Switch Century IIB between heading sources?

[AA5Bman]

Question first: Let's say I get my old Century IIB hooked up a G5 DG/HSI such that it is providing heading info to the autopilot in quasi-GPSS mode. Let's say I'm flying along on GPSS and then get vectored to shoot an ILS approach and ILS is on a *separate* indicator (i.e. a completely separate Nav/Comm than what is hooked up to the G5), is there a way to "switch" where the heading input is coming from, so that I can switch the heading input from the G5 to the NAV (ILS) and get the autopilot track down the ILS too?

Background:

Alright everyone, thanks for your patience, I'm finally figuring out how to repair and improve my panel... but I gots one more question!

My situation is that I have a dead attitude indicator and a Century IIB autopilot. I was going to replace the AI with a G5 (dual G5s actually was the original idea). However, it took some digging to confirm, but it turns out that "attitude based" autopilots like mine need input from the attitude indicator, and the G5 AI won't provide it.

I briefly went down the path of considering dual G5s and a new GFC500, but when the quotes came in between $30k and $50k, I got real and realized that's not going to happen.

Where I'm at right now is I think I'll fix the existing AI and keep my Century IIB. I'm tempted to put the G5 in for the HSI/DG anyway for a couple of reasons, one of which is that it will drive the *heading* portion of the autopilot (not attitude - that will still come from my old AI), and my old autopilot will get, essentially, GPSS functionality. Pretty cool, right?

So here's the question: let's say I get my Century IIB hooked up to the G5 DG/HSI and am flying along using GPSS, but then I get vectored to fly a ILS and my ILS is on a *separate* indicator (i.e. a completely separate Nav/Comm than what is hooked up to the G5), is there a way to "switch" where the heading input is coming from, so that I can switch the heading input from the G5 to the NAV (ILS) and get the autopilot track down the ILS too?

 

[Bob Weber]

With a CIIB you have no vertical guidance tracking to consider, GPSS will navigate that autopilot as accurate as "raw data" in LOC NORM.

Another thing to check is what model 1C388 coupler is in your system, DG or HSI?

 

[simtech]

A GAD 29b will allow the G5 to talk with the century unit.

 

[Bob Weber]

A GAD 29b will allow the G5 to talk with the century unit.

My apologies, do you presently have a DG, or an HSI. Your radio coupler may need to be changed if you want the CRS output of the G5 to work.

 

[AA5Bman]

My apologies, do you presently have a DG, or an HSI. Your radio coupler may need to be changed if you want the CRS output of the G5 to work.

I currently have a standard old school DG, but not the G5 (yet). It has a heading bug which the autopilot will follow, but I also have this radio coupler:

https://www.google.com/search?q=cen...ECA0QAQ&biw=1024&bih=659#imgrc=LYU823ynNolacM

that allows me to switch the autopilot from tracking the heading bug on the DG to the localizer on my NAV1. Maybe it would keep working more or less the same way after the G5 install?

FYI I don’t currently have the G5 or, for that matter, an IFR GPS. I have been wanting to put an IFR GPS in the plane, and since I have to get an indicator that works with it too, I figured I’d go with the G5 HSI, which only costs a little bit more than a new indicator but gets me a lot more functionality. I have considered going with something like a GTN 635 or GNC 355, because I’m not sure I see a reason to pay for the nav functions of a 650, for instance, given that Localizer and Glideslope will be available on my existing Nav/Comm that will become my backup. I just am hoping that I won’t have to hand-fly ILS approaches and there’s a way to switch the autopilot input from the G5 to the localizer on Nav 2.

 

[Bob Weber]

Having the DG rather than an HSI means you have to step to a different radio coupler in order for the intercept and track function to work with the course select function out of the G5/29b. It will function by using the HDG bug with the G5 selected to HDG vs GPSS, provided it can legally be interface that way.

What aircraft and what future plans for it. I personally would fly the G5/CII in GPSS whenever I was not being vectored.

The raw data tracking in LOC NORM mode, though far more settled than tracking a LOC, is still less stable.

I'm mixed about the inability of the G5 to switch between navs, frankly I think it's a step backwards, in the sense it will now require another set of contacts and connections to facilitate the function.

 

[AA5Bman]

Which coupler, @Bob Weber? I thought the G5 would use the existing radio coupler in heading mode?

Aircraft and future plans - it’s a Cessna. I fly IFR rarely and fairly simple stuff, like punching through a marine layer and not much more (where I live, it’s either typically great VFR or mountains and icing). My big goals are to, fix my currently broken AI (thought I was going to go G5, but realized I’d lose my AP which I didn’t like), get a new Comm (my current Comm1 kinda stinks), and get a reasonable-cost GPS navigator. All that said, knowing myself, I can be almost be 100% sure that someday in the next 5 years I’ll probably convince myself to sell this plane and buy something new, so investing huge dollars in this plane is shortsighted.

