GTN CDI/GPS "button" vs. Skyview HSI "Source"

This one is a tough one.. Please HELP ! I am working on my IFR Proficiency Now, as you can tell..

When on an ILS, the CDI button should be initiated when joining the localizer to ensure that you have CDI sensitivity vs. VNAV sensitivity that the WAAS GPS would normally be providing during an ILS approach. This is common practice I believe for the GTN650 and 430. Always switch to CDI when joining the localizer? That said, the GTN has two separate Arinc out sources to my Skyview. First is the ?Nav? Source which sources direct from the Nav frequency on the GTN, the second source is the ?GPS? source which sources the GPS information from the GTN. Either of these sources is selectable via the HSI on the Skyview.

So my question is I have a couple options to ?legally? fly the ILS with this equipment.

1) If my Dynon HSI is set for GPS Source while enroute and upon joining the localizer I switch the HSI source to be ?Nav?. This now disregards the GPS Signal and uses the frequency tuned into the GTN Nav. In this case the HSI is flying the actual Localizer signal. This is legal even if the GPS was left on VNAV, i.e. the CDI button was not pushed.

2) If upon joining the localizer I switch the GTN from VNAV to CDI by hitting the CDI button, the GPS then puts out the ?Nav? Signal using CDI sensitivity from the GPS Source, so I could continue to fly the Dynon HSI on the ?GPS? mode. (i.e. without switching the HSI source to ?Nav?.)

3) Alternatively I could Switch both (Skyview HSI and GTN) Modes.. Skyview HSI to ?Nav? and GTN GPS by selecting the CDI button. In this case the Skyview HSI would be flying the ?Nav? signal from the Nav frequency tuned into the GTN Nav, and also the Readout on the GTN and the GPS source from the GTN would put out CDI sensitivity for the Localizer approach. (note in this last option, the HSI wouldn?t care because it was already tuned to the ?Nav? source from the GTN)

Is someone close enough to this problem to Confirm the above ?

Thx

Chad,
The CDI button doesn't do anything when hooked to a SkyView, so it doesn't matter what you do with it.

The CDI button is used when you have an external mechanical HSI or CDI. This changes what gets sent out the port that goes to that. But with an EFIS, both signals are always present, and the HSI SRC on SkyView is what switches. This is what lets us do something like show a bearing pointer to a VOR while you fly the GPS.

SkyView always tells you accurately what you are following. If it says GPS, it's GPS, and it will be magenta. If its VLOC, it will be green and it is a VLOC. There is no way for it to say GPS but be showing you the VLOC.

If you want to fly a GPS approach, SkyView must be in GPS mode. In this mode the terminal, approach, or enroute sensitivity is automatic and always right (and is shown in the HSI window). It doesn't matter how or when you switched into this mode.

If you want to fly a ILS, you must switch the HSI to VLOC. There is no sensitivity adjustment in VLOC modes, since they are angular.

Now, this does mean that if you want to use the GPS to enter an ILS, but fly the ILS, you need to switch modes manually on the SkyView as you intercept the ILS.

We do plan on an option in the near future where the HSI will behave more like an external HSI: All you choose on SkyView is "GTN" or "GNS" as your HSI SRC, and then you switch modes with the CDI button on the navigator. This will be optional, but has the advantage that the navigator can switch from GPS to ILS for you as you intercept and then go back to GPS if you go missed. We'll also be synchronizing flight plans between the navigator and your SkyView, and showing the whole approach visually on the SkyView map, which will be a really nice upgrade for our IFR customers.

Good luck with your new rating!

--Ian Jordan
Dynon Avionics

I highly recommend that you disable the CDI key when using the GTN/GNS series with an EFIS system that does not mirror/follow the CDI key of the external navigator.

In my opinion this is the only legal and safe way to do it. You do not want any confusion when it comes to the source you are following.

How to disable this button is covered in the installation manuals of the GTN/GNS series navigators.

There is some great info here. Can anyone tie this into a GTN650 with a AFS4500? I think the answer is no but should the CDI key be disabled with this setup?
George

Thanks all, Makes perfect sense.

Do you know if the CDI button effects the sensitivity of the GTN on screen localizer readout? It should right?

My hope was to use that on screen Localizer readout as a backup/crosscheck to the Skyview HSI. If it reflects appropriate sensitivity based on the CDI/GPS toggle, then I would want to leave that toggle in place and use it correctly rather than remove it altogether so I can reference it as backup.

Thanks