GPS Navigator Comparison

[midlifeflyer]

It arose out of a combination of an online discussion and me trying to find things to occupy extra time sitting at home. If you are equally bored, you might like it. It compares the GTN, GNS, and IFD in a single task - holding where the holding fix is a point some distance from a navaid or waypoint.

 

[Bob - S50]

Great video.  Thanks.

One comment about the IFD.  It appears to have the same issue the GTN would have if we used the GTN to manually create a waypoint (holding fix) using radial and DME.  You'll notice that the route you were on was a 308 degree course.  But when you created the fix as the LIB 128/15, the inbound course to the holding fix became 302 degrees and the course from that fix to LIB became the 314 course.  There is now a kink in the road.  You are not really holding on the current course.  To truly hold on the current course you would have to go back and modify the holding fix.  Since it was off by 6 degrees, I would probably try making the fix the LIB 122/15 and see if the inbound and outbound courses became the same.

The problem is caused by the fact that the GPS does not know the actual magnetic variation.  Instead it uses an algorithm to calculate an estimated magnetic variation for its current position which can be off by several degrees.  That's not usually a problem when navigating fix to fix because it's doing the calculations using true headings and then converts to an approximate magnetic heading for our benefit.  But when creating a fix starting with magnetic, the difference between actual and assumed variation becomes significant.

That's the same problem we run into when we are cleared by ATC to a radial/DME to intercept an airway and we use the GPS to create a user waypoint for that fix.  It most likely is not really at the correct point.  That's one case where I would use my trusty old KNS80 to create the intercept point and navigate to the airway before using the GPS to navigate.

[gsxrpilot]

That's an excellent video Mark! You do a great job of very clearly explaining and showing the whole process.

There's certainly room for an IFD update here. As this seems like a simple fix. I'll reach out to Avidyne and see what they say.

Thanks!

[carusoam]

Go MSers!

You guys ARE good!

Thanks for sharing....

Best regards,

-a-

[MoonFlyer68]

5 hours ago, Bob - S50 said:

Great video.  Thanks.

One comment about the IFD.  It appears to have the same issue the GTN would have if we used the GTN to manually create a waypoint (holding fix) using radial and DME.  You'll notice that the route you were on was a 308 degree course.  But when you created the fix as the LIB 128/15, the inbound course to the holding fix became 302 degrees and the course from that fix to LIB became the 314 course.  There is now a kink in the road.  You are not really holding on the current course.  To truly hold on the current course you would have to go back and modify the holding fix.  Since it was off by 6 degrees, I would probably try making the fix the LIB 122/15 and see if the inbound and outbound courses became the same.

The problem is caused by the fact that the GPS does not know the actual magnetic variation.  Instead it uses an algorithm to calculate an estimated magnetic variation for its current position which can be off by several degrees.  That's not usually a problem when navigating fix to fix because it's doing the calculations using true headings and then converts to an approximate magnetic heading for our benefit.  But when creating a fix starting with magnetic, the difference between actual and assumed variation becomes significant.

That's the same problem we run into when we are cleared by ATC to a radial/DME to intercept an airway and we use the GPS to create a user waypoint for that fix.  It most likely is not really at the correct point.  That's one case where I would use my trusty old KNS80 to create the intercept point and navigate to the airway before using the GPS to navigate.

Yesterday when I first saw this video, I also noticed the kinks in the course when using the reciprocal course and distance.  That prompted me to pull out my iPad and try this for myself.  Something I found that makes it it just a little easier is using the rubber band function for creating a waypoint.

If they want you to hold at 15nm prior to a fix, start by setting an altitude restriction like was demonstrated in the video.  Them from your map screen go to where that point is on your course line.  Zoom in to make it a little bit easier to be as exact as possible, and press and hold the course line at the altitude crossing point to activate the rubber band function.  Release your finger as close as you can to that point.  You will then be prompted to use a standard generic name or a custom name.  I just used the generic name RB001 and accepted that.  Now you can build your hold at RB001.