So, my solution to the above was a GNC355 (addresses a new Comm and IFR GPS), a G5 DG/HSI (addresses the need for a new indicator, and I thought I’d get GPSS out of the deal, too). I thought this route would preserve my existing AP, get a new Comm and GPS while keeping the investment to a reasonable minimum (it’ll still be $15k or so!).

My only concern with this approach - I thought - was that I’d lose the tracking function for the localizer if I have to shoot an ILS, but it sounds like maybe there’s more?

 

[Bob Weber]

Which coupler, @Bob Weber? I thought the G5 would use the existing radio coupler in heading mode?

Aircraft and future plans - it’s a Cessna 205. I fly IFR rarely and fairly simple stuff, like punching through a marine layer and not much more (where I live, it’s either typically great VFR or mountains and icing). My big goals are to, fix my currently broken AI (thought I was going to go G5, but realized I’d lose my AP which I didn’t like), get a new Comm (my current Comm1 kinda stinks), and get a reasonable-cost GPS navigator. All that said, knowing myself, I can be almost be 100% sure that someday in the next 5 years I’ll probably convince myself to sell this plane and buy something new, so investing huge dollars in this plane is shortsighted.

So, my solution to the above was a GNC355 (addresses a new Comm and IFR GPS), a G5 DG/HSI (addresses the need for a new indicator, and I thought I’d get GPSS out of the deal, too). I thought this route would preserve my existing AP, get a new Comm and GPS while keeping the investment to a reasonable minimum (it’ll still be $15k or so!).

My only concern with this approach - I thought - was that I’d lose the tracking function for the localizer if I have to shoot an ILS, but it sounds like maybe there’s more?

Your solution is exactly what I would do given the plan for the aircraft. I can check what interface is published for the G5 to the CII, if you can use your 1C388, giddy up. GPS approaches accomplished in LOC modes of autopilots tend to be a little clunky due to the balance needed between the CDI and the CRS. Another oddity of the GPS approach is the step down on beam width rather than the "cone" of a LOC signal.

 

[SbestCFII]

I have the G5 PFD bust have avoided getting the HSI unit and the GAD29. The CII is basically bullet proof. When I installed the G5 in my Warrior, I relocated the vacuum attitude gyro to the TC slot and move the TC to the far left slot. I don't mind setting the DTK with the old DG bug and engaging the coupler in Omni (for cruise) or Loc (for approach) and it works equally well whether you have selected GPS or VLOC as the NAV source on the GNS/GTN unit. I'd like altitude hold, but not for what it would cost (the STEC altitude hold only is ludicrously expensive). The newer STC'd autopilots units really only work with GPS and I like that the Century works well with either GPS or NAV, with minimal hassle. I really don't mind not having altitude hold. With all the bell and whistle stuff I could put in the plane, controlling at least one axis of flight is always a good reminder that I am still a pilot and not Otto/George.

 

[AA5Bman]

Thanks for the help @Bob Weber. I was following you right up until:

GPS approaches accomplished in LOC modes of autopilots tend to be a little clunky due to the balance needed between the CDI and the CRS. Another oddity of the GPS approach is the step down on beam width rather than the "cone" of a LOC signal.

Can you clarify what this means? My understanding is that when the G5 works with an autopilot like the Century IIB, you leave the IIB in "heading mode" (on the radio coupler, i.e. not in LOC mode) and it will calculate the intercept angles, turns, and what-not and transmit them to the autopilot as if they were a "heading" instruction. Maybe your comment was more general in nature about using LOC mode to track a GPS heading in general, separate and apart from the Century IIB / G5 interface?

@SbestCFII: really appreciate your real-world experience. Can you provide a little more detail on your setup? I have a couple questions:

1) Can you clarify your setup? I'm assuming it's something like a GTN650 + G5 HSI/DG that is "driving" the Century IIB, more or less like what I'm considering.
2) When you are tracking a NAV/LOC, is that coming from the GTN, being displayed on the G5, and then the G5 is "driving" the heading/tracking of the autopilot, or is that LOC indicated on a different, separate CDI like a GI106A? If it is going through the G5, my understanding was that you would use "heading" mode, not LOC?
3) You said "I don't mind setting the DTK with the old DG bug and engaging the coupler in Omni." I'm not following this - you said "old heading bug," maybe I misunderstood and you're not using a G5 for DG/HSI and for driving an autopilot?

Appreciate it, all. I feel like I'm spamming the board with all these questions, but I've found that this is the best source of information on the subject - Garmin kicks me back to their dealers for anything more than a basic question, and the dealers are unsure and so swamped with ADS-B installs it's hard to get their attention to really dive into this kind of thing. Thanks again.

 

[Bob Weber]

Can you clarify what this means? My understanding is that when the G5 works with an autopilot like the Century IIB, you leave the IIB in "heading mode" (on the radio coupler, i.e. not in LOC mode) and it will calculate the intercept angles, turns, and what-not and transmit them to the autopilot as if they were a "heading" instruction. Maybe your comment was more general in nature about using LOC mode to track a GPS heading in general, separate and apart from the Century IIB / G5 interface?

