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Incorrect VOR DME approach course?

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jimmyz80 View Drop Down
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    Posted: 28 Jan 2017 at 1:43pm
Maybe I'm missing something really obvious here...  I'm still adjusting to my IFDs for instrument flying, and last night I observed something really curious when flying a VOR DME approach.  The approach in question is the VOD/DME-A at KBUY:

http://155.178.201.160/d-tpp/1701/05694VDA.PDF

Everything went smoothly flying the DME arc with my HSI course pointer auto-slewing me around as it should.  But then when it turned me inbound to the VOR, it decided not to use the charted final approach course of 209 degrees.  Instead the fixes in the flight plan were configured as 215 degrees, my course pointer was automatically set to 215 degrees, and then I was auto switched over to VLOC mode.

This caused a few ugly things to happen.  First, the 215 degree inbound course to the VOR is behind you at that point, so your AP/FD now swings you hard left to re-intercept the course in VLOC mode..  Second, the 215 degree course doesn't get you anywhere near where you're supposed to end up.

I flew the approach a second time and tweaked the course pointer to 209 after it had auto-set to 215, and everything went as expected.  Any ideas what's going on here?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PA20Pacer Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jan 2017 at 8:50am
Hi Jimmyz80-

I am going from memory here, but believe that I have seen a similar thing happen when flying a VOR approach where the VOR radial is not aligned with the actual magnetic course. This happens when the earth's magnetic field has shifted since the VOR was installed. In that case, the GPS course used to fly the designated VOR radial will be different than the radial.

In my case, I was using GPS guidance to fly the procedure turn and inbound leg to the VOR, and the HSI (Aspen) auto-slewed to the proper GPS course for the procedure turn inbound leg, but when the IFD540 switched to VLOC mode, I had to manually select the proper VOR radial on the HSI.

Based on this experience, I concluded that when using VOR navigation, it is necessary to manually select and verify the course set on the HSI.

Regards,

Bob
Bob Siegfried, II
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jimmyz80 Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jan 2017 at 1:10pm
I was thinking that it may be something along those lines, since I know the VOR radials are not updated as time goes on.

If this is the case though, I feel like the IFDs are doing something very dangerous by setting up your course pointer for GPS navigation, yet switching automatically into VLOC navigation.  Especially since the course pointer setting has no effect during GPS navigation (other than in OBS mode), but has an important effect for VOR navigation.  It seems like it would have been much safer to set it to the charted course on a VOR approach.

I checked my Garmin GTN simulator and it appears to load the correctly charted course of 209 degrees on this approach, whereas my fancy new IFDs will steer me six degrees off course unless I catch its mistake mid-approach when it gums things up.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jimmyz80 Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jan 2017 at 1:16pm
In fact if my math is correct, I believe the IFD would put you 1.6NM off course at the MAP on this approach.  This area is fairly flat and free of obstacles, but I imagine there are areas and approaches where blindly flying the IFD's suggested course could be deadly.

Edited by jimmyz80 - 29 Jan 2017 at 1:23pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ac11 Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jan 2017 at 1:33pm
I don't have a lot of experience yet, but this sounds like a database error. You probably want to bring this up with Jeppessen. I can see a one degree magnetic correction, but not 6. Do you have current charts loaded? Does the chart data match the nav data on the IFD itself?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jimmyz80 Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jan 2017 at 1:52pm
I guess I'm not 100% clear on what you're asking me to compare. All of the databases on the IFDs are up to date, and the FMS page clearly shows final approach courses of 215 vs 209 on the published chart.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ac11 Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jan 2017 at 1:57pm
Nav data and chart data are not the same. I wasn't clear from your post if you were using a chart on the IFD versus a paper chart or some other source. Sounds like you get both. Did you get a chance to view the chart on the IFD and compare with the map page, which, like the FMS, would be nav data?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jimmyz80 Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jan 2017 at 2:12pm
While I was flying I was comparing to both the approach plate I have in Foreflight, as well as the approach plate I have available on my Avidyne MFD (both current).  I didn't however check the chart on the IFD since I assumed it would match the MFD, being from the same Jepp data cycle.

But clearly the nav database on the IFD is either wrong, or the IFD is trying to do something "smart" and adjust the course away from what's published.  Either of those scenarios are a bit scary.


Edited by jimmyz80 - 29 Jan 2017 at 2:13pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PA20Pacer Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jan 2017 at 2:18pm
Hi ac11-

There are many situations where there is a significant difference between a VOR radial and an actual magnetic course. Another example is the VOR approach to 1C5 off the JOT VOR. The VOR radial to be used for VOR navigation to the MAP is 42 deg, but the actual magnetic course (which would be flown by a GPS) is 47 deg.

I still do not fully understand how the IFD540 handles this situation for a VOR approach. In this case, on the IFD540 sim, a course of 42 deg is shown as the desired track between the VOR and the FAF, but the course actually flown by the sim is around 46 or 47 deg. After the FAF, when the sim is using GPS navigation, the correct magnetic course of 47 deg is shown. To fly this correctly, the VOR radial of 42 deg must be set on the OBS when VOR navigation is to be used, but the sim does not have a VOR capability. I will need to try this next time I fly and see whether the HSI auto-slews to 42 deg or 47 deg during the leg from the VOR to the FAF, and whether the auto-slew course changes at the FAF.

