Retaining backup CDI

[camorton]

I'm wondering if there is any value in retaining an analog CDI for use as a backup or cross-reference when installing an electronic MFD like the Garmin GI 275 MFD. I have a conventional nav/com (KX155) and I'm installing and IFR GPS (GPS175). I know I can connect both to the 275 and toggle between the two nav sources for course guidance. I was going to replace the existing CDI (KI209) with the 275. But now I'm thinking it might be better to keep the old CDI in the panel.

When I'm flying a VOR or ILS approach (using the KX155), I could load the overlay approach on the GPS and the 275 would provide a cross-check for course guidance - secondary to the analog CDI. If I only have one CDI, it would have to be in either NAV or GPS mode. When it's in NAV, the GPS overlay would only be displayed on the GPS screen itself, right?

On the other hand, maybe the extra display just adds more to scan, and in the real-world focusing on one CDI is better? Interested to hear what others think.

C.

 

[guest user]

I would vote to keep it.

You already have it, so no real cost associated there. Plus, you're already comfortable with using it, so no need to train in using that in case of an emergency (yes, you should keep up training anyway)

Unless you're dealing w/ a lack of space, or a huge weight penalty, I say keep it.

 

[WDD]

Guest User has great points.

IMHO - I wouldn't bother with a toggle switch for the 275. I would just have the G175 drive the 275, and the KX155 drive your analogue CDI.

Can't imagine flying without two CDI's.

BTW - what are you using for your second com?

 

[asicer]

Can't imagine flying without two CDI's.

I guess you haven't flown a G1000.

 

[midlifeflyer]

When I'm flying a VOR or ILS approach (using the KX155), I could load the overlay approach on the GPS and the 275 would provide a cross-check for course guidance - secondary to the analog CDI. If I only have one CDI, it would have to be in either NAV or GPS mode. When it's in NAV, the GPS overlay would only be displayed on the GPS screen itself, right?

Sort of.

Depending on the installation, the GI275 has the ability to display one or two bearing pointers in addition to a CDI. If you can read a bearing pointer, the second CDI is irrelevant - that's what you see in a G1000 (and its derivatives). For an ILS, you would be able to have the CDI on green needles and a pink bearing pointer for GPS course guidance. If you were flying an LPV with a co-located LOC, you can have a green bearing pointer for the LOC for additional situational awareness. For a VOR approach, the required "raw VOR data" could be the bearing pointer while you happily fly the overlay.

Whether the second CDI has any value to you in those situations, only you can answer. Here's a pic from the GI275 manual.

 

[chemgeek]

The whole point of electronic gyros (GI275 or G5) is to concentrate your scan in one, information-rich and integrated space for NAV1. I don't see the extra value of duplicating that with a mechanical CDI. It would be better to pair the old CDI with a redundant nav source (NAV2). That's what I did: Dual G5s for NAV 1, separate conventional CDI for NAV 2. This way I have essentially 3 CDIs which should cover me for any reasonable nav emergency. (4 if you count the additional CDI page on the GNS430W.)

 

[Low Level Flyer]

When I upgraded my panel, to include a GTN 650xi, dual G5s, and a GNC 255A, I decided to retain my old analog King KI-209 CDI. I have the guidance from the GNC 255a sent to the KI-209. Like you, I already had the CDI so decided to retain it. I choose to keep it because I liked the idea of my second NAV going to a separate indicator in case the G5 failed.

 

[Mongoose Aviator]

I wanted to keep my old analog CDI. Avionics shop said that was a No Can Do.

@chemgeek - I forget about the 430 CDI. How well could that be used during an approach if the PFD/MFD that are expected to be displaying the CDI both went inop? Is the 430W CDI just for enroute or does it also have the lateral and vertical guidance for an approach?

 

[WDD]

The GTN 650 can only send to one G5/CDI. So if you were asking to have it sent to your G5 and your old CDI then that is why they said no.

But no reason a separate nav source (such as your second nav radio) can feed the old analogue CDI.

 

[chemgeek]

I wanted to keep my old analog CDI. Avionics shop said that was a No Can Do.

@chemgeek - I forget about the 430 CDI. How well could that be used during an approach if the PFD/MFD that are expected to be displaying the CDI both went inop? Is the 430W CDI just for enroute or does it also have the lateral and vertical guidance for an approach?

Horizontal guidance only. But that would be good enough in an emergency to fly an LNAV or LOC approach. But chances of two G5s failing at the same time is very remote.

 

[Mongoose Aviator]

Horizontal guidance only. But that would be good enough in an emergency to fly an LNAV or LOC approach.

Absolutely. I will try a practice approach using the 430W. Thanks for the pointer.

But chances of two G5s failing at the same time is very remote.