You are absolutely correct, GPSS mode out of the G5 presents itself just the same as the 52D54 DG HDG signal that's feeding it now.

It should perform magnificently. I would toggle between GPSS and HDG on the G5 and leave the CII in heading permanently.

If you operate the CII in LOC NORM mode, with the G5 fed by a 530W, with a 1C388-2, or-3, It will recognize the CRS setting of the G5. and "balance" the offset of the CDI input received from the 530W with the CRS offset out of the G5.

There is a spec the coupler is set to on the bench to balance it at 30°or 45°of CRS offset with full scale deflection of the CDI, full left CDI 30° right CRS for the intercept.

As the CDI deflection diminishes, the system rolls toward the CRS for a smooth intercept.

There is a crosswind integrator that allows the CRS to drift off in order to keep the CDI centered going on as well.

What we noticed with the GPS approaches flown by raw data on an approach mode on the autopilot, was any CDI offset allowed by an alignment issue would cause a wing to dip when the GPS toggled thru the approach sequencing and stepped the beam width down, causing the CDI to deflect further.

If there is an approved interface, and you had a 530W, you may be able to use the HDG output as the CRS if you chose to use LOC NORM mode, this would eliminate the need for another radio coupler.

 

[AA5Bman]

Okay, I get it @Bob Weber - I'm following you re: "balancing" inputs. I actually read about this in the autopilot manual (imagine that) and the idea is that the autopilot is trying to balance out the difference between the track/course selected on the heading bug, and the radial and CDI deflection. When they're all lined up and the needles are centered, all good. But when the CDI is deflected, it will try to balance out the difference by flying an intercept angle.

Now... all that said, the above only matters when you're in NAV or LOC mode and getting heading information from NAV (VOR/LOC) equipment. I don't think this applies to a GPS course signal on a G5, only a NAV signal coming from a VOR/LOC. I am assuming that this would have to be on a separate indicator and not on the G5 (like a GI106A or KI209). If you were displaying the NAV (again, VOR/LOC specifically) through the G5 DG/HSI, would you put the G5 in GPSS and the autopilot in heading, and it would track the VOR/LOC course as if it were or the same as a GPS track, or something else?

What if the VOR/LOC was not being fed through the G5 but was running through a separate indicator head, could you switch between the GPSS from the G5 and tracking the VOR/LOC on the separate indicator? If so, how? (this is OP question).

Finally, I found this on another post and verified in the GAD29B manual, supported Century autopilots for the G5 are:

Century 1C388*
Century 1C388M*
Century 1C388C*
Century 1C388MC*
Century 1C388-2*
Century 1C388-3*
Century 2000
Century 21
Century 31
Century 41
Century IV

* Radio Coupler P/N indicated and based on aircraft installation
could be interfaced in a Century II or III.

Note for the Century II it lists a bunch of radio couplers. I'm guessing that answers the question you had @Bob Weber regarding Garmin support. Is that pretty much all of them (the radio couplers, that is)? I'm guessing (hoping) I don't need a new radio coupler.

 

[Bob Weber]

Okay, I get it @Bob Weber - I'm following you re: "balancing" inputs. I actually read about this in the autopilot manual (imagine that) and the idea is that the autopilot is trying to balance out the difference between the track/course selected on the heading bug, and the radial and CDI deflection. When they're all lined up and the needles are centered, all good. But when the CDI is deflected, it will try to balance out the difference by flying an intercept angle.

Now... all that said, the above only matters when you're in NAV or LOC mode and getting heading information from NAV (VOR/LOC) equipment. I don't think this applies to a GPS course signal on a G5, only a NAV signal coming from a VOR/LOC. I am assuming that this would have to be on a separate indicator and not on the G5 (like a GI106A or KI209). If you were displaying the NAV (again, VOR/LOC specifically) through the G5 DG/HSI, would you put the G5 in GPSS and the autopilot in heading, and it would track the VOR/LOC course as if it were or the same as a GPS track, or something else?

What if the VOR/LOC was not being fed through the G5 but was running through a separate indicator head, could you switch between the GPSS from the G5 and tracking the VOR/LOC on the separate indicator? If so, how? (this is OP question).

Finally, I found this on another post and verified in the GAD29B manual, supported Century autopilots for the G5 are:

Century 1C388*
Century 1C388M*
Century 1C388C*
Century 1C388MC*
Century 1C388-2*
Century 1C388-3*
Century 2000
Century 21
Century 31
Century 41
Century IV

* Radio Coupler P/N indicated and based on aircraft installation
could be interfaced in a Century II or III.



Note for the Century II it lists a bunch of radio couplers. I'm guessing that answers the question you had @Bob Weber regarding Garmin support. Is that pretty much all of them (the radio couplers, that is)? I'm guessing (hoping) I don't need a new radio coupler.

It looks like your in!