In any case, setting and verifying the course any time VLOC navigation is used will keep one out of trouble. 

Regards,

Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ac11 Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jan 2017 at 3:19pm
Thanks Bob for that info. I am new to GPS approaches, so I will pay close attention to that.

Jeff
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PA20Pacer Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jan 2017 at 3:40pm
Hi Jeff-

All of this information is indeed interesting, but it is essentially transparent when doing a GPS approach, as the navigator will load the correct legs and courses without any "fine tuning" by the operator. 

The difference between VOR radials and magnetic GPS courses first became widely appreciated when the FAA implemented GPS overlays for VOR approaches (mid-90s), and people noticed the difference between the charted radial and the GPS course. Overlay approaches are few and far between these days.

The "gotcha" that started this discussion came up when the automation associated with the IFD540 and an auto-slewing HSI did not do what the user expected during the transition from GPS to VLOC course guidance. One could have an extensive discussion about how the automation "should" work, but verifying the selected course on the HSI when using VOR navigation should always keep you out of (this particular) trouble with any navigator.

Regards,

Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ac11 Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jan 2017 at 4:24pm
Thanks Bob. Understood.

Jimmy, I agree with your assessment, and encourage you to bring this up with Jeppessen. My vote is that the Nav database is wrong. I'd love to hear the results if you do follow up, as it would be a good learning experience.

Jeff
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oskrypuch Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jan 2017 at 5:57pm
It is pretty common for VOR radials to be off, partly due to magnetic shift, but mostly due to maintenance issues. In-air indicated radials are constantly flight checked, any variations are noted and charts updated. Five degrees is not uncommon.

Not applicable for an approach, but airway VOR derived radials as marked on charts, may also not match a GPS "radial" due to the curvature of the earth, and the difference between a great circle and a rhumb line.

* Orest



Edited by oskrypuch - 29 Jan 2017 at 5:59pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jimmyz80 Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jan 2017 at 7:46pm
But my original point still stands, that your navigational equipment shouldn't pick the "correct" course for GPS navigation, and then tell you to use it as a VOR radial.  That's flat out wrong and dangerous, especially when you've told the navigation equipment to fly a charted approach, and that's not at all what it's actually doing.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oskrypuch Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jan 2017 at 10:13pm
Well, the course has to be set to something, what would you suggest? That data for the inbound course is not available.

I suppose an annunciation SET COURSE on a ground based approach might be an idea.

IAC, checking that the proper approach is loaded, and checking the course and navaid/ID (as applicable) is a critical step for the intercept.

* Orest

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Post Options Post Options   Thanks (0) Thanks(0)   Quote jimmyz80 Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jan 2017 at 11:19pm
The data is available and is written right on the approach plate in multiple places: 209 degrees.  Garmin units get it right and Avidyne units get it wrong.  Clearly the correct data is available.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ac11 Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jan 2017 at 11:53pm
I've learned something today. I flew the VOR-A approach to KVCB tonight and had a similar experience. Inbound course via plate was 241. The GPS track and tuned radial was 245. I had to adjust course manually.

I agree Jimmy. This doeasn't seem to be appropriate behavior. The IFD, when switched to VLOC, should get published radial.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jimmyz80 Quote  Post ReplyReply Direct Link To This Post Posted: 30 Jan 2017 at 12:14am
AC11, thanks for confirming. :)

Just for the sake of completeness and illustration, here are a few photos I've assembled:


The first few photos are from me tinkering with my IFDs in the hangar tonight, and you can clearly see the FMS page giving an incorrect final approach course.

The next few photos are from the GTN simulator on my iPad.  You can see that on the approach preview screen it calls out the final approach course of 215 degrees, but then when you load the approach it's smart enough to know you're flying a VOR approach and need the published course of 209 instead. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chflyer Quote  Post ReplyReply Direct Link To This Post Posted: 30 Jan 2017 at 9:58am
What status was showing (upper right) when you were a) flying the DME arc and b) turning from the arc to final course? Was it still in GPS->VLOC or had it changed to only VLOC? See p5-12 of the PG.

My experience is that the IFD540 does the switch from GPS to VLOC very late which often means that one needs to be lined up on the final course before the VLOC becomes fully active alone (green). This is also true for an ILS, but in that case the auto-slew isn't important because the ILS guidance isn't dependent on mag heading set. With a DME arc, the intercept angle is about 90 degrees, so if the info on p5-12 is considered it is likely that GPS heading output will be in effect for about another 5 seconds following intercept. 

The GPS guidance is likely 215 due to VOR/mag/GPS differences as mentioned by Bob. This should change to 209 when using the VOR for guidance rather than GPS. 
Vince
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AviSimpson Quote  Post ReplyReply Direct Link To This Post Posted: 31 Jan 2017 at 4:35pm
Unfortunately, there is often a delta between the mag var values that existed at the time the procedure was designed and the current information available in the navigation databases.  The behavior described in this thread is due to the manner in which the IFD accounts for these mag var differences.  Fortunately, the newest versions of the nav database contain the procedure design mag var data, so in the future we will no longer have to dynamically account for these differences. You can expect to see this change in a minor software release after 10.2 and future navigation databases. 

