Altitude data source for garmin 400w
 

I have a kt76a transponder and a grt mini x. Which one do I use to feed the altitude data to the 400w ? I think all my other wiring is complete. Thanks !

Since no one answered, I'll try.
I do not know for your transponder. But the data comes from the efis. So a direct efis to gps connection will work. The efis also sends altitude data to the transponder (assuming you are not in Canada and have a blind encoder). Some transponders, like the GTX327 I have, can re-send the data out, in which case that can go to the gps. It's daisy chained, so if the transponder is off, the data does not go thru to the gps. I do not know if your transponder has this serial data out capability.

Ok, curiosity has gotten the better of me. What is the purpose of sending altitude data to the GPS?

I have a 430W and am not aware of how it would use altitude data sent from another device.

On some approaches you can use lower minimus if you have Baro-aided VNAV. Also some GPS' can use air data to compute fuel-at-waypoint and similar data.

Mostly so its E6B-like functions work. Wind direction and strength, for example (there's more than just altitude data sent). Not absolutely necessary to have it.

VNAV is a handy function on a cross country. The 430 manual says this:
"Altitude input is useful for advisory vertical navigation (VNAV) calculations."

Well thanks guys, have to tell my wife I learned something today :cool:,

Thanks for the replys. Just figured i should hook everything up I possibly could because I'm NOT opening this panel up again for surgery again till I can afford a g3x touch ! Tired of panel surgery but looking forward to starting my instrument flying now. I still need to wire my King gs indicator to the garmin but I'd rather leave it alone and upgrade my mini x to an ap so I can do the annunciation on the efis screen and then I can run an ils in conjunction with a gps approach at the same time.

I still need a heated pitot too. Blast it never ends but I'm getting. Closer

The air data is useful for a few functions in the 430 as mentioned but in reality now it's pretty useless functionality compared to any EFIS you are likely to have plugged into it.

Re approaches though and a bit off track:

You certainly can't APV Baro-VNAV with a 430w in Australia and my understanding is that with WAAS in you don't need the baro to conduct LPV or LNAV/VNAV. So I'm guessing that in the U.S. There is no such thing as a APV Baro VNAV as far as GA aircraft are concerned? Just curious.

Do not confuse Garmin "Lnav+ V" with "LNAV/VNAV". The latter takes equipment rarely found on small GA aircraft, including a GS generated by a horizontal gps position and a barometric altitude. Feeding baro altitude into a 430W is not enough for a lnav/vnav approach.

Thanks bob
Yes that's my understanding. That is the realm of FMS etc.
we don't even get LNAV + V here and likely never will.

Is the airdata required for LNAV+V or LPV over there? Or is I just a function of WAAS ( which we missed out on too :-( )

lnav+V is just a Garmin thing, the GS is advisory only, it's still an LNAV non precision approach. For LPV we just need WAAS, no air data. It used to be that pre WAAS some boxes used baro data as part of their integrity check.