 

[NealRomeoGolf]

@AA5Bman have you done any more research with this? My plane is coming out of the shop with Dual G5s and a GNC355 and they are having AP trouble with my Century 21. I have an old KX155 with glideslope as radio two and my ILS nav. They are having a hard time getting the AP to recognize the different inputs. I thought the G5 HSI can swap between inputs and then the GAD 29B uses the nav data accordingly. Maybe I am wrong.

 

[Jmcmanna]

I have G5’s and Century IIb autopilot. The HSI will only see data from Nav 1. Anything from the GPS source is seen as “heading” by the autopilot’s lateral guidance selector. If I want to follow a VOR or ILS, I have to twist the lateral guidance selector to the proper selection to follow it.

Can you put in ILS/VOR inputs from Nav2 to the G5 HSI? I don’t know. But the autopilot will not follow the ground based signals in HDG mode like it does the heading bug or magenta line in GPSS mode.

 

[NealRomeoGolf]

I have G5’s and Century IIb autopilot. The HSI will only see data from Nav 1. Anything from the GPS source is seen as “heading” by the autopilot’s lateral guidance selector. If I want to follow a VOR or ILS, I have to twist the lateral guidance selector to the proper selection to follow it.

Can you put in ILS/VOR inputs from Nav2 to the G5 HSI? I don’t know. But the autopilot will not follow the ground based signals in HDG mode like it does the heading bug or magenta line in GPSS mode.

Then what is the point of wiring a second nav source to the GAD 29B? In the G5 install manual they have an example of a GTN650 and a GNC255 both going into the GAD29B. If the G5 can’t see Nav2, what’s the point?

 

[NealRomeoGolf]

And btw, I do see this in the G5 user guide:

“The G5 will only display data from the #1 navigation source.”

My question above still stands.

 

[NealRomeoGolf]

Avionics shop just answered my question. My KX155 is analog and a GNC255 is digital. GAD 29B only takes digital signals, so my KX155 is not hooked up to the GAD 29B.

 

[AA5Bman]

@NealRomeoGolf It sounds like you have your answer but I’m still working on mine. I’m scheduled into the avionics shop in the middle of January, so I’m hoping they can sort it out. My question was a little different than yours: can the Century IIb track, in NAV or LOC mode, the analog signals coming from your NAV 2 that is display on a separate indicator (i.e. not the G5).

@Jmcmanna You implied that you have dual G5’s, but I assume you still have your original AI in the panel somewhere? Just wanted to confirm in case you figured out a way to ditch it and not lose the attitude input the IIb needs.

Also, @Jmcmanna are you saying that if you are Tracking a GPS course displayed on the G5, you do it in heading mode on the IIb (this is my understanding as well). If, however, you need to track the VOR navigator coming from that same GPS/NAV/COM unit and displayed in the G5, the IIb needs to be in NAV or LOC mode? I didn’t realize this - I thought the G5 “converted” but inputs to a GPSS-style data stream that the IIb tracked in heading mode. Are you able to track a VOR that is displayed in a separate indicator head (not the G5)?

FYI my planned setup is GTN650, single G5 (HSI), GTX345, and possibly a GNC255 (may just keep my MAC1700 NAV/Comm as backup but I really don’t like that radio!).

 

[NealRomeoGolf]

@NealRomeoGolf It sounds like you have your answer but I’m still working on mine. I’m scheduled into the avionics shop in the middle of January, so I’m hoping they can sort it out. My question was a little different than yours: can the Century IIb track, in NAV or LOC mode, the analog signals coming from your NAV 2 that is display on a separate indicator (i.e. not the G5).

My shop is trying to hook up a switch to be able to change inputs between the GAD 29B and the KX155. I will let you know if it works.

 

[AA5Bman]

My shop is trying to hook up a switch to be able to change inputs between the GAD 29B and the KX155. I will let you know if it works.

you mean inputs to the Century, I assume, not inputs to the G5, right?

 

[bnt83]

The only thing that changes with a G5/GAD29B install is what your autopilot heading input is connected to and removal of any exiting CDI belonging to a radio that is now displaying on the G5.

Ex, in autopilot NAV mode the analog lateral error from GTN/GNC/SL30 etc is wired directly to the autopilot control head AND is displayed on the HSI.

autopilot in heading mode, you can track either GPSS or magnetic heading (also GPS track if available)

dual navigator inputs to G5 HSI system only apply to those with GFC500 autopilots and not approved for any other purpose IIRC.

Quite possibly some need the autopilot controller or a different one.

 

[NealRomeoGolf]

you mean inputs to the Century, I assume, not inputs to the G5, right?

Correct.

 

[Jmcmanna]

@Jmcmanna You implied that you have dual G5’s, but I assume you still have your original AI in the panel somewhere? Just wanted to confirm in case you figured out a way to ditch it and not lose the attitude input the IIb needs.

