[RV8R999]

Im trying to figure if I do in fact need this external switch/annunciator and why it would be important?

My panel is GPS only so there isnt a NAV to switch to/from. THe EFIS CDI will be 100% GPS 100% of the time.

I have a GNC-300XL approach cert GPS. I can see needing this for a NAV/GPS but why GPS only?

all the other functions can be controlled from the GNC directly....

Hmmmmm????

 

[Dave Cole]

GNC-300XL needs an annunciator panel

I also have a GNC-300XL, my take on it is that the annunciator/switch panel provides a pilot interface with the 300XL that the unit itself does not. This is what the annunciator panel will do for you:

1. Allow you to switch the approach to ARM mode. The GPS will not do that automatically.
2. Allow you to switch waypoint sequencing from AUTO to HOLD when you are required to fly vectors and do not want the GPS to sequence automatically.
3. Annunciate the approach mode as ARM (armed).
4. Annunciate the approach mode as ACTV (active).
5. Annunciate the waypoint sequence mode as AUTO or HOLD.
6. Annunciate WPT (waypoint) arrival.
7. Annunciate MSG (message) available on the GPS.

Not only do you get all of that functionality, but the TSO for IFR GPS use requires that the GPS be installed in accordance with the manufacturer's requirements. One of those requirements is that an annunciator panel be installed.

I am making my own annunciator since, as you have pointed out, the GRT provodes the HSI fucntion for NAV and GPS, so and external switch is not required. To simplify construction, I am going to use two of the four-circuit annunciator circuit boards that VX Aviation sells, along with some LED indicator lights and some lighted pushbuttons.