I was asked on the Grumman Gang about GPS installations and how I could get away with not having a switching/annunciator panel. Here's my response:


>>> One thing most of these mfrs don't point out on the IFR certified units is that an annunciator
>>> panel is required for certification and that is another $900 plus installation.

In a manner of speaking, yes the annunciators are required, but no, the switches are not.

To start with, I haven't read the TSO completely, so take my experience with a grain of salt and do your own research. But I have a UPSAT (nee Apollo/iiMorrow) GX-60 installed and I do not have the switching mechanism. Following is a basic transcript of a discussion I had with a Gangster last month:

>>> The ACU is a switching panel with annunciators etc built in. In our plane it would replace the a/p on off and source select, plus add gps annunciator lights and (I think) do the auto-shunt when you select a loc freq on nav1. Do you have one?

No, I don't. I got around the TSO requirement for the lights by integrating them into the Mid-Continent display. My GX-60 installation does not require the switching mechanism, because the GPS CDI display is used ONLY for the GPS, and not toggled between the GPS and the ILS. So, it's a clean installation.

For the a/p, I still use the standard SPST switches for the C1, I just have them labeled GPS/NAV2 vs. NAV1/NAV2.

>>> which mid-continent indicator do you have? is it a 3 1/8 or smaller (forget the smaller dimensions).

Smaller. You can see a pic of it on the web page. [http://www.gatm.com/Flying/minimalist.html]

>>> what did it cost?

Unknown. At the time, iiMorrow was offering a special deal: a free annunciator panel with every GX-60 purchase. I reworked that into the Mid-Continent display for a little more money. It was basically no additional cost to me.

>>> if it has all the right annunciators I may go that route if you don't need to relay shut for a/p disconnect when loc freq tuned in because then I can display info from both at the same time.

Well, that depends. If you're going to display both VOR/ILS and GPS onto the same display head, you're bound by the TSO for all the switching [stuff]. Remember, I only have a single VOR, which is being displayed on its own CDI. The Mid-Continent is used exclusively for the GPS.

>>> so the regulation is only for displayed data, not for auto-pilot inputs?

Both! They want to make sure you're looking' at what you THINK you're lookin' at, flyin' what you THINK you're flyin'. It works the autopilot at the same time: wherever the indicator is switched, so goes the a/p.

>>> do you remember the model of you indicator?

I think it's the MD40-65

>>> so how do you comply if you have your gps connected to the autopilot but have no relay?

Because there's no ambiguity in the a/p switch I have now. When it's switched to GPS, only the GPS is moving the autopilot. When I switch it to NAV2, only the VOR is moving the autopilot.

See, the TSO is designed such that it's expected you'll keep your existing two VHF nav radios, but all three units would be channeled to only two total CDIs in your panel. So, when you install the GPS it's expected that you'll channel the GPS output to one of your VOR CDIs.

For example, let's say you have two KX-155 radios and a KI-209 head (w/glideslope) and a KI206 CDI head. You install a GPS and channel the output of the GPS to the #1 KI-209. You now have an ambiguity: when you're looking at the #1 head, are you looking at the VHF output or are you looking at the GPS output?

To resolve this you have to install an annunciator/switch panel which coordinates all this. Not only will it tell you what you're looking at, it will control the output to the #1 CDI. Even further, the TSO requires that if you have selected an ILS frequency that the associated CDI and autopilot output MUST switch over to the VHF radio. This requires that all these devices talk to each other.

To further complicate all this, your autopilot gets its input signal not from the radio (VOR/GPS) itself, it gets it from the CDI which has an output. Thus, whatever your CDI is saying (go left/go right) it's telling the autopilot the same thing. Therefore, the annunciator/switch panel is switching both the CDI and the autopilot, although not directly.


Now, in my case I don't need all that stuff. I have a single IFR GPS/COMM (my #1 nav and my #1 comm) and a single Terra VHF radio with glideslope (my #2 radio and my #2 comm). Since each has its own CDI (the GPS uses the MD40-65 and the Terra uses its own digital CDI) I don't need a switching mechanism. Everything is dedicated.

For the autopilot, I have both CDI outputs channeled to a single switch to the autopilot (GPS/NAV2) thus I control through a single switch which output goes to the autopilot.

You could basically do the same thing by adding a dedicated CDI for just the GPS and making your autopilot switch a rotary three-position switch to select the input.

Makes sense?

>>> is your altitude encoder wired up to the gps?
>>> do you need that for approach approval?
>>> anything special you need to do for that or any additional equipment needed?
>>> do you need dual encoders?

Yes, and for enroute/terminal as well.
Yes, see below.

Your encoder must have "serial" capability, meaning that it has to have a GPS interface. The regular transponder (without serializer) sends only "gray code" to the transponder. It's really an archaic system. GPS input however requires a serial data interface to be able to know what its altitude is.

You'll have three options:

- you already have an encoder with serializer, so all you need to do is hook it up to the GPS
- you don't have an encoder with serializer, so you buy a converter which converts the gray code to serial interface and works in conjunction with the existing encoder
- you buy an encoder with serializer
I went route #3 myself, instead of adding another box under there. You'll need to find out if your transponder is currently capable of the output to GPS.

>>> bloody hell. its more and more expensive the more I look at it!
>>oh well, its not as though I actually NEED furniture or food is it ?
>>(grin what did I expect)

Here's some examples:


You'll notice that the price difference between a serialized encoder (about $400) and the non-serialized one (about $200) is about the same as what you can get for your used one (about $100) and a converter (about $350).

Better to save some weight and complexity and replace it.

Once it's all done (I was lamenting the money just like you are) you'll wonder how you ever did without the IFR GPS. Great SituAware and it's saved my sanity on several approaches to date. I even like the "non-precision" GPS approaches better than ILS.

>>> I like your electrical system ideas...how do you plan to get it approved? http://www.gatm.com/Flying/electrical/index.html

They're not really my own ideas, just derivations of info available from Bob Nuckolls and the Aeroelectric Connection (http://www.aeroelectric.com/).

I think the best way to get this all approved is what I call the "Liberal Party" way.

Remember how the massive Nationalized Health Care Program was summarily rejected by the masses 6 years ago; folks didn't want a massive social program for Health Care? So, instead what has the Federal Gov't done instead? Instead of proposing a massive one-stop program, they are carefully and slowly inching up to it by "protecting" the elderly, then the children (how can you be against children?!?), then the parents of the children, then the poor, then the uninsured, etc. Eventually you've got what you want, in a sneaky way.

We do it the same way. Instead of a massive one-stop all-in-one complete entire electrical system package dropped on the desk of some poor FSDO schmuck, you do it step-by-step. First you get a 337 for the battery, then you get a 337 for the ground wire for safety (how can you be against safety?!?), then you replace the gauges, then you replace the alternator wire, so forth.

Slowly and surely, once it's all completed, you can step back and say, "Hey! Isn't that what Billary Care was all about? How did we get THAT approved...?)"


Any avionics gurus care to comment?