Also, @Jmcmanna are you saying that if you are Tracking a GPS course displayed on the G5, you do it in heading mode on the IIb (this is my understanding as well). If, however, you need to track the VOR navigator coming from that same GPS/NAV/COM unit and displayed in the G5, the IIb needs to be in NAV or LOC mode? I didn’t realize this - I thought the G5 “converted” but inputs to a GPSS-style data stream that the IIb tracked in heading mode. Are you able to track a VOR that is displayed in a separate indicator head (not the G5)?

FYI my planned setup is GTN650, single G5 (HSI), GTX345, and possibly a GNC255 (may just keep my MAC1700 NAV/Comm as backup but I really don’t like that radio!).

Yes, I still have the vacuum AI. And yes to the second question. My autopilot will follow the G5 heading bug or anything with a magenta line with the autopilot in HDG mode. If I want to track anything with a green line (VOR, LOC, etc...) it has to be appropriately selected by the Century AP knob. I do, obviously, have to switch my GPS from GPS to VLOC mode in this case, so I don’t know if it is because of the signal there or with the G5 itself. My AP cannot track anything from Nav 2.

 

[Clip4]

I realize APs cost a lot of $, but maintaining an old Century with modern instruments is nuts.

 

[Jmcmanna]

Maybe, but mine is rock solid....no reason to rip it out now.

 

[MajorTurbulence]

I agree. Why rip out a perfectly good and trouble free legacy autopilot. I have a Piper Autocontrol 3B (Century 2). It has not required maintenance since new 39 years ago except for the connectors years ago. Every other dysfunction has occurred because of a bad AI or DG input. Connecting the autopilot to my Aspen enabling GPSS, or choice of Nav source has been transformative.

 

[NealRomeoGolf]

@AA5Bman here is what my panel ended up like. Partner flew it home today but didn't get to test much. I won't be stateside for a few more months so I won't get to personally fly it for a while. They added my autopilot switch in the old NAV2 header hole. I can switch the NAV source from either GPS or the Nav2 radio. Of course HDG mode can only come from the G5 (which would be the GPS setting). I believe I can use APR on either setting since Nav2 is an ILS capable radio. Again, need to test it to make sure that is how it ended up. And yes, the second G5 is supposed to be an HSI. That got changed after the picture was taken.

 

[asicer]

@AA5Bman here is what my panel ended up like. Partner flew it home today but didn't get to test much. I won't be stateside for a few more months so I won't get to personally fly it for a while. They added my autopilot switch in the old NAV2 header hole. I can switch the NAV source from either GPS or the Nav2 radio. Of course HDG mode can only come from the G5 (which would be the GPS setting). I believe I can use APR on either setting since Nav2 is an ILS capable radio. Again, need to test it to make sure that is how it ended up. And yes, the second G5 is supposed to be an HSI. That got changed after the picture was taken.

View attachment 81602

Say, that looks a lot like what @WannFly panel would look like if he opened up his checkbook.

 

[WannFly]

Say, that looks a lot like what @WannFly panel would look like if he opened up his checkbook.

You would have to pry the GNS 480 out of my cold dead hands

 

[AA5Bman]

@AA5Bman here is what my panel ended up like. Partner flew it home today but didn't get to test much. I won't be stateside for a few more months so I won't get to personally fly it for a while. They added my autopilot switch in the old NAV2 header hole. I can switch the NAV source from either GPS or the Nav2 radio. Of course HDG mode can only come from the G5 (which would be the GPS setting). I believe I can use APR on either setting since Nav2 is an ILS capable radio. Again, need to test it to make sure that is how it ended up. And yes, the second G5 is supposed to be an HSI. That got changed after the picture was taken.

View attachment 81602

Nice! Just want to make sure I understand this, because I’m about to buy a new car’s worth of gear and stick it in a couple tiny boxes in my panel, so I’m doing my best to get it right!

I asked this question before and you responded positively, confirming that you can toggle inputs to the IIB via that switch so that you can track GPSS inputs in heading mode from the G5, or VOR inputs via Nav mode from your separate Nav 2. Imagine you’re tracking a GPSS course in the G5 and need to switch to tracking down an ILS. In your installation mode, you would switch the autopilot to Nav (or LOC) mode and hit that switch to move the autopilot control over to the Nav, right? But why do you have to have that switch at all given that you can’t track anything from the G5 in nav mode - it seems that switching from Heading to Nav on the autopilot selector would be enough to do it?

Also, since the GNX series navigators don’t have a VOR, in what scenario would you be switching to VLOC (just out of curiosity)?

 

[NealRomeoGolf]

Nice! Just want to make sure I understand this, because I’m about to buy a new car’s worth of gear and stick it in a couple tiny boxes in my panel, so I’m doing my best to get it right!

I asked this question before and you responded positively, confirming that you can toggle inputs to the IIB via that switch so that you can track GPSS inputs in heading mode from the G5, or VOR inputs via Nav mode from your separate Nav 2. Imagine you’re tracking a GPSS course in the G5 and need to switch to tracking down an ILS. In your installation mode, you would switch the autopilot to Nav (or LOC) mode and hit that switch to move the autopilot control over to the Nav, right? But why do you have to have that switch at all given that you can’t track anything from the G5 in nav mode - it seems that switching from Heading to Nav on the autopilot selector would be enough to do it?

