OK, I have finally been able to get back to this after having some oil pressure issues. I have confirmed that it is the 540 itself, after enabling the GPS CDI.
The Aspen image shows the issue. I am flying a track 112 degrees off from the first leg of the RNAV 05 approach into CYFD, waypoint MIBLA. In the image, the course pointer has just swung to show the desired track from MIBLA to OVUTU. The magenta line continues to MIBLA, the white line is the track to OVUTU. The course deviation is zero. The IFD540 cross track distance and deviation are also zero and DTK is 142, so the Aspen and 540 are in agreement. To me, that takes the Aspen out of the picture. The behavior is being driven by the 540. (That is good news from a troubleshooting point of view.)
I verified the 540's GPS behavior on the IFD simulator. It's the same using both the old sim (for 10.1) and new sim (for 10.2). When approaching the waypoint, the DTK changes to the course for the next leg, the deviation is zero.
What I have determined is that, as long as I am on the curving GPS track that provides the turn anticipation, the deviation is zero. That may be logical since I am "on course", but it disagrees with the DTK.
I asked my buddy with a G1000 to try this. At a similar point approaching the IAWP, the course pointer/DTK change to the next leg, but the deviation is NOT zero, it is showing the actual deviation from current location to the track of the next leg. I think that is correct behavior. At least it is more logical to me.
I find the 540's display of zero deviation to be non-intuitive. I suppose I can learn to live with it, but I'm looking for either a way to change the behavior, or an explanation of why this is better or correct behavior. It doesn't look that way to me.
I still need to try this in the air with a localizer. I know that, when I am flying procedure turns and returning to the localizer, the deviation works as I expect, guiding me to the localizer. Yet, when I am not doing a procedure turn and just intercepting the localizer, the deviation works differently. That has been confusing. I need to verify it again, and capture some images. I am expecting that the zero-deviation behavior will occur while in GPS mode, but the actual deviation will appear when the 540 changes to VLOC. I want to try making the change to VLOC manually. That would provide a workaround for intercepting localizers, but I'd still be faced with this unwanted behavior in GPS mode. To me, it should always work the same way.
|