Merge nav1 and nav2 CDI into one GI-275?

[shawnd]

(moved from general to avionics/panel - still looking for some opinions/guidance)

I have been given the option to remove the second VOR CDI (King KI-208) as part of an upcoming GI-275 AI+HSI installation. This would be possible because you can have the GI-275 HSI show both NAV1 and NAV2 navigation sources simultaneously. Goals are to train for and fly the aircraft in IFR in the near future. For the experienced IFR folks in the crowd, how important is it to have the second separate CDI? Most approaches I imagine will be RNAV GPS approaches but for those rare times that require possibly following one radial and identifying intersections along the way using another radial, I could guess it may help? Ex: https://flightaware.com/resources/airport/KEMT/IAP/VOR-A. I believe the merged CDI solution would also allow it but might be a bit tougher to spot when it centers? Don't know since I have never played with this scenario on an unit before. I have a GTN-750 that could help identify those intersections as an alternative. I believe we can also utilize ATC's help to identify the intersections along the route.

Reason why this is attractive is the 6.5"x6.5" space that I will get by removing the second CDI will let me bring the engine monitor next to the 6 pack requiring less head movements during flight (which I am told is a good thing during IFR conditions).

As always, any help and suggestions appreciated!

[PeteMc]

Are you talking about the Bearing Pointer?  Yes, you can get an Arrow that points towards the other Nav's Wpt, but that's not the same as two simultaneous CDIs (if that's what you're thinking).  Also, what difference does it make if you can display all of these Nav devices on your 275 if it fails?  What is your backup instrument?

I have a GI-106B still in my panel that is connected to my GPS #2.  So on any approach I have that CDI mirroring what is happening with my primary CDI that is (usually) being fed by my GPS/Nav #1.  My theory is that if something goes flaky with my #1 or my Aspen, I still have course guidance already right in front of my face.  Granted I'd have to decide with what the real time situation was, but if hard IMC I'd probably go Missed.  But that probably will mean that I still have to track the inbound Path/LOC until reaching the MAP before starting any turns etc.  (All stuff you'll learn about once you start your Inst. training.)

So if there are no issues feeding the KI208 with whatever GPS/Nav radios you have, and you have the panel space, I'd suggest keeping it maybe in one of the lower side holes in the panel).  You can go to the expense of making it switchable if you want, but I didn't see the need since it's just a backup.

 

Edited March 9, 2023 by PeteMc

[PT20J]

You don't need it for normal operations. But the question is: How will you navigate if the HSI fails?

[shawnd]

Getting two GI-275s with support for revisionary mode. (should have clarified). So in this case, AI will be able to show the CDI at least from one source. Shop is checking if it can do 2 but that might be too much data in the 3.25" screen.

[shawnd]

@PeteMc thanks for the detailed scenario - do agree that its nice to have as a backup. But I am addicted to monitoring my CHTs and EGTs meticulously given my engine runs cooler than normal (after a near OH as well). I prefer the JPI 900/930 for ease of readability which is why the extra panel space would be nice to have. The alternative is to keep the KI-208 and use a smaller 275 EIS.

[Flash]

On 3/9/2023 at 10:58 AM, shawnd said:

(moved from general to avionics/panel - still looking for some opinions/guidance)

I have been given the option to remove the second VOR CDI (King KI-208) as part of an upcoming GI-275 AI+HSI installation.

I considered this same question and decided to keep my second CDI as a backup to the GI-275s. It's true that the GI-275s can show bearing data from two Nav radio sources, and I was hoping I could have that AND the second CDI, with my legacy King No. 2 Nav radio feeding both the CDI it currently feeds and the GI-275s, too. But (1) it's not clear that you could get an installer to do that, and (2) somebody who tried it said on another thread that it didn't work. So you're making a choice between taking advantage of the power of the GI-275, which reduces how many instruments you have to scan, and the availability of a backup CDI if the GI-275s fail. I chose the latter.

The previous thread was Can KX-155 drive both a KI-206 and a GI-275?

Edited March 12, 2023 by Flash
added name of previous thread

[jlunseth]

There have been a few episodes of system failures. It seems really nifty to have everything connected in a system with reversionary capabilities. The problem is that creates a single system which is a single point of failure if something in the system goes wrong, such as a software or electrical failure. I elected to keep my second GPS, my 430AW, separate from the system created by my GTN750Xi and dual 275s. It is connected to its own CDI. I also have an inexpensive backup AI which is separate from the main Garmin-based nav system that includes the 275’s. 