Also, since the GNX series navigators don’t have a VOR, in what scenario would you be switching to VLOC (just out of curiosity)?

I don't know why it does not work without the switch. They tried without, it didn't work. They tried with, it works.

 

[AA5Bman]

Hi everyone - thought I’d follow up with a conclusion to this post for anyone that finds themselves in a similar situation.

I ended up putting in a GTN650xi, a GI275 (like an updated G5 for anyone not familiar), and a GNC225 (#2 Nav/Comm). My goal was to be able to track VOR/LOC signals with my Century II autopilot from *both* the #1 (displayed on GI275) and #2 (displayed on KI206) NAVs, and get heading bug and GPSS steering from the GI275.

We figured it all out (with a little help from @NealRomeoGolf and his mechanic), and it works amazingly well!

Here’s what you need to know:

The Century II takes two inputs for steering: a heading bug input and a Left/Right input from the Nav source. In order to intercept and fly a VOR course, it needs both inputs (yes, it still needs the heading bug input to fly a nav course, this is because it compares the nav signal with the heading bug signal to “calculate” an intercept angle). The heading bug input will always come from the DG (in this case the GI275), and the Left/Right input will come from the Nav. If you want to be able to switch between two different Nav sources, all you have to do Is put in a switch that selects either the Nav 1 or Nav 2 Left/Right source. The “heading bug” input bypasses the switch and goes directly to the autopilot. When tracking a VOR signal, the autopilot is always in NAV mode (or LOC, or basically any setting that is NOT “heading”). When tracking a heading bug or GPSS, the autopilot is always in “heading” mode.

So here are the available modes:

For tracking the heading bug alone: GI275 in heading bug mode, AP in heading, “Left/right switch” on either (because heading bypasses the switch)
For tracking GPSS: GI275 in GPSS mode, AP in heading mode, switch in either
For tracking #1 Nav: GI275 in heading mode, AP in Nav, switch on #1 Nav
For tracking #2 Nav: GI275 in heading mode, AP in Nav, switch on #2 Nav

That seems like a lot, but when you get your head around it, it’s not really that complicated. The fact of the matter is that the AP tracks GPSS steering so well that it almost doesn’t matter - what you’re mostly going to use is GPSS and simple heading bug inputs, in which case the autopilot is always in heading and you just select GPSS or heading on the GI275.

One final word: there isn’t a ton of actual user reviews out there in the GI275, so I’ll just let you know that my experience with it has been awesome. The HSI map page is amazing - it will plot your course just like a moving map of a GPS, so you can see your turns and holds, name and distance to next waypoint. It will display terrain and traffic, and it’s really nice having it all in front of you and not having to turn your head to look at the GPS. Really a great unit. There’s been a lot of speculation about whether or not the unit has enough screen space to fit all the info, and while it’s compact, it’s not crowded and all the info is easily readable without squinting or anything. It’s nice to be able to switch back and forth between the map page when you want the big picture / situational awareness and the HSI when you want to focus (like on the final approach of an ILS). I didn’t have an indicator that would work with the new GPS, so I had to put something in and was pretty much set on the G5. The GI275 was released pretty much the day my plane went in for upgrade and I let myself get talked into the new unit. For the incremental cost, it was definitely worth it.

 

[GRG55]

@AA5Bman thank you for taking the time to post the followup!

This thread, and your post above have been very helpful as I am contemplating something similar.

 

[Steve Flangan]

@AA5Bman, @NealRomeoGolf - thanks for the such a thorough write up. One thing that I lost track of was the radio coupler you finally used. Did you stay with the original 1C388? and didn't have to upgrade to the 1C388-2 or -3?

My situation is similar but different given you switched to a 2nd Digital Nav/Com.

My current setup is
- Apollo GPS no nav/com nor approach (end of life)
- 2 x MX170B NAV/Coms.
- Century IIB with 1C388 Radio Coupler
- King 209 (NAV 1 - ILS/LOC and Glide Slope)
- King 208 (for GPS and NAV2 (LOC)

NEW HW:
- G5 DG/HSI with GAD29B
- GNX375 GPS (not an NAV/Com, but is ADS-B in/out transponder

My shop is telling me they can only connect the GNX375 to the G5 HSI (for Display) which I expected, BUT - only will be able to use the Autopilot for the GPS, no longer can have AP coupled ILS/LOC approaches. Mostly due to the Digital vs Analog Radio challenge / GAD29B. Keep in mind the GNX375 does not have NAV so no ILS/LOC coming from it.

Also mentioned I need to pick up 1C388-2 for even that to work.

He is suggesting to forgo the G5 and pick up an GI-106 for the new RNAV, and then I can keep all 3 sources tied to the AP.

What is your thoughts on this?