In the meantime, we have the following recommendations when flying VOR based approaches:

1) If the approach is authorized for a GPS overlay, fly it using the GPS.

2) If the approach is not authorized for GPS overlay, set the "GPS->VLOC Capture" user setting to "Manual", manually set the course pointer to the charted course if necessary, and then continue to fly the approach normally using VHF deviations.

We will issue a formal SIL addressing this, but wanted to get the word out on the forum as soon as possible.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote FlyingCOham Quote  Post ReplyReply Direct Link To This Post Posted: 31 Jan 2017 at 5:53pm
Simpson, very clear, concise, and to the point.  THANKS!!!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jimmyz80 Quote  Post ReplyReply Direct Link To This Post Posted: 02 Feb 2017 at 8:00pm
Out of curiosity, how will the new databases handle the possibility of flying the same approach in both VLOC and GPS mode?  In this approaches case, will it correctly display and use a course pointer set to 209 for VLOC mode and also in GPS mode?  Will it toggle back and forth between 209 and 215'ish when you switch the IFD from VLOC to GPS?

How does Garmin seem to have it properly handled already, without waiting on the databases?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AviSimpson Quote  Post ReplyReply Direct Link To This Post Posted: 03 Feb 2017 at 9:40am
With updated software and database content, the each leg will be defined as "Fly Course 209".  In GPS mode, the system will fly them as course legs using 209 as the course.  In VLOC mode, the IFD will set the pointer to 209.  Basically, there will be no reference to 215.

We're not sure how the Garmin works internally, so we can't answer that part of your post.  However, if you'll notice in the GTN screenshots that you posted, they're not perfect either.  In their preview screenshot, they show those legs as 215.  When it gets loaded into the flight plan, then they show 209.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote jimmyz80 Quote  Post ReplyReply Direct Link To This Post Posted: 04 Feb 2017 at 1:06am
One last question.  Is this a fix that will show up in the pending 10.2 release, or will it be even further out than that?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chflyer Quote  Post ReplyReply Direct Link To This Post Posted: 05 Feb 2017 at 4:45am
Simpson answered this question in his reply. A minor release means that it will be 10.2.x where x is >0

Originally posted by AviSimpson AviSimpson wrote:

You can expect to see this change in a minor software release after 10.2 and future navigation databases. 

Vince
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kentucky Captain Quote  Post ReplyReply Direct Link To This Post Posted: 15 Feb 2017 at 4:49am
I didn't even realize that KBUY had a VOR approach.  We fly into Burlington to see my wife's parents on a regular basis.  After a 3 hour flight from here in KY I'm really interested in getting on the ground but still usually do a GPS or ILS to Rwy 6 since that always seems to be which way the wind blows.  The weird thing I noticed was on the ILS.  We were cleared direct GSO for the approach.  If I remember correctly, the course out of GSO on the IFD540 didn't match what I saw on Foreflight.  I think Foreflight had us at least a 10 degree different intercept to the final for the ILS than what showed on the IFD.

I did do the VOR A to Siler City during my instrument training and it uses the same VOR.  Now that is an interesting approach.  We were doing at least 10 approaches a day and I was on day 4 or 5 and had stopped paying much attention  to the airports since we were almost always missed approach.  It was very surprising breaking out of the overcast to see the runway almost 90 degrees to us at the MDA.

Where do you fly out of?  I did my instrument training out of KJNX and took my checkride at KTTA.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jimmyz80 Quote  Post ReplyReply Direct Link To This Post Posted: 15 Feb 2017 at 3:23pm
Originally posted by Kentucky Captain Kentucky Captain wrote:

I didn't even realize that KBUY had a VOR approach.  We fly into Burlington to see my wife's parents on a regular basis.  After a 3 hour flight from here in KY I'm really interested in getting on the ground but still usually do a GPS or ILS to Rwy 6 since that always seems to be which way the wind blows.  The weird thing I noticed was on the ILS.  We were cleared direct GSO for the approach.  If I remember correctly, the course out of GSO on the IFD540 didn't match what I saw on Foreflight.  I think Foreflight had us at least a 10 degree different intercept to the final for the ILS than what showed on the IFD.

I did do the VOR A to Siler City during my instrument training and it uses the same VOR.  Now that is an interesting approach.  We were doing at least 10 approaches a day and I was on day 4 or 5 and had stopped paying much attention  to the airports since we were almost always missed approach.  It was very surprising breaking out of the overcast to see the runway almost 90 degrees to us at the MDA.

Where do you fly out of?  I did my instrument training out of KJNX and took my checkride at KTTA.

I didn't realize it had one either until my local CSIP took me over there for some approach practice. :)  I have my SR22 based at KTTA and am also a member of Wings of Carolina over there.  Where in KY are you based?  I have a customer I visit in Louisville once in awhile, and I actually bought and picked up my SR22 from someone who had it based at KLOU over there.
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