The only 2 things I will say about this method are to be sure you verify your point in the FMS page.  My first couple attempts the point was on course but the DME was off by about .5nm.  The second suggestion is to use the IFD100 app for this.  The iPad screen will be much easier to work with especially if like me you have the smaller screen IFD440.

I do agree that there should be a "create point along track" function added to the IFD software. I know other systems use a fix +/- distance format for creating a point along a track.  Should be easy to implement if you ask me.

To the OP.... great video.  Videos like these challenge us to learn our systems better and that always enhances safety.

[midlifeflyer]

15 hours ago, MilitaryAV8R said:

If they want you to hold at 15nm prior to a fix, start by setting an altitude restriction like was demonstrated in the video.  Them from your map screen go to where that point is on your course line.  Zoom in to make it a little bit easier to be as exact as possible, and press and hold the course line at the altitude crossing point to activate the rubber band function.  Release your finger as close as you can to that point.  You will then be prompted to use a standard generic name or a custom name.  I just used the generic name RB001 and accepted that.  Now you can build your hold at RB001.

That was one of the great comments on the video. And I agreed that's a good alternative. The thing about these navigators is there is always another way to skin the cat. 
 

In this scenario, I wouldn't worry too much about the kink. We - the pilot and ATC - are not that concerned with being a few degrees off. With good old VOR, our tolerance is going to be up to 8 degrees when comparing two VOR (4 degrees of each other), we're not that concerned with obstacles, and ATC is far more interested in hanging out than exactitude.
 

I'm really pleased the video has engendered this kind of discussion, both here and elsewhere. That's what it's really all about.

[Bob - S50]

4 hours ago, midlifeflyer said:

In this scenario, I wouldn't worry too much about the kink. We - the pilot and ATC - are not that concerned with being a few degrees off. With good old VOR, our tolerance is going to be up to 8 degrees when comparing two VOR (4 degrees of each other), we're not that concerned with obstacles, and ATC is far more interested in hanging out than exactitude.

 

I'm really pleased the video has engendered this kind of discussion, both here and elsewhere. That's what it's really all about.

Remember, if the course really was off by 6 degrees, at 15 DME, you would be about 1.5 miles away from where ATC was expecting you.  That is not insignificant.  The larger the DME, the greater the error.

[kortopates]

Mark, Nice video. I am curious about the whole DME fix for holding aspect. In my area on the west coast, DME fixes are a thing of the past and all use waypoint names which will be in a GPS database. If one actually got a DME fix, the controller should be able to give a name or in all likelihood its going to be on their present route somewhere ahead and should be easily found on an enroute chart. Is this really contrary to your experience? But with a GPS waypoint name all of challenges go away with the modern GPS's and we merely go back OBS mode with the GNS. 

I am also of the opinion creating a user waypoint is too much work. With the thought IFR flying should be kept as simple as possible and with only a couple minutes till the hold, I'd rather ensure I have the hold understood and correct entry figured out while flying without needing to draw it out, and use the GPS distance to determine the holding point - just like the old school method. To me its akin to spending too much time on the automation rather than flying the aircraft. 

Bob is right about the differences between magnetic variation used between the VORs and GPS - but its really the GPS that has correct  or most upto date magnetic variation - not VORs. which are often years out of date. But we're stuck with the VORs  being off since they are so costly to periodically update and recalibrate. I'll still use GPS OBS off a VOR for a airway radial as long as I am reasonably close to the VOR to make the difference immaterial like your example. But when far enough away to make it important the issue is easily solved by adding another waypoint on the airway to define it and just activating the leg - issue solved. 

Thanks for the video, I learned some stuff about the IFD.

[midlifeflyer]

Good question @kortopates.