 

[AA5Bman]

@Steve Flangan I'm not sure I can answer your question, to be honest. There are enough differences between our setups and this knowledge has grown a little stale for me, so I'm not sure the connections I'm making in my head are really applicable or correct.

But, I'll make and attempt and do my best with a couple of thoughts:

1) I did not have to update my radio coupler. I'm not sure if it is 1C388 or 1C388-2 or 1C388-3, but it looks exactly like the one in the ebay link at the bottom of this post below. I may be able to dig this out of my logbooks if you really need to know - I will check if you need me to. Mine does look exactly like the one listed in the link below which is listed as a -2 (but maybe they all look the same?).

2) I think what you're saying is that the shop is reporting that your signal can go GPS -> G5 HSI -> Gad29b -> Autopilot (with the coupler in there, I think it's really like GPS -> G5 HSI -> Gad29b -> IC388 -> Autopilot), but what you can't do is go both GPS -> G5 HSI -> Gad29b -> Autopilot AND ALSO NAV2 (VOR) -> Autopilot (again, probably more like NAV2 -> IC388 -> Autopilot). I don't know if they are saying this is because one signal path goes through the GAd29B and one doesn't, or because your NAV2 (VOR) MX170B is analog and not digital. My NAV2 VOR signal is now coming from a GNC255, so if the problem is that this is a digital NAV whereas the 170b is analog, they may be correct but I just don't know. I doubt it is really the case that the Century II can't take input from an analog NAV, because the Century II comes from an era before digital NAVs probably even existed. Furthermore, my old VOR was almost certainly analog, and it worked, albeit sloppily.

Continuing, if they are saying that it won't work this way because the two signal paths are different (i.e. GPS -> G5 HSI -> Gad29b -> Autopilot vs. NAV2 -> 1C388 -> Autopilot), they may be technically correct, and we spent some time figuring out this problem. What we ended up doing is creating a "switch" that allows me to switch between different signal sources for the autopilot, so now I can "switch" between them. I haven't seen a wiring schema, but I would guess that conceptually the paths are now something like GPS -> GI275 (in my case) -> Gad29b -> SWITCH -> 1C388 -> Autopilot, and the other path is something like NAV2 -> SWITCH -> 1C388 -> Autopilot. The switch is just a simple panel-mounted "up" or "down" toggle switch where the options are "up" for GPS/NAV1 and "down" is "NAV2". In my case the NAV1 signal is coming from the 650xi, and if I want to switch between GPS and NAV1 (as opposed to NAV2), I actually do this in the GI275.

3) In retrospect, here are some thoughts about things I would do differently, or lessons learned. First, the reality is that the GPSS steering in GPS mode is SO GOOD that in all scenarios flying actual IFR, I'm not going to use anything but the GPS. Not even my NAV1. Even to the point where I'd much rather shoot an ILS with the autopilot tracking the GPS-derived "for VFR navigation purposes only" localizer path while using my NAV2 indicator as "primary" for monitoring, than to actually shoot the ILS with the autopilot tracking the actual NAV1 or NAV2 derived localizer. I realize this is probably borderline legal, so I hereby state for the record: I don't actually do this! In any case, the accuracy and tracking ability of the GPSS is 100x that of either NAV signal to the point that there is no practical reason to use them. As a follow-on to this point, a lot of ink and dollar signs were spilled on my decision to even bother having a NAV2 at all and you can find a lot of other posts hand-wringing over this decision on POA. In the end, I probably spent $5,000 to make the NAV2 happen. In retrospect, that was $5,000 wasted. It's kinda nice to know that I have the full meal deal panel, but it is not - and has not been - practically useful in literally any way. The only thing the GNC255 does that is nice is that it automatically calculates density altitude (haha!), so I leave that value on the primary screen and that's about all I use it for! Finally, the complexity of switching between all these different systems is way more difficult to remember than you might think, and if you screw up any one of these settings, the autopilot will do something you didn't intend, potentially to frightening affect. For instance, say you're tracking along on a GPSS derived course and now you want to shoot an ILS on your NAV2. Here are all the things that I have to do to make that happen:

To go from GPSS tracking to NAV2 ILS you would:

- Set Loc frequency and validate
- Dial in the LOC course on the NAV2 indicator
- Dial in the LOC course on the heading indicator
- Autopilot OFF (ideally, or it will momentarily start tracking something wrong between the next two steps)
- Switch the "SWITCH" from the "up" (GPS/NAV2) position to "down" (NAV2)
- Set 1C388 in LOC mode
- Autopilot ON

Once you do all that, the Autopilot will start tracking the localizer - sloppily. And if you don't get that sequence just right, perhaps you fly into the mountain. The reality is that the autopilot performs so much better tracking the "VFR Only" localizer course or shooting an RNAV approach, that it's better just to dial the localizer in to NAV2 and monitor it while shooting the "VFR Only" localizer course on GPS (I don't do this!), or just request an RNAV approach (I do this!) and keep flying on the GPS.

