A/P Questions

I'm in panel final planning/purchasing phase and a little confused. Help me wizards...

It is my understanding the popular experimental autopilots (Trutrak, Trio) accept only digital GPS inputs, i.e., RS232. I believe I have seen several panels configured with a NAV and a GPS together with two selector switches. One switch appears to select either the NAV or GPS for CDI/HSI display info and the 2nd appears to select which signal drives the A/P.

My questions are:
are they driving the A/P with a non-digital NAV signal?

If not are you flying your A/P assisted approaches with using a legal 'ground based navigation signal' NAV or questionably legal GPS?

Is there a converter to enable analog NAV info to drive RS232 devices?

Or have I missed some important info along the way?

The less expensive models only use the GPS course mode for external navigation signals.

You have to step up to more expensive models to get GPS steering options and even more expensive models to get NAV modes.

Inside those there are also options for vertical GPS steering etc. so you have to be very carefull in what you pick out to ensure it will integrate and function the way you expect it to.

For standard GPS course following all you need is RS232
For GPS steering, you need ARINC interfaces
For NAV, you need either ILS/VOR ARINC interfaces or Analog connections to the AP.

The SL-30 has an RS232 serial port that outputs just about everything it supports but it is a different protocol than what is used for the GPS NAV data therefore the GPS enabled AP's will not work with it.

The older analog Navaid wing leveler/AP could be configured with a switch and the smart coupler to either use a GPS or a Nav Radio over the analog lines. It does not support vertical anything or GPS steering.

Some EFIS systems have the ability to take just about any kind of navigation signal and transfigure it and spit it out the ARINC lines to the ARINC enabled AP's as GPS steering commands. This gives you NAV/ILS tracking without having to pay for the more expensive NAV enabled AP.

There is more to it than this, but these are some main points to concider. This is a major source of disapointment for some who pick stuff thinking it will do xyz and find out that it will only do x.

In all cases, you have to find an AP that has the modes you want (or an EFIS that can fake these modes and send them out the ARINC lines) then you have to ensure that you have the GPS and Nav radio gear and connections/signals to support those modes.

Dynon is introducing a new AP that can do the things more advanced AP's do and then some and will take in GPS NAV, GPS STEERING,ILS/VOR and ARINC data. This negates the need for the external more expensive AP.

Hope this helps.

Brian pretty much hit the nail on the head - I know he's been doing his homework lately! The only thing I can add is that while the Dynon AP has promise and is exciting....you can't buy one yet - and we don't know exactly when you can. Lots of exciting hype, but the fact is I can't sell you one as it sits today.

So, here's the short simple if you're buying anytime soon related to what you might want it to do:

GPS only: TruTrak and Trio - all of their products.
GPS + ARINC: TruTrak Digiflight IIG (or VSGV) and up only.
GPS + ARINC + Analong NAV: TruTrak AP-100 or Sorcerer.

Now like Brian said, if you have an EFIS like a GRT, Chelton or AFS, they will take NAV signals and convert them to ARINC to drive "quasi/overlain" coupled approaches to an AP like the Digiflight IIVSGV. Also, the Digiflight II VSGV will couple directly to the GNS-430W, 480 and 530W to shoot coupled GPS approaches (but not nav). If you fly direct coupled GPS WAAS approaches driven from the 430W and 530W will blow you away compared to an ILS, but that's an entirely different discussion. The common reason for the "AP source" switches is for allowing the mid/higher end trutraks to couple directly to the radio, or switching GPS/ARINC feeds from one or more boxes to the AP in the basic TruTrak and Trio models.

In the future we know for sure this is going to change. Dynon's product will start out doing the lateral stuff (GPS + ARINC + NAV when using an HS-34 and AP74) but no vertical guidance. When the AP-76 is out (we don't have any sort of date on that yet) then you can add vertical guidance. Also don't think that TruTrak and Trio are asleep at the wheel. I wish I could say more, but stop by each of their booths at SnF for some more surprises!

Anyway, that's how it all sits as of today. One thing for sure, it's confusing to sort through and it's always dynamically changing. But, give us a call or shoot me an email with what you want/plan on doing with your plane and naturally we'll figure out what is best for you. We sell, service and install all of them so we can help you decide which works for your own particular situation. There is not one single solution (no matter what the mfgrs tell you) that is best for all of the people all of the time!

My 2 cents as usual.

Cheers,
Stein