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Course Guidance on VOR Approach
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    Posted: 24 May 2015 at 11:38am
While exercising my new IFD540, I loaded and shot the VOR 9 approach to Lewis University Airport (KLOT). The VOR approach plate shows a course of 066 degrees from Joliet VOR (JOT) to the missed approach point. The approach as loaded by the IFD540 (verified by the simulator) shows a course of 066 to the FAF (TAYEK) and then 073 to the MAP. Since the course is flown using the VOR, the correct course will be flown as long as 066 is set as the course on the OBS of the CDI used for the approach. However, the auto-slew feature of the IFD540/Aspen combination set the course on the Aspen at 073 after Tayek, which led us to a point well south of the actual MAP. (Actually, my recollection is that the auto-slew set the course to about 070, but I would have to go out and fly it again to be sure.) 

I suspect that this discrepancy may be related to the fact that the actual magnetic course between the VOR and the MAP can be different than the VOR radial due to changes in magnetic variation over time. In any case, the behaviour seems to be in conflict with the note on the top of page 5-14 in the IFD540 Pilot's Guide, which suggests that the course sent to an EFIS should be within one degree of the course on the published approach chart.

It would be good to know exactly why this discrepancy occurs, and if it is due to the difference between the actual magnetic course and the VOR radial, it would be good to note that in the Pilot's Guide to reinforce that the course set for a VOR approach should be verified and that the auto-slew feature should not be relied upon to set the correct course for a VOR approach.

Any thoughts are welcome.

By the way, the combination of the IFD540 and the Aspen is awesome!

Regards,

Bob Siegfried, II
Bob Siegfried, II
Brookeridge Airpark (LL22)
Downers Grove, IL
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Bob,

My 540 install is still a week from completion... so in anticipation, I have also been wondering whether the 540 'auto-slew' feature would drive the Aspen PFD.   Conversely, the Aspen PFD will NOT act as a Resolver CDI to drive the 540 in OBS or vLOC mode (hence external CDI needed for this until Aspen software updates at years end).

So, I am wondering, if you have tried the 540/Aspen PFD for an ILS approach, how well does the Auto-Slew work to set the Aspen Inbound Course Pointer?

Thanks for any insight.

Tom Wolf
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Bob,

Since your post, I flew the 540 on the iPad Sim and noted the same discrepancy you saw.  Changing the Map Settings to Magnetic Heading (rather than True), the indicated headings/bearings are accurate to published charts (within 1-deg).

Speaks maybe to using Magnetic Headings...

Tom Wolf 
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Originally posted by n7ifr n7ifr wrote:

Speaks maybe to using Magnetic Headings...

Tom Wolf 

Just curious, why would you set to TRUE headings (vice MAGNETIC), if you are not in the far northern airspace, or are you?

* Orest
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Originally posted by n7ifr n7ifr wrote:

Bob,

My 540 install is still a week from completion... so in anticipation, I have also been wondering whether the 540 'auto-slew' feature would drive the Aspen PFD.   Conversely, the Aspen PFD will NOT act as a Resolver CDI to drive the 540 in OBS or vLOC mode (hence external CDI needed for this until Aspen software updates at years end).

So, I am wondering, if you have tried the 540/Aspen PFD for an ILS approach, how well does the Auto-Slew work to set the Aspen Inbound Course Pointer?

Thanks for any insight.

Tom Wolf
KDVT - Phoenix 

I don't have an external CDI for the 540, the deviation signals are connected only to the ASPEN. I have full function, including OBS. If I change the course on the ASPEN, with the 540 set to OBS, I will see the course line change on both.

Are you referring to something else?


I normally intercept an ILS inbound by flipping to GPSS mode when the XTrack is 0.5nm (sure wish we had the IFD540 HEADING mode back!), and then flip to APPR once the GPS -> VLOC indicator has transitioned. In that instance, the auto-course works fine.

I did on one occasion end up with a backcourse indication on the ASPEN, where the course had NOT properly transitioned and was more than 90* off, but I'm not sure exactly how that happened, I have not been able to replicate it.

* Orest



Edited by oskrypuch - 24 May 2015 at 6:07pm
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Hi Tom-

Thanks for the thought.

I just checked the bearing reference in my simulator, and it was set to magnetic. Is there another setting to which you might be referring?

Regards,

Bob
Bob Siegfried, II
Brookeridge Airpark (LL22)
Downers Grove, IL
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Hello again Tom-

The auto-slew feature seemed to work great for ILS approaches. Also, the Aspen course setting seems to drive the IFD540 fine when in OBS mode. I did have to make sure that the IFD540 was not set up to disregard the course input from the Aspen. That setting is on one of the maintenance mode setup pages on the 540.

Regards,

Bob
Bob Siegfried, II
Brookeridge Airpark (LL22)
Downers Grove, IL
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This is good news from both of you.
Orest, the 540 manual (p 5-10) indicates that without an external CDI driving the 540, two functions will not work from the Aspen:
   . the course line in OBS Mode, has to be turned from the 540 GPS/Nav knob
   . the To/Fr lines on a tuned VOR can not display on the 540 or be rotated. 

So, at least it sounds like on the first issue - Aspen can drive the OBS Magenta course line directly from the Aspen.

Bob,
As for the Auto-Slew, this is great news also that the 540 will drive the Aspen course pointer once the 540 transitions to vLOC mode (ILS or VOR approach).

Thanks.

Tom Wolf
Phoenix, Az
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Bob/Orest - Question on the Aspen controls:

When your 540 is switched from GPS --> to OBS mode

  .  What then is the Aspen source: still in GPS1 source mode, or does that cause a switch to vLOC1?
  .  To rotate the magenta line (on both 540 & Aspen HSI), what control do you rotate: HDG Bug (GPS1 mode), or if in vLOC1, is it the CRS knob?

Thanks.

Tom Wolf
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Hi Tom-

If I understand your questions correctly:

1. When the 540 is switched to OBS mode, the GPS data still provides the course guidance to the aspen CDI.
2. When in OBS mode, the magenta line on the Aspen and the 540 are set using the course knob on the Aspen.

Regards,

Bob
Bob Siegfried, II
Brookeridge Airpark (LL22)
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Thanks Bob.  
That is exactly what I needed to know ... Will use this for last minute checking with my 540 installer.  The 540 integrates an amazing amount of info/avionics!

Tom
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