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DME error? What did I do?

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260SE View Drop Down
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    Posted: 20 Aug 2020 at 12:04am
We had something goofy happen on an instrument training flight, and hopefully it was pilot programming error.  IFD-540.

My airport doesn’t have a DME arc, so we decided to fly a pretend 9 DME around our VOR.
-I set up the VOR frequency in NAV1.  
-I then used my HSI to fly the arc (connected to NAV1).
-I also programmed my IFD for Direct-To the VOR so I could see my distance from the station (DME)
-I then turned the IFD’s CDI knob to VLOC, but did not turn the IFD OBS knob at all.  I had no plans to use the maps page on the IFD, so I just used my HSI.

What happened next and why I’m not sure, but we ended up pointing the plane outbound because it seemed like we were getting blown towards the VOR according to the DME mileage.  What we think might have happened is that the IFD was showing us cross-track distance to the radial we wanted, rather than distance to the station.  It took us a few turns away from the station and a few radials later and a recheck of winds aloft calmness to figure out the distance in the IFD magenta waypoint box was not our distance to the station.

What did I screw up?


Edited by 260SE - 20 Aug 2020 at 12:09am
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Bob H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bob H Quote  Post ReplyReply Direct Link To This Post Posted: 20 Aug 2020 at 9:58am
Hard to know.  What distance were you monitoring?  TkDist or Direct Dist?

Edited by Bob H - 20 Aug 2020 at 9:59am
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94S View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 94S Quote  Post ReplyReply Direct Link To This Post Posted: 20 Aug 2020 at 11:22am
The IFD displays Along-Track-Distance in the FMS waypoint blocks.  So, if you were using the FMS waypoint blocks for your distance, the distance along the programmed Direct To route would decrease as you proceed around the arc.  

What you want to use in your case is either the "To Waypoint Direct Info." or "To Waypoint Direct Distance." data block (one just gives more info than the other).  They can be placed in the fixed left side data blocks so they are visible on the FMS page, or in the scrollable right side data blocks visible on the MAP page.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 260SE Quote  Post ReplyReply Direct Link To This Post Posted: 20 Aug 2020 at 11:41am
Awesome, thank you.  I had the “To Waypoint Information” block on the right of the Map page.  I thought it displayed distance similar to “To Waypoint Direct Info”, but I now see that’s not the case.  The former shows “TkDist”, the latter “Dist”.
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Bob H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bob H Quote  Post ReplyReply Direct Link To This Post Posted: 20 Aug 2020 at 11:56am
Also, note that instead of modifying a datablock on either the FMS page or MAP page, just go to the nearest VOR page and get the distance and bearing information right there.
Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AZ Flyer Quote  Post ReplyReply Direct Link To This Post Posted: 21 Aug 2020 at 10:44am
Originally posted by 260SE 260SE wrote:

Awesome, thank you.  I had the “To Waypoint Information” block on the right of the Map page.  I thought it displayed distance similar to “To Waypoint Direct Info”, but I now see that’s not the case.  The former shows “TkDist”, the latter “Dist”.

I discovered this is also the data block to use to see distance from the hold point when flying the outbound leg (for a distance-defined holding pattern).
Stan
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