The point of all this is just to say that... it might not be worth the headache and expense to figure any of this out. If you don't plan to put a GPS in that will allow you to shoot a WAAS approach - I don't care if it is an old 430 or a brand new 750xi - that's absolutely where your efforts and money should be focused, because that is 100% worth the upgrade. The rest of it? Not so much...

https://www.ebay.com/itm/1248684368...hFzM64FP2vBTuWCNwOo8yisRxK9vo7nxoC7OIQAvD_BwE

 

[NealRomeoGolf]

@AA5Bman, @NealRomeoGolf - thanks for the such a thorough write up. One thing that I lost track of was the radio coupler you finally used. Did you stay with the original 1C388? and didn't have to upgrade to the 1C388-2 or -3?

My situation is similar but different given you switched to a 2nd Digital Nav/Com.

My current setup is
- Apollo GPS no nav/com nor approach (end of life)
- 2 x MX170B NAV/Coms.
- Century IIB with 1C388 Radio Coupler
- King 209 (NAV 1 - ILS/LOC and Glide Slope)
- King 208 (for GPS and NAV2 (LOC)

NEW HW:
- G5 DG/HSI with GAD29B
- GNX375 GPS (not an NAV/Com, but is ADS-B in/out transponder

My shop is telling me they can only connect the GNX375 to the G5 HSI (for Display) which I expected, BUT - only will be able to use the Autopilot for the GPS, no longer can have AP coupled ILS/LOC approaches. Mostly due to the Digital vs Analog Radio challenge / GAD29B. Keep in mind the GNX375 does not have NAV so no ILS/LOC coming from it.

Also mentioned I need to pick up 1C388-2 for even that to work.

He is suggesting to forgo the G5 and pick up an GI-106 for the new RNAV, and then I can keep all 3 sources tied to the AP.

What is your thoughts on this?

You'll need the signal switch like mine did. I'm not buddy buddy with this avionics shop I used so I'm not sure I could call in the favor again like I did for @AA5Bman . You're welcome to show your shop my panel picture. It has the toggle switch in the nav2 CDI hole a few posts above.

 

[Steve Flangan]

Gents - Thanks for the reply.

@AA5Bman
If you never had an HSI in the plane prior, chances are you had the 1C388M, I believe the -2 was for Century HSI, and -3 for King HSI. All the RCs look the same.

The Garmin GNX-375 is a WAAS RNAV GPS - just without NAV/COM. It was an option I went for to get RNAV and ADSB IN/OUT w/Transponder all in one purchase.

I dug a little deeper and the issue my installer has is with the 1C388M (vs the 1C388-2) is the Selector will have to stay in heading mode and will not operate with course/datum corrections. The Omni, NAV, LOC, and BC buttons should operate normally.

 

[Steve Flangan]

@NealRomeoGolf

So your Nav2 an Analog radio and you are able to get both the Digital (GPS) and Analog (NAV2) working with the Autopilot? And the toggle switches which source (NAV1=GPS, or NAV2 - KX155) control the Autopilot?

Any idea what the connections are behind the panel - my guy says not able to do Digital and Analog once we replace the orig DG, the Autopilot wont support the older Analog radios with the G5 providing the Heading input from GAD29, maybe more of a legal vs technical challenge for my installer, not sure. Thanks

 

[asicer]

Gents - Thanks for the reply.

@AA5Bman
If you never had an HSI in the plane prior, chances are you had the 1C388M, I believe the -2 was for Century HSI, and -3 for King HSI. All the RCs look the same.

The Garmin GNX-375 is a WAAS RNAV GPS - just without NAV/COM. It was an option I went for to get RNAV and ADSB IN/OUT w/Transponder all in one purchase.

I dug a little deeper and the issue my installer has is with the 1C388M (vs the 1C388-2) is the Selector will have to stay in heading mode and will not operate with course/datum corrections. The Omni, NAV, LOC, and BC buttons should operate normally.

It's been a while since I've looked at all of the manuals, but I seem to recall that there are 2 possible paths from the GNX375 to the Century IIB. The first is the old-style lateral left/right that goes straight from the GPS to the 1C388M radio coupler. When the radio coupler is in one of the NAV/OMNI/LOC modes, it follows that left/right input. The second path is GPSS where the GNX375 tells the G5 (via the GAD29b) which way and how much to turn. The G5 then translates that into a heading signal for the IIB to follow, much like a human constantly twiddling the heading knob. For that path to work, the G5 needs to be in GPSS mode and the IIB needs to be in HDG mode.

As @AA5Bman pointed out, you probably won't ever use the first path if the second path is available to you. If your current radios are already connected to the IIB and available by selecting one of the NAV/LOC/OMNI modes, you can probably just leave that alone and connect the G5 to the heading input. When you want to hold a heading, select HDG on the 1C388 and select Heading on the G5. When you want to follow the GPS, select HDG on the 1C388 and GPSS on the G5. When you want to follow the analog radio, select LOC/NAV/OMNI on the 1C388.