing as slowing down to approach speed and configuration too early, especially when you have a big turn coming up. Had we been down at 105-110 KTAS before the first turn started, the turn radius would have been much smaller and the outbound course would have been intercepted in plenty of time to perform a "normal" PT.

- PLAN AHEAD! An approach briefing is more than just reading the altitudes and heading off the chart. Know where you are on the chart. How are you going to get into the approach? What altitude? What are you going to have to do to make that altitude? When to slow down? How much turn? Lead it or don't lead it?

- Don't give up CONTROL to the machines! If you don't know what "it" is doing, whether "it" is the GPS or the autopilot, take over and fly it by hand. I had no idea how this was going to turn out, and I wouldn't have wanted to find out in actual IMC.

- As much as you can, KNOW your equipment and how it functions. Sadly, I looked in the GTN750 pilot's guide and couldn't find much about how it calculates turn anticipation, or at what point it starts showing it (note that the first picture above doesn't even show the turn yet).

Lots to learn in this flight, but that's one of the main purposes of the "long IFR XC" in training. I'd say mission accomplished!


  1. Russ,

    I had a similar story. In my case, I was IMC and disconnected the autopilot. I've looked at two situations on my G1000 Perspective similar and have found:


    For 1, the KEWB OGFIG distance is 13 nm and a 158 turn.
    For 2, the KEWB KTAN distance is 12 nm and a 169 turn.

    For 1, the turn starts 10 nm from KEWB, for 2 the turn starts at KEWB.

    Have you found any formulas or rules for turn anticipation?

    Jan Krouwer

    1. Hi Jan, thanks for reading!

      I do not know the exact formula that Garmin (and others) uses for the turn anticipation, however I can try to make an educated guess. The book "Aerodynamics for Naval Aviators" has a section on how to calculate turn radius at various true airspeeds. I discuss this in my other blog post "Flying 'teadrop' procedure turns", at

      I have to assume that the GPS takes your current groundspeed, converts that into a turn radius at a standard rate of turn (or less at higher speeds), then calculates where that turn radius would require you to start turning to end up on the next course.

      So, the greater the speed or the greater the angle of turn, the earlier that turn must be started. Have both a high speed and a large turn? That turn anticipation distance could be impressively large!

  2. Thanks - I'll take a look. Still question while turn anticipation is used for 1 (above) and not 2.