IFD course presentations on VOR 27 to 1G5
Printed From: Avidyne
Category: Avidyne General
Forum Name: IFD 5 Series & IFD 4 Series Touch Screen GPS/NAV/COM
Forum Description: Topics on Avidyne's IFD 5 Series and IFD 4 Series Touch Screen GPS/NAV/COM
URL: http://forums.avidyne.com/forum_posts.asp?TID=2597
Printed Date: 30 Apr 2025 at 3:24am
Software Version: Web Wiz Forums 12.01 - http://www.webwizforums.com
Topic: IFD course presentations on VOR 27 to 1G5
Posted By: compasst
Subject: IFD course presentations on VOR 27 to 1G5
Date Posted: 30 Oct 2023 at 11:29am
I am wondering where the IFD (both the 540 in my aircraft and the 100 on my iPad) get the course depicted on this approach when loaded into the IFD. I have uploaded images of the approach plate and of the presentation on the IFD - note the IFD-depicted course to COLEN INT is 282 degrees yet the IFD- and the plate-depicted course for the rest of the approach to the runway is 273. Both courses emanate from the same source - the ACO VOR. How can COLEN exist on two radials from the same VOR?? What am I missing here? The GFC 500 is also confused as it cannot track properly to execute this approach.  |
Replies:
Posted By: dmtidler
Date Posted: 30 Oct 2023 at 3:03pm
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The depicted course depends on type of navigation the IFD leg
uses. The IFD appears to generally a form of two types of navigation (Fly
Direct and Fly Course). Fly Direct navigation calculates a great circle between waypoints
or to a waypoint during a Direct to operation. Fly Direct legs appear to always
depict the IFD calculated magnetic course. The leg between ACO and COLEN is a
Fly Direct leg and 282 is the IFD calculated magnetic course out of ACO to COLEN
(which overlays the ACO 278 radial out of ACO). Fly Direct is indicated on the
expanded FPL tab for the COLEN waypoint. Fly Course calculates rhumb line courses to/from a waypoint.
Fly Course is used in OBS mode, parts of some procedures, as well as holding
courses (Hold). The depicted courses for Fly Course are normally magnetic; except
when using OBS mode to/from a VOR waypoint or generally when the waypoint is based
on a VOR or VOR radial while part of an active procedure; then the VOR
declination appears to normally be factored in so that the depicted IFD course
matches charted VOR radials. In this approach, the legs from COLEN to NVRTL and NVRTL to RW27
are Fly Course legs (as indicated on the expanded FPL tab) and their respective
IFD displayed courses take into account the ACO declination so they match the ACO
278 radial depicted on the approach chart. Unless they overlay a true north or south course or overlay the
equator, Fly Course legs are not great circle and may not precisely connect two
waypoints in the same manner a VOR radial or Fly Direct routing (both great circle)
do. If you zoom in on the map view of COLEN or NVRTL you will notice that the start
of the leg from COLEN to NVRTL as well as the start of the leg from NVRTL to
RW27 do not exactly line up with COLEN or NVRTL respectively; this will lead to
a slight left correction to stay centered on the GPS course after crossing each
of these waypoints if this approach were flown as an overlay using GPS steering
instead of in VLOC. Navigating this approach with a GFC 500 in VLOC from ACO would
most likely require adjusting the auto-slewed VOR course to 278 to match the
charted VOR radial.
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Posted By: compasst
Date Posted: 30 Oct 2023 at 9:04pm
| Thank you so much for the thorough explanation. It didn't cross my mind, but makes perfect sense to me. |