I have the Aspens but yeah, your comment is 100% on and I agree. Each Aspen is independent and they each have their own antenna/RSM along with each having an internal battery which is supposed to give about 30 minutes of backup electrical power. So trying to think of a valid scenario where the aircraft is still operational, the 430W is still operational, the pilot is operational, but both Aspens are inop at the same time navigating in IMC is a bit of a stretch. But putting that aside (lol), I really like the idea of trying a practice approach using the CDI on the 430W.

 

[camorton]

Depending on the installation, the GI275 has the ability to display one or two bearing pointers in addition to a CDI.

Pretty sure this feature is found on the HSI version but not the MFD/CDI version that I am installing.

When I upgraded my panel, to include a GTN 650xi, dual G5s, and a GNC 255A, I decided to retain my old analog King KI-209 CDI. I have the guidance from the GNC 255a sent to the KI-209. Like you, I already had the CDI so decided to retain it. I choose to keep it because I liked the idea of my second NAV going to a separate indicator in case the G5 failed.

I'll probably go this route for now. At least until I upgrade the KX155. When I replace that, maybe I'll for the HSI version.

Thanks,

C.

 

[Ed Haywood]

So trying to think of a valid scenario where the aircraft is still operational, the 430W is still operational, the pilot is operational, but both Aspens are inop at the same time navigating in IMC is a bit of a stretch.

In theory, an error in software code could manifest itself in two identical devices simultaneously. Think a Y2K-type glitch. That is admittedly far-fetched. But some people do ascribe to the view that a backup AI of a separate make is a prudent hedge against that possibility.

 

[RVJohnK]

I'd keep the KI-209. I'm nearly done updating my panel to almost all glass, but I'm keeping the same KX155 + KI-209 combo you have. I was already a little over budget and already had a G5, so couldn't justify the GI 275. The GI 275 is very cool though.

I also like that the KI-209 still looks the same whether the plane is running or not. If nothing else, you still see the needles even when everything else is turned off.

 

[Steamflyer]

WAAS Nav1 -> GI275

King/255A Nav2 -> BK209/GI106

Fly one plane with a duplicated primary nav (G5 and GI106 showing the same thing) and hate it. Wiring to switch between the two is not worth it IMHO.

 

[murphey]

As a long time software engineer….I hate single point of failure systems. I’ve got the G5 HSI connected to the GPS 175, then the SL30 talks to the analog CDI.

 

[Steamflyer]

As a long time software engineer….I hate single point of failure systems. I’ve got the G5 HSI connected to the GPS 175, then the SL30 talks to the analog CDI.

Exactly.

 

[Eric Brunelle]

I put in (2) gi 275's and a 750, but kept my 430W and CDI. The 750 does not talk to the CDI, but both the 750 and 430 talk to the gi-275's (A/I and HSI). So, I can do an ILS with the 430 and CDI. I can do RNAV approaches with either the 750 or the 430. The Bearing Pointers are great - good cross check on the CDI.

The installer wanted me to ditch the CDI, but I like having two sources for navigation. I like my setup - to each his own.

 

[Eric Pauley]

As a long time software engineer….I hate single point of failure systems. I’ve got the G5 HSI connected to the GPS 175, then the SL30 talks to the analog CDI.

Totally agree. The default panel designs are largely designed around physical redundancy, but not logical redundancy. You may have two independent G5s, but what happens when some intern writes a corner case in the code that fails both at once? When designing my new panel I was very explicit about having backups that have as much software diversity as possible. This means a GNC255 with analog CDI separate from the GTN650, and an analog turn coordinator as a worst-case backup gyro. If that fails I may replace it with a backup 3rd-party digital AI like an AV-30.

 

[Mike Gagnon]

I put in (2) gi 275's and a 750, but kept my 430W and CDI. The 750 does not talk to the CDI, but both the 750 and 430 talk to the gi-275's (A/I and HSI). So, I can do an ILS with the 430 and CDI. I can do RNAV approaches with either the 750 or the 430. The Bearing Pointers are great - good cross check on the CDI.

The installer wanted me to ditch the CDI, but I like having two sources for navigation. I like my setup - to each his own.

Does the 430 “talk” to the 275 and CDI simultaneously, or do you have to switch which it is connected to?


Sent from my iPad using Tapatalk

 

[Eric Brunelle]

Does the 430 “talk” to the 275 and CDI simultaneously, or do you have to switch which it is connected to?


Sent from my iPad using Tapatalk

The 430W talks to both the 275 and the 106 CDI at the same time for VOR and LOC functions. The 430W does not talk to the 106 CDI AT ALL for GPS functions. So I have complete VOR and LOC/ILS functionality with the 430W and 106 CDI if the 750 or 275 fails.
There are no switches for talking between the CDI and 275. The only physical switch is the GPSS. The 275's allow you to choose where they are getting the primary signal from - GPS1, GPS2, LOC1 or LOC2. Those are touch screen buttons on the 275.

I like to use the 430w and CDI to cross check VORs while using the 275 as primary navigation. The bearing pointers are also a cross check for the CDI.