[ANSWERED] Track Indicator (i.e., "donut") on PFD does not appear to be showing the actual track

[richjb]

So, this is a geeky issue that you may not see unless you're flying VOR or ILS green needles.  I was flying the ILS or LOC Rwy 18 approach to Pine Bluff, AR (KPBF) looking at the teardrop course reversal.  I was testing fly it in response to question I was asked, and I needed to fly it using just the VOR and the ILS green needles.  A technique we learned a long time ago was when the CDI was centered, fly a heading that keeps the track indicator on the head of the CDI needle.   That way, your track is following the course set in the CDI and you should not drift since the track indicator donut would be showing zero drift angle.  Per the Collins CL650 manual, the track pointer shows the ground track over the earth.  So, if it is aligned with the course arrow when CDI is centered, there should be no drift and CDI should remain centered.   This is of course if you are using HDG mode to track the CDI instead of NAV/VOR mode, which is sometime easier if you're flying these types of approaches.  Not everything is LNAV and VNAV! 

 

While flying outbound on the teardrop course reversal (PBF R-022) for this approach, I was able to find a heading that kept the CDI centered.  However, the track indicator donut was several degrees off to the right:

You will have trust me that did fly a heading long enough to find one that kept the CDI centered for a period of time.   With track pointer showing as it is above, my ground track should be tracking to the right and away from the PBF R-022, and the CDI needle should be moving to the left.    It's not. The CDI is remaining centered. 

The MagVar for KPBF is 4 degrees East.  Is it possible that in HS CL650 the Track Pointer is showing True Track instead of Magnetic Track?   It looks like the track donut is at 026 degrees, just ever so slightly left of the heading bug of 027.  Going by the saying "east is least", subtract 4 degrees from a true track of 026 and you get 022, which is the PBF radial I am tracking.  That's the only thing that comes to mind as to what might be happening here. 

If needed, I can try some more experiments. However, at first glace it looks like the track donut is showing True Track instead of Magnetic Track. 

Thanks for your assistance!

Rich Boll

Edited April 25, 2022 by skiselkov

[skiselkov]

I had a look at this and it doesn't appear to be a bug or badly implemented track indicator. What you are seeing is the accurate magnetic ground track donut. The problem appears to be that the database-provided magnetic variation for the Pine Bluff VOR/DME is incorrect. You are quite correct that the databases say it's 4°E. However, and here's a funny one: the Pine Bluff airport near to the VOR/DME is is declared as 2°E:

Now, as to how the airplane knows its magnetic heading - the IRSes contain a magnetic variation database, which is valid from 2020 through to 2025. The IRS feeds the current date + position and this database computes the actual magnetic variation expected at that position and that's what you're seeing on your avionics. And here's the kicker: the magnetic variation database says the actual magnetic variation should be around 0.3°WEST:

Similar results using the IGRF magnetic model:

You can run the computations for select positions yourself here: https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml

Investigating enroute charts, there's a 3 degree declination change over the span of a mere 65 miles. This seem to suggest that the Pine Bluff VOR/DME is misaligned and needs to be realigned IRL:

To summarize: doesn't look like a bug to me. Seems more like either the navaid is misaligned IRL, or the navigational database needs to be updated.

Edited April 25, 2022 by skiselkov

[skiselkov]

Just for completeness, here's the magnetic variation computed for the exact location of the VOR/DME (not that a mile or two would make difference, but just in case there are questions):

[richjb]

Hi Skiselkov,

Explanation makes perfect sense and the issue is actually quite well known in the real aviation community.  MagVar discrepancies between the aircraft, the IRU, the procedure source (i.e., the 8260 Forms) have been a known source of problems on instrument procedures for a long time.  It resulted in a real mess in ORD last Fall on the STARs. it's also the the reason why we lost a bunch of CAT II and CAT III approaches in the US a few years ago for older airplanes where the 

The fact that you have modeled these MagVar discrepancies between the avionics and the procedure source data (i.e., 8260 Forms in the US) is another feather in your cap.  I've never seen this discrepancy modeled in a flight simulator product before, so I did not even occur to me that this could be the issue. 

Been a lot of talk of going to True in various part of the world.  Airbus and Dassault are pushing for it.  

Again, excellent job with the HS CL650!   I withdrawal the issue. 

Thanks!

Rich Boll

[richjb]

To continue...

For Pine Bluff VORTAC, here's the latest 8260-2 Form:

MagVar is shown to be 4E.  Last date is shown to be 2020.  

 

 

 

 

[richjb]

"To summarize: doesn't look like a bug to me. Seems more like either the navaid is misaligned IRL, or the navigational database needs to be updated."

No bug...you are just simulating the real world, real well!

Rich 

[skiselkov]

3 minutes ago, richjb said:

To continue...

For Pine Bluff VORTAC, here's the latest 8260-2 Form:

Interesting. I wonder how they're measuring it. Wonder if it's some local magnetic disturbance that's not captured in the world magnetic model database.

[richjb]

Part of the problem is that the world is tipping on its side, and it's happening faster each year.  Doesn't bode well for those southern vacation spots in a hundred years or so! ;-) 

The reality is that in the US we're not updating the VOR MagVars that often, part it is due to the ongoing decommissioning process in the US called the VOR Minimum Operational Network.  Adjusting for MagVar requires changes to the actual VOR and the FAA is just behind the problem as they work through the VOR MON program.

Here's an FAA Aeronautical Charting Meeting agenda item discussing the MagVar discrepancies that can happen between conventional procedures and the MagVar in the FMS: Hist_11-01-296 (faa.gov).  Here's one on use of MagVar on DMEs: 14-02-284-ACF_DME_Facilities.pdf (faa.gov)

This InFO explains the differences between conventional procedures and FMS tracks: InFO 12009, Magnetic Variation Differences Between Ground-Based Navigational Aid (NAVAID) (faa.gov)

Much of the issues are involved outdated MagVar tables in the IRU, collected by the field and used in procedure development and documentation along with the procedure not being updated.  There's been much talk in in various industry groups, including the Aeronautical Charting Meeting, which is going on as I type, about moving towards using the Airport Reference Point as the source MagVar for all instrument procedures tied to that airport. 

All of these issues go away if move to True.  Magnetic track, course, and heading is presented solely for pilot convenience.  The FMs and IRU computes everything in True, then converts it to Magnetic so it agrees with the magnetic compass.  Going to True would mean no more runway repainting due to runway number changes resulting from MagVar changes.  It would mean no more mis-match between procedure tracks as MagVar changes. 

Rich

 

 

 

 

 

[richjb]

Case in point for PBF VORTAC:

The Facility (i.e., VORTAC) MagVar was last updated in 1980!

Rich

[richjb]

and from the FAA's electronic National Aviation Resource (NASR) database:

[skiselkov]

48 minutes ago, richjb said:

Case in point for PBF VORTAC:

The Facility (i.e., VORTAC) MagVar was last updated in 1980!

Rich

Ok, this is the thing that explains it. Outdated magvar measurement. As you can see below, the IGRF magnetic model agrees that in 1980 the magnetic variation at the VOR was indeed 4 degrees east:

[richjb]

  I have to laugh at this.  I have worked on various industry & FAA working groups for last 18 years.  We see these discrepancies flying, and now they're following me into the flight sim world!  Gotta love it!

Thanks for hunting this down! Again, kudos for HS CL650!

Rich Boll