I don't think a distance fix is any more or or less likely than a DME fix except to the extent that, as you say, with RNAV capability, there are far more options available. But we do get things like altitude instructions of this type ("cross 20 miles east of"). I even got one once VFR. So I think this one of those not-common-but-realistic situations which require more system knowledge. 

On the creating a waypoint, it's really a personal preference thing. Pre-GPS DME holds required us to watch the readout and that is absolutely a option with new equipment. Don't even have to hit the OBS button if the hold is a distance on present course. 

Speed. I did a similar video for a flying club using an "old" G1000 - identical to the GNS - and received a similar comment. So I went back to the raw video. The process from activating the cursor to the waypoint inserted in the flight plan, while taking the time to explain it step by step, took 70 seconds. Better than half that when just doing it.

And, for me, the situational awareness offered by a magenta line which shows the inbound course and stops at the holding fix, is golden. On the entries, the first thing is that, like the video, I'm pretty sure enroute holds are, by their nature, almost always direct entries. They are for traffic management when things get backed up inbound. And if not, I'm a former member of the hold-challenged fraternity - couldn't get a simple direct entry right on a bet - who found his answer in drawing holding clearances. A screen showing the inbound course and my airplane tells me almost literally everything I need to know to fly it.

I have a video showing hold drawing in which I use a distance hold precisely because it is more complex, but here is  a basic hold NW of a waypoint on the 330 radial (or inbound course of 150, depending on how the instruction is given). Add an airplane heading to the holding fix anywhere on that drawing, and I'd bet most will be able to tell you the proper entry within 2 seconds unless they are a little OCD about the 70 degree direct/parallel boundary. 

 

[PTK]

Thank you for the very informative video Mark! It’s amazing how a very simple task: a direct hold on present course serves to reveal the sharp contrast in these navigators! In IMC the box should help relieve workload. Certainly not add to it as one of them so poignantly but predictably does!

 

[kortopates]

@midlifeflyer Mark, getting crossing altitude restrictions a DME distance before a waypoint or VOR like your example is routine at my TRACON. Just not for Holds and all our controllers assume that every aircraft is RNAV equipped regardless of how filed - since 90% of them really are. The rare pilot that isn't has to tell them if they can't accept direct to a waypoint ahead of them. As an example on the holding instructions, some months ago I was flying back to SOCAL from Utah about to cross over Las Vegas. A B737 had just departed and had a bird strike and was asking for an immediate return to landing. The controller cleared the B737 for any runway and started giving him vectors around while dumping everyone else off his frequency. The new controller I got was too busy for me to even check in - he was rapidly giving out holding instructions one after another to no less than 15-20 aircraft (at the time it seemed more like 30 to me). Every single one was to an RNAV waypoint. He used the same waypoint multiple times separating aircraft by altitude. You could tell after he used up his favorite or common waypoints and he was then stretching for additional waypoints that would still be easy for both the pilot and controller to position and keep separated - but he kept coming up with waypoints - never used a single DME fix. That's just one data point and I am sure different controllers do it their own way. I asked a controller friend afterwards and he said they avoid DME fixes - since pilots are so crappy at them. 

I do find that pilots are terrible at holds unless they do regular IPCs. They tend not to practice what they never get in the air - although rare but not out of the ordinary. I use every IPC to make sure they can do a unpublished arbitrary hold. Although many could figure it out if they had time to draw it out, but that's not realistic while flying and IMC - many don't have an AP either. So I really push the thumb or POD method if they don't already have method. I also teach an advanced IFR class at the local college which is all RNAV based and by far the hardest  thing for them is figuring out how to do holds correctly without the pressure of time and can still draw it out; especially when its not a direct entry straight ahead. My conclusion is that Holds remain the single biggest challenge for IR pilots; especially when they have to do it in the cockpit with only a 3 minutes ETA to the hold.  They're not trivial for controllers either - My Las Vegas controller managed to get couple wrong in that Cardinal direction didn't match the radial  or reciprocal bearing he gave out in a few of them and he had pilots waiting to get a chance to ask him to clarify. (that's a nice side benefit of non-dme based holds since the cardinal direction is superfluous to the radial for a hold on a VOR or Waypoint and therefore it provides an easy check that we got the right idea of where the hold is). Anyway, first time I had experienced that in the air but its a good example of how you can even get a hold in good VFR weather.