[donkaye]

There are advantages to keeping the mechanical backup (or keeping it on a separate circuit along with the number 2 Nav source) and there are disadvantages.  After weighing both sides, I chose to eliminate the mechanical CDI.  That does mean that the G500TXi (or in your case GI 275) is the sole source of certified navigation.  In my case, if the TXi fails, then the G5 will take over and the autopilot will function off of the GTN 750.  If the 750 fails and I still have the TXi, the AP will operate from the 650.  If the whole electrical system fails, then attitude will be controlled by the G5 and navigation will be from either the Aera 760 or the iPad.  Keeping the system as one unit means having the AP if I have the TXi and the 750 fails.  Certainly there are more combinations of failure modes, but for me keeping all units working as one system provide more benefits for major failure modes than separating them.  If you separate them, then for sure you don't have the AP if you have a major system failure in your primary navigation system.

[carusoam]

simple explanation for the new to IFR ideas…
 

Simple IFR navigation….

1) Use WAAS based gps to navigate between Vors and their intersections… 

2) All defined by GPS waypoints…

3) Displayed on the HSI for ease of navigation… and vertical guidance.

 

Old style IFR navigation…

1) Use VOR based navigation to define waypoints…

2) BK made a box for this just before GPS became the norm…  KNS80 that looks like a KLN98 that was GPS based…

3) All data sent to the HSI for ease of navigation…

4) Same effect… different radio sources.  Ground vs. satellite.

 

Electronic displays…

1) have added multiple pointers…

2) different ways to define intersections…

3) Doesn’t improve the amount of data available…

4) Doesn’t improve normal intersection based navigation…

 

Going back to VOR based navigation…

1) Great back up when GPS is unavailable.

2) more data and choices are better for some people…

 

Always have two methods of finding the ground in low IMC…

1) Traditional ILS…

2) WAAS based gps….

3) Have a way to get to better conditions…

 

Essentially,

1) select a nav box that can define waypoints… intersections, VORs, ILS 

2) Send that data to the HSI

3) Select a back up plan, independent VOR/ILS display and data source…

Pitfalls of VOR navigation… (old analog displays)

1) if you lose situational awareness…

2) it is possible to turn the wrong way on a V-route and keep navigating the wrong way… and find mountains…

3) DME was a nice addition to the old analog system…
 

For how the pros do it…. See DK’s post above!

 

PP thoughts only, not a CFII… (one part normal system ops, another part is failure of each box and how to get down safely.

Best regards,

-a-

[shawnd]

8 hours ago, Flash said:

The previous thread was Can KX-155 drive both a KI-206 and a GI-275?

Thanks Flash, adding a link to your thread so others can follow if interested:

 

[shawnd]

Thanks everyone. After reading through the feedback, I am going to keep the KI-208 installed and connected to KX-165 for now.The GI-275 system will be connected to the GTN750. I am contemplating having the installer add a switch to instruct the KFC-150 AP which NAV Source to use, cons being getting confused in the rush of things and have AP be driven by nav2 while I am looking at Nav1). Might be better to practice and hand fly if nav1 fails.

Here's what I am thinking for failure scenarios:

1. Two installed GI-275s with revisionary mode and battery backup. I will always have attitude information if any unit fails.
2. If the HSI fails, the ADI will show heading and lateral/vertical guidance, see picture below (agree w/ @kortopates thoughts from the other thread )
3. If I loose power from the aircraft, battery backup on the ADI will kick in to keep one unit operational
4. If the GMU-11 magnetometer fails, can use magnetic compass but likely ask for help with course correction from ATC. Will ask if its possible to add a backup one. Moot point if it requires system power.
5. If GTN fails, KX-165 becomes the navigation source
6. If KX-165 fails, don't care but expedite landing

 

Reason to keep the KI-208 for now is that there is not much point to removing it. If I am going with the bigger displays, probably have the integrated EIS as well. Once I get the IR and have flown for a bit, I will be looking at adding a bigger display like the G3X and the existing 275s will be come backup at the time adding sufficient redundancy. Similar to @donkaye's setup. Also looking at 650 upgrade for the KX-165.