Strange LPV/ILS Observations, Thoughts?

[Marauder]

I shot a couple of approaches with a safety pilot in my plane last week and he shot a few pix of my panel during the approach. I was looking at the photos today and noted something strange. It didn’t affect the outcome of the approach, but it has me wondering why it would be different.

 

I was flying the LPV approach to runway 29 at KMQS. I had the Aspen PFD set to track the Garmin GTN’s approach for the airport. On the second Aspen, I had the second HSI and second Nav set to track the ILS to the same runway.

 

The final approach course for both approach is 293 degrees. What I noted was that the LPV was showing a final course of 294 degrees. Any idea why this would be different that 293? It is an auto slew, so the data should be coming from the GTN. Unfortunately, I did not note the course on the GTN but I have no reason to suspect it would be different than what is on the Aspen.

 

Also, I noticed a slight difference in glide slope presentations between the LPV & ILS. Does anyone know the reason why there might be a slight variation between glide slopes?

 

 

 

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[FoxMike]

Could be a rounding issue or on the ILS could be an alignment issue.  Hardly worth the time to figure it out.

[DualRatedFlyer]

AIM 1-1-19

l. Conventional Versus GPS Navigation Data

There may be slight differences between the course information portrayed on navigational charts and a GPS navigation display when flying authorized GPS instrument procedures or along an airway. All magnetic tracks defined by any conventional navigation aids are determined by the application of the station magnetic variation. In contrast, GPS RNAV systems may use an algorithm, which applies the local magnetic variation and may produce small differences in the displayed course. However, both methods of navigation should produce the same desired ground track when using approved, IFR navigation system. Should significant differences between the approach chart and the GPS avionics' application of the navigation database arise, the published approach chart, supplemented by NOTAMs, holds precedence.

Due to the GPS avionics' computation of great circle courses, and the variations in magnetic variation, the bearing to the next waypoint and the course from the last waypoint (if available) may not be exactly 180° apart when long distances are involved. Variations in distances will occur since GPS distance-to-waypoint values are along-track distances (ATD) computed to the next waypoint and the DME values published on underlying procedures are slant-range distances measured to the station. This difference increases with aircraft altitude and proximity to the NAVAID.

[DXB]

No idea if it is related, but I notice your two magnetometers read differently by 7 degrees, with the RNAV reading higher. 

[Bob - S50]

47 minutes ago, DXB said:

No idea if it is related, but I notice your two magnetometers read differently by 7 degrees, with the RNAV reading higher. 

I assume you are talking about magnetic heading in which case I think there is only a 3 degree difference (289 vs 292).

[Marauder]

No idea if it is related, but I notice your two magnetometers read differently by 7 degrees, with the RNAV reading higher. 

As Bob pointed out, there is only a 3° difference. The magnetic difference shouldn’t have played a role in the difference in course from the GTN.


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[Bob - S50]

22 minutes ago, Marauder said:

 

As Bob pointed out, there is only a 3° difference. The magnetic difference shouldn’t have played a role in the difference in course from the GTN.


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No but as DualRatedFlyer pointed out, that's just the way our GPS systems work.  They calculate a magnetic variation for the current location.  That can often vary by a few degrees from the actual magnetic variation.  However, since the GPS is navigating to a lat/long that we identify by name, it still goes to the right place.

[kortopates]

No but as DualRatedFlyer pointed out, that's just the way our GPS systems work.  They calculate a magnetic variation for the current location.  That can often vary by a few degrees from the actual magnetic variation.  However, since the GPS is navigating to a lat/long that we identify by name, it still goes to the right place.

Exactly, just as the AIM describes, GPS is not using a coded magnetic course but a true one with a magnetic variation for the airport. The ILS is in truth course less - it doesn't matter where you leave the dial - it's a directional dual lobe beam. Bigger variations will be seen between VOR airway courses and the the same GPS track because their magnetic variation can not be updated as frequently as the airports because it cost so much to realign the VOR signals. But airports are updated much more frequently.


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[jlunseth]

I see small differences like that, between the CRS on the plate, and what the GPS says to fly, routinely.  I haven’t looked into it, but always assumed it was the difference between magnetic nav. and GPS nav.  In my aircraft, the best way to fly the final approach is to set the course needle and put the AP in APCH mode.  I always use the course setting on the plate, even though the GPS (I have a 430) may be giving me a course that is a degree or two different.  I can fly the final approach by hand by not engaging the AP, so I have the course needle and flight director to follow, or by engaging Otto and letting him do it.  

[M016576]

The obvious problem is that you’re using an inferior and more expensive garmin GTN unit.  If you were using an Avidyne IFD, all the numbers would be perfectly aligned on your approach with the charts.