GTN - Actual vs Desired Track and Heading

[David Megginson]

Another thought - the GTN is more precise, more granular - but maybe too much. I might be over correcting chasing that granularity (314 vs 311), vs following the HSI.

An interesting side point — a couple of weeks ago, I was heading direct to an initial waypoint for a practice approach, and for some reason, the roll steering commands from the GTN to my autopilot just couldn't cope with the moderate turbulence (the rolling kind rather than the bumpy kind). I switched my A/P to heading mode, and it was much easier to stay on the magenta line by following the heading bug than having it follow the GPS inputs.

I haven't experienced that often before, but it can happen. Never be shy about uncoupling and flying a heading instead (I had to do that once during an IPC as well, in the middle of an ILS approach, and the examiner never even noticed).

 

[WDD]

If you want to make it easier, without actually covering up the GTN, practice flying VOR radials such as Victor airways and don't program any flight plan into the GTN at all (so it won't show a DTK). Maybe set the GTN displayed fields to not include the actual track while you're at it. Force yourself to fly the needles without the crutch of DTK and TRK being shown all the time.

Fly without any Magenta? I've heard such stories from the elders of the ancient days - most thought those fantastical stories of just using a VOR, ADF, or DME were fables handed down from long ago. Is flying without any Magenta even possible? Sounds dangerous. Then again, maybe flying Amish can be done.

 

[Ryan F.]

The technology got better, but the technique hasn't changed. You always want to fly a heading to maintain a track. You never fly the CDI (the "needle"). You always fly a heading and observe the effects on the CDI.

Generically, if you're paralleling your DTK of 090 with TRK of 090, with 3/4 deflection of the needle to the right, you're north of course by whatever that deflection works out to for your navigation source. Could be a quarter mile, could be a few miles, etc. You have to determine a new heading which will make the airplane fly the correct track to re-intercept your desired track. In zero wind, your track equals your heading, so maybe fly anywhere from a 100 to a 120 heading to re-intercept depending on how far north of course you are at the moment.

Since the wind is virtually never consistently calm, you're probably going to have to select a heading which accounts for the wind. Say there's a 20 knot wind from from 180 degrees magnetic. While you were paralleling the DTK of 090 with a TRK of 090, you were crabbing into the wind. Maybe your heading was actually 100. Given the wind will affect your ground track, you'll need to maybe fly a 140 heading in order to get a 130 TRK to rejoin your DTK of 090. (Or maybe a much shallower intercept if you're only north of course by a half mile.)

The overarching point here is that you're always flying a heading -- never a track. Your heading indicator remains primary when tracking a course. An HSI makes this easy because your crab is depicted in a very intuitive way vs. a DG and a separate CDI.

 

[MBDiagMan]

I started getting a better handle on instrument flying when I started simply matching the TRK to the DTRK. I started drawing Track Logs that looked like they were drawn with straight edge. Thought I was doing great until I started trying it in choppy air. I then discovered that the refresh rate in the GPS allowed me to get seriously off track in choppy air. At that point I started using the needle more and using the bug on the DG. By flying the bug and keeping it set such that flying the bug keeps the TRK aligned with the DTRK, even in choppy air you can draw some pretty darn straight track logs. The TRK/DTRK along with the bug and the needle, with practice, can keep that needle centered really well.

 

[David Megginson]

Fly without any Magenta? I've heard such stories from the elders of the ancient days - most thought those fantastical stories of just using a VOR, ADF, or DME were fables handed down from long ago. Is flying without any Magenta even possible? Sounds dangerous. Then again, maybe flying Amish can be done.

Wait until you learn about "bracketing" (no, it's not a medieval torture device).

 

[chemgeek]

I learned to fly instruments prior to GPS. But if you have a GPS installed, there is a clever hack to establish the correct reference heading to hold the desired track: if you turn to orient the magenta line vertical on your moving map, and bug that heading, voila you have your reference heading. No mental gymnastics. Take your cut to get back on centerline and then return to your bugged reference heading, and you will be in the ballpark to keep the CDI centered thereafter. Of course, the reference heading may change slightly as you change altitude, but it's always good to have a quick starting point. Might as well use the technology to simplify things if possible. Once you are centered up on the CDI display or G5 or equivalent, maintaining reference heading with small corrections as needed is the way to go, as others have made clear.

 

[coma24]

My primary focus for lateral guidance during approaches is the TRK/DTK/XTK figures on the GPS. There is a slight increase in cognitive workload because your brain is having to process the numbers and do the comparisons rather than just visually interpreting a CDI, however, I've found it to be less work overall than the traditional method of only using the HSI and reacting to needle movement. The HSI approach is functional, no question, however in cases where crosswinds change dramatically during an approach (such as 40kt direct xwind at 3000 AGL shearing to 10kt xwind from opposite direction at ground level, thank you Sullivan County, NY, for that little gem a few years ago), DTK/TRK/XTK lets you stay on top of it in real time. Without it, the heading you were flying 20 secs ago is no longer cutting it, and you genuinely have no idea by how much.

So, all things being equal, I've found DTK/TRK/XTK to be a huge win. In known calm conditions, I will sometimes relax for a little while and transition to using the HSI for long periods of time, almost out of guilt and making sure I'm well-versed in both techniques, and to keep an honest tally of which technique is best for me, and under what circumstances.

Granted, on precision approaches, I'll tend towards leaning on the HSI a little more and crosscheck with the GPS figures. That allows me to focus on lateral and vertical on a single instrument. Otherwise, it is, admittedly, a pain in the ass to be parsing the lateral guidance from the GPS while using the HSI for vertical.

I realize how odd all of that might sound, but maintaining 0.00nm, 0.01nm (60ft) or at worst 0.02nm (120ft) lateral deviation during the approach pays dividends at breakout, and, again, removes the enormous pain in the ass of variable crosswinds which would otherwise lead to a needle chasing situation. In short, using the needles only is more of a "oh, we're slowly drifting off course, let's try a correction and see how it goes..." whereas using the GPS is, "these numbers are a little bit of work to read and compare, but BOY do they work fast and with minimal correction."

 

[MBDiagMan]

I learned to fly instruments prior to GPS. But if you have a GPS installed, there is a clever hack to establish the correct reference heading to hold the desired track: if you turn to orient the magenta line vertical on your moving map, and bug that heading, voila you have your reference heading. No mental gymnastics. Take your cut to get back on centerline and then return to your bugged reference heading, and you will be in the ballpark to keep the CDI centered thereafter. Of course, the reference heading may change slightly as you change altitude, but it's always good to have a quick starting point. Might as well use the technology to simplify things if possible. Once you are centered up on the CDI display or G5 or equivalent, maintaining reference heading with small corrections as needed is the way to go, as others have made clear.

My son in law does the same thing, well, he used to before automating his Baron. He would put TRK up and use the magenta line as his needle. Guess what? You can do the same with North up and STILL follow the line and keep it on track if that’s what you choose to do. THEN, guess what? Your GPS then doubles as a normal chart with a north at the top to help with situational awareness.

I don’t have autopilots beyond a wing leveler in either of my planes, so I have to hand fly. I also don’t have modern GPS, a 420 non WAAS in the little plane and a 430 W in the Mooney. I never leave the magenta line screen as primary. I use the NAV screen that gives me a high resolution needle at the top and all the other pertinent information such as TRK, DTRK, ETE, GS, DIS as well as the progressive approach information. For me the magenta line screen is for occasional reference.

The good news is that our GPS technology, even the old stuff like mine, provide so many features that all of these methods are workable allowing for each of us to have our personal choice to get the job done.