2nd CDI on the B-model

This is a question mainly for the guys owning the B model.
I have always wondered what the 2nd CDI’s purpose on the B model is. Since this CDI is only connected to the Garmin 420 all one can track is GPS waypoints. The Garmin already has a CDI indicator plus you have that big screen in front of you where you immediately see if you’re getting off track on your way to the next GPS waypoint. So, my question is, is this not a waste of money and real estate, or is there more capability in this CDI?

Looking forward to your comments.

Cheers,

Placido

This is a question mainly for the guys owning the B model.
I have always wondered what the 2nd CDI’s purpose on the B model is. Since this CDI is only connected to the Garmin 420 all one can track is GPS waypoints. The Garmin already has a CDI indicator plus you have that big screen in front of you where you immediately see if you’re getting off track on your way to the next GPS waypoint. So, my question is, is this not a waste of money and real estate, or is there more capability in this CDI?

Looking forward to your comments.

Cheers,

Placido

Placido,

I’m sure someone will speak up if I’m misinformed, but I believe that somewhere in the regs it mentions that for a GPS to be IFR-certified (not sure just for approaches or even en-route), it needs to have a CDI that is within the normal scan area that a pilot would use. I remember reading in some AOPA publication that most of the time this means having an external CDI, since apparently the FAA doesn’t consider the radio stack as part of the normal scan. (And considering how low the 420 is on the B model this might be good advice).

So I think the answer is that in order to get IFR certification for the 420 intallation, they needed an external CDI.

Steve

So I think the answer is that in order to get IFR certification for the 420 intallation, they needed an external CDI.

This is basically right, but you would be sure glad to have it when you are shooting a GPS approach in the soup after your #1 GPS died.

Clyde and Steve,

If the legal requirements are such as mentioned by Steve then there’s not much to do.

As to the #1 GPS dying: which GPS is driving the ARNAV screen? Is it only #1 or both. If it’s both couldn’t you just track your course on the big screen? This might be a totally dumb question, since I’ve got only the VFR ticket, so pardon my ignorance.

Cheers,

Placido

So I think the answer is that in order to get IFR certification for the 420 intallation, they needed an external CDI.

This is basically right, but you would be sure glad to have it when you are shooting a GPS approach in the soup after your #1 GPS died.

As to the #1 GPS dying: which GPS is driving the ARNAV screen? Is it only #1 or both.

It’s not a dumb question, but the answer is that only the #1 GPS drives the big screen. So if you lose the 430 you lose the Arnav. The HSI and therefore the autopilot also work off the #1 GPS/NAV only.

Clyde and Steve,
If the legal requirements are such as mentioned by Steve then there’s not much to do.
As to the #1 GPS dying: which GPS is driving the ARNAV screen? Is it only #1 or both. If it’s both couldn’t you just track your course on the big screen? This might be a totally dumb question, since I’ve got only the VFR ticket, so pardon my ignorance.

Cheers,
Placido

Placido,
Only the #1 GPS drives the Arnav; if it goes the Arnav can’t pick up position from GPS #2.
Even if it could, I’m not sure I’d want to track the course just by looking at how close the “airplane” figure is to the pink line. It would depend on what range was set, and really doesn’t seem like a great way to navigate. Particularly if one was already stressed due to GPS #1 going out. Even without an external CDI, I’d probably rather follow the Garmin CDI display rather than try to follow the pink line on the screen. Of course, it depends what the goal is. If you’re VFR enroute, following the pink line seems more realistic than if you’re trying to shoot an approach in the clouds.
Also, after posting that previous message, I found an http://www.aopa.org/whatsnew/newsitems/1999/991213gps.htmlAOPA document, which, in the course of discussing using GPS in lieu of DME & ADF, mentions this:
Neither IFR enroute or approach receivers require a separate CDI by the TSO 129A specifications. However, both probably do require a separate conventional CDI display, or connection to an existing VOR-CDI, to meet Advisory Circular 20-138 installation requirements for display within the pilot´s field of vision/scan.

AOPA’s web site also has the full text of AC 20-138 in the member’s section, but it’s pretty dense reading. Of course, do you (noticed your .ch email address) and Clyde have to follow FAA regulations?

Steve

Thanks for the clarifications.

Cheers,

Placido

As to the #1 GPS dying: which GPS is driving the ARNAV screen? Is it only #1 or both.

It’s not a dumb question, but the answer is that only the #1 GPS drives the big screen. So if you lose the 430 you lose the Arnav. The HSI and therefore the autopilot also work off the #1 GPS/NAV only.

As to the #1 GPS dying: which GPS is driving the ARNAV screen? Is it only #1 or both.

It’s not a dumb question, but the answer is that only the #1 GPS drives the big screen. So if you lose the 430 you lose the Arnav. The HSI and therefore the autopilot also work off the #1 GPS/NAV only.

Question. Since we are talking about what amounts to a partial panel situation I was wondering if the #420 has the capability of displaying the “DG” on the same page as the map like my handheld 195? With the loss of an HSI I could see this as being better than trying to fly the whiskey compass. My little handheld is amazingly accurate and an IFR box I assume would be even more dependable. A CDI mounted in the scan plus this electronic “DG” would be almost comfortable, at least while I was trying to get to the nearest VFR area or down through the clouds.

M. Myers

Since we are talking about what amounts to a partial panel situation I was wondering if the #420 has the capability of displaying the “DG” on the same page as the map like my handheld 195

No, but there is a NAV page that shows the compass and various other info. It’s only two clicks of the small knob to change back and forth from the map to the compass page.

In any case, the chances of losing both the HSI (or DG) and a GPS is probably quite small - if you have the two GPSes you could just have one on the map page and one on the compass page.

When you look at it, there is a lot of redunancy - about the only thing in the B model that is not backed up in some way is the ILS.

Of course, do you (noticed your .ch email address) and Clyde have to follow FAA regulations?

TSO C-129 is also the basis for IFR GPS certification in Australia, however the same requirement regarding being in the scan would apply, and I do not believe an IFR GPS installation without a CDI would stand much chance of approval. When getting VH-CRF signed off for GPS-NPA approval here (it required a flight test amongst other things) the lack of an annunciator panel in the pilot’s scan was raised, and there was some suggestion that we might have to add it (the Garmin 430 does not require this under C129 because of the annunciator built into the display), but the conclusion of the guy who signed it off was that the combination of the HSI flag drop on waypoint passage, and the situational awareness given by the big moving map, made up for the lack of the annunciator panel, though you could still miss seeing a RAIM loss during the approach.