Creating a waypoint at the intersection of two radials on a GTN

[PT20J]

There are still some obstacle departure procedures around that require flying a VOR radial and then intersect a radial from a second VOR. Sure, I have dual VORs and the G3X has dual bearing pointers and there is always OBS mode on the GPS - there are several setups to fly these procedures that are not in the GTN database. But, it is so much easier to let the GPS navigator do the work. Although my GTN Xi won't let me enter a user waypoint at the intersection of two radials, ForeFlight will easily do this. Just enter VOR1radial1/VOR2radial2. When I upload the ForeFlight flight plan to the GTN via the G3X (Flightstream would be another way to upload), the GTN creates a user waypoint at the lat-lon for the intersection. If you don't have a way to upload flight plans from ForeFlight, just select the waypoint on ForeFlight and note the lat-lon and enter that in the screen for creating a user waypoint in the GTN. ForeFlight uses decimal degrees for lat-lon and Garmin defaults to deg-minutes, but there is a convert button on the user waypoint data entry page to change modes so that you don't have to do the math.

[midlifeflyer]

Doable (see video). But not really necessary. Typical is ATC giving a heading to intercept so all you need is to define the leg.

 

[PT20J]

There are locations out in the mountainous west where, due to terrain or gaps in radar coverage, you are on your own nav for quite a while after departure.

[Marc_B]

@PT20J The GTN should allow you to create user waypoints that are either defined by lat/long, by a radial/distance, or by radial/radial.

edit: check out 5:26 minute mark from @midlifeflyer's video.  He's doing the same as above but using radial distance rather than radial/radial or lat/long.

And if you get cleared to a fix via radial, fly heading... you can always use the "Direct To" button.  When you hit direct to you can select the waypoint, but you can also choose the course on the same screen.  But the only option on this is the "course to" so you have to use the inverse heading...i.e. if you are cleared fly heading 100, intercept the 360 radial to X..." you'd choose "180" on the course to in order to fly the 360 radial from.  (by default the course to is filled with the actual direct course to the waypoint, but it's cyan meaning user adjustable)

[PT20J]

3 minutes ago, Marc_B said:

@PT20J The GTN should allow you to create user waypoints that are either defined by lat/long, by a radial/distance, or by radial/radial.

You're right, I missed that button somehow. 

The point of entering the waypoint is that it allows the GPS to do its normal TO-TO leg sequencing and frees you from any further knob twisting or button pushing.

[Marc_B]

23 minutes ago, PT20J said:

The point of entering the waypoint is that it allows the GPS to do its normal TO-TO leg sequencing and frees you from any further knob twisting or button pushing.

Ya, it's interesting that Foreflight allows you to graphically edit a flight plan and when you drop the rubberband gives you the option to either use the Lat/Long where you dropped it OR choose the nearest waypoint.  If you click on the map on the GTN, Garmin does allow you to graphically edit the flight plan as well but only gives you the option to use waypoints in the database (including existing user waypoints you created).

[Marc_B]

although I guess it does give you the option to create your waypoint there as well...  So if you were trying to do this you'd have to then "Create Waypoint" at the map pointer, then you could "Graphically Edit FPL" and rubber band your flight plan track to that waypoint or just add it to the Flight Plan by name.

I believe that the GTN only allows 1000 user waypoints to be created so there is a "temporary" button that will scrub the waypoint from the unit at power off.  VS if you had a mountain pass you wanted to keep as a waypoint permanently.  It doesn't default to temporary, so you have to manually choose it.  Interesting that Garmin typically tries to identify map pointers as "Radial/Radial" rather than "Lat/Long"...whereas ForeFlight typically creates lat/long points when you graphically edit and rubber band your flight path.  (EDIT: although this could have been because I previously had created a radial/radial waypoint so it defaulted to "last used" to create a waypoint on the map pointer...not sure)

[Ibra]

1 hour ago, PT20J said:

The point of entering the waypoint is that it allows the GPS to do its normal TO-TO leg sequencing

That would be neat 

3 hours ago, PT20J said:

There are still some obstacle departure procedures around that require flying a VOR radial and then intersect a radial from a second VOR

I think historically, these radial/radial were never coded as official waypoints (or even as internal computational navigation fix, CNF) because they do require “dual magnetic variation” inputs and handling which is not supported in TSO146 (in US, this is moot as VOR magnetic lines are frozen anyway, the better word is “magnetic declination”: original variation at the time the VOR was last aligned or installed)

The GPS will naturally handle, VOR-VOR legs and VOR-DME legs on VOR airway as these can be coded as track-to-fix (TF) or along-track-distance (ATD) irrespective of magnetic variation: one flies the true line between two fixes that happen to be VOR…

The older GPS TSO129, did not specify which magnetic variation (airport, vor, internal, published…) has to be used to get magnetic course although most of them use VOR published one on VOR radial or tack legs, in TSO146 like GTN, it’s explicit that you need to use the published VOR magnetic variation, however, it’s a issue when you have two VOR radials (it’s theoretical one: they could interpolate or use the closest one and likely it does not matter that much) 

* Direct-to-VOR+OBS or Course-To-VOR will use published VOR variation or declination 

* Direct-To-Waypoint use internal magnetic model when it shows magnetic course 

* Radial/Radial, well you can see a debate with 3 choices to pick from, it seems ForeFlight does solve this quietly and easily behind the scenes for you (*)

See 2.2.1.3.12 Magnetic Course, RTCA DO 229D specs for WAAS GPS, it’s a shame we have to pay for this document but here is some info

https://www.euroga.org/forums/flying/2784-gnss-approach-availability-in-europe?page=3#post_44004

(*) None of this is relevant or matters in practice, especially, when flying departures or terminal procedures (30nm around airport ARP?), however, someone was concerned about errors on 1000nm radial/radial that cross the North Pole …

 

Edited March 25, 2024 by Ibra