Incidentally, a tool one of my students turned me onto when he was having a hard time with holds is the Pilot Cafe Hold Trainer that runs on IOS devices (and perhaps Android). Cost $4. It will give random hold instructions and will help you learn the Thumb or POD method really well just using your HSI or DG - I find students that spend a few hours doing lots of holding practice with the tool get proficient at them right away. So now I recommend it to all my students. Many of them now have them figured out by our second session after they have practiced with it, then its just learning how to use the OBS to navigate to the hold and set the inbound course. (I only allow GTN programming of the holds with IPC clients since a number of my local DPE's insist the IR candidate be able to do it manually on a checkride). 

Sorry for the long winded discussion on this, but its a very interesting topic to me because I do find pilots have a really hard time with holds. I meant to add earlier, you're very right though, in that with some practice you can create the user waypoint for the hold in a minute and I agree having the magenta line for the hold is indeed golden to maintain situational awareness while your checking weather for another approach or figuring out where you may go to next etc.

Keep the IFR Refresher articles coming - always enjoy your writing!

 

[midlifeflyer]

Thank you about the comment on my articles. I try to provide good information, but my real intent in all is is to get pilots to think. So disagreeing with me is just as good as agreeing.

Perhaps interesting: I had an instrument student who from Day 1, could just visualize holds. Early in his training, I unintentionally gave him a bad holding instruction. Literally as soon as the words were out of my mouth, he said, "You can't do that." He was right.

[kortopates]

20 hours ago, midlifeflyer said:

Thank you about the comment on my articles. I try to provide good information, but my real intent in all is is to get pilots to think. So disagreeing with me is just as good as agreeing.

Perhaps interesting: I had an instrument student who from Day 1, could just visualize holds. Early in his training, I unintentionally gave him a bad holding instruction. Literally as soon as the words were out of my mouth, he said, "You can't do that." He was right.

Too funny, I've had exactly one student that could visualize holds during our instrument training - a big exception to the norm. He had taken all the college pilot classes we offered and was one of my better students. I was pretty amazed with his ability to figure them out. I screwed one up and he quickly corrected me before I could! Never had anyone like him since. A couple years later though and we were recently working on his commercial in his A36 and he had become rusty on the IFR skills, so we started finishing our commercial maneuver training flights with a hold and an instrument approach back in to knock the rust off. But now since he wasn't thinking instrument flying regularly, he could no longer visualize the hold entry and was getting it wrong. For the first time he was interested in learning the thumb method to quickly determine the proper hold entry. He also got the Pilot Cafe Hold Trainer app to practice. I thought the day would come that relying on visualizing it would become too difficult when he wasn't so focused on instrument flying. 

I like all the articles, most do get thoughtful introspect which is most appreciated.  

[Ross Taylor]

For others who wanted to look into the app recommendation by Paul, I'll add that I didn't find it at first when I searched the Apple app store for it.  Try searching under "Pilotscafe" with an S in there.  I actually just got it, with a couple other apps, in their "Pilotscafe Training Bundle 1" and it looks handy.  Thanks @kortopates for the recommendation - I'd not seen that before.  

[midlifeflyer]

I got a surprise today. Avidyne is doing webinars on using the IOS simulator app. I registered to get some tips. One of the participants asked about a distance hold which made me wonder whether he saw my video. The surprise cam in the response. The presenter's answer was that he had recently seen a video about it!

Guessing that Avidyne has a search set up to be notified when a video mentions their product.

[MoonFlyer68]

I will be on the webinar in just over an hour.  Looking forward to using some of this "sit on my A@@ at home time" and get some tips and tricks to use my system better.  It will be interesting to see what might get said about this topic.

You have opened Pandora's Box my friend !!

[midlifeflyer]

1 hour ago, MilitaryAV8R said:

I will be on the webinar in just over an hour.  Looking forward to using some of this "sit on my A@@ at home time" and get some tips and tricks to use my system better.  It will be interesting to see what might get said about this topic.

You have opened Pandora's Box my friend !!

Doubt they will say anything unless a listener brings it up.

[midlifeflyer]

I had so much fun with this, I'm working on another. Also a comparison, also a user waypoint, but this time I'm using a real clearance which includes a fix based on a VOR radial/distance. I originally came across the clearance in a YouTube video and thought it posed an interesting scenario. 

If you want, you can look at the clearance in advance and think about how you would enter it with your system. It's from Winston-Salem NC to Charlotte. If you are a Foreflight user it will come up as one of the piston ATC routes.

Cleared to Charlotte via direct to the Greensboro 240 degree 7 mile fix, direct MAJIC, direct...

 

[Bob - S50]

2 hours ago, midlifeflyer said:

I had so much fun with this, I'm working on another. Also a comparison, also a user waypoint, but this time I'm using a real clearance which includes a fix based on a VOR radial/distance. I originally came across the clearance in a YouTube video and thought it posed an interesting scenario. 

If you want, you can look at the clearance in advance and think about how you would enter it with your system. It's from Winston-Salem NC to Charlotte. If you are a Foreflight user it will come up as one of the piston ATC routes.

Cleared to Charlotte via direct to the Greensboro 240 degree 7 mile fix, direct MAJIC, direct...

 

At only 7 miles from the VOR, even a 3 degree difference between computed magnetic variation and actual magnetic variation would result in an error of less than .4 miles.  That being the case I'd create a temporary fix and put it in my route of flight.  Just to be more accurate, I'd probably also program my trusty old KNS80.  Once airborne and receiving GSO, I'd use the KNS80 for navigation and verify reasonableness with the GPS.  Once I got within about .5 miles of the fix I'd just go direct to MAJIC to allow for the lead turn.

[midlifeflyer]

Ready...

 

[Bob - S50]

Nice.  I have a GTN650 and never thought of creating a fix directly from the flightplan page.  I always assumed I needed to create the waypoint and then add it to the flightplan.

[midlifeflyer]

10 minutes ago, Bob - S50 said:

Nice.  I have a GTN650 and never thought of creating a fix directly from the flightplan page.  I always assumed I needed to create the waypoint and then add it to the flightplan.

That seems to be a Garmin thing. You can go to the user waypoint page or directly from the flight plan. If you play with this a bit on your simulator, I think you will find this way a bit faster. As you say, if you do it from the flight plan, it immediately pops it in the spot. As you know, if you do it by creating it first, there are steps to get to the creation screen and then you have to get it into the flight plan manually. 

The "long way" works OK in the IFD because of differences in the Avidyne flow and logic. It's faster to get there in the IFD. But the bigger thing is that when you say D→ to a waypoint which is not already in your flight plan,  Avidyne assumes you want to put it in your flight plan so it puts it there. Garmin doesn't - it leaves D→  waypoints which are not already in the flight plan outside the flight plan.   

[midlifeflyer]

7 hours ago, Bob - S50 said:

At only 7 miles from the VOR, even a 3 degree difference between computed magnetic variation and actual magnetic variation would result in an error of less than .4 miles.  That being the case I'd create a temporary fix and put it in my route of flight.  Just to be more accurate, I'd probably also program my trusty old KNS80.  Once airborne and receiving GSO, I'd use the KNS80 for navigation and verify reasonableness with the GPS.  Once I got within about .5 miles of the fix I'd just go direct to MAJIC to allow for the lead turn.

I remember the first time I flew with a KNS80. Thought it was incredible.