1973 Cessna 172M. My plan is adding a Garmin 175 to replace the ADF and installing a Garmin 275 to replace my DG? Thoughts and feedback are greatly appreciated.
Wow. That panel is about as classic as it gets. I would guess that the only thing that was done to this plane's panel since it was new was to do the upgrade/slide-out replacement of the original Cessna radios with the MX (?) flip-flops.
Now, to answer your question, I'm gonna ask you some questions, because we simply don't have nearly enough information to give you a good answer yet!
1) What's your budget?
2) How long do you plan to keep the airplane?
3) Do you fly IFR or VFR, and in either case, do you want the plane to be IFR equipped?
4) Do you fly enough cross country that you might want an autopilot?
5) Do you fly internationally at all (rural Canada/Mexico)?
6) What are your motivations and goals for your panel upgrade? What do you feel like you're missing right now, and what capabilities are the most important to you when you're done?
Now, I'll throw a few thoughts out there anyway:
With regard to budget (1), you can do some pretty nice things at a fairly low budget (maybe under <$10K) if you want to just have an updated panel and ADS-B. You can do a slightly nicer panel at a fairly low budget with a bit more capability that's still missing some things and won't net you much in the way of resale value... Or, you can go up to probably $100K if you want to be really fancy!
As far as keeping the airplane (2), if you're selling it soon, just let the next owner put in what they want and buy the plane you want. If you're keeping it for a little while, get something a little nicer... And if this is your forever plane and you want to go all out, then go all out.
If you're strictly VFR (3) with no aspirations to fly IFR and mostly a short-range, $100 burger or poke-holes-in-the-sky-for-fun kind of flyer (4), keep it simple - Spend your money on avgas and things that help you as a VFR pilot.
If you don't fly in Canada/Mexico/Alaska, ditch the ADF. It is the heaviest piece of avionics you have, by far, and utterly obsolete. Even 23 years ago when I was learning to fly, the local DPE said "The only thing the ADF is good for is listening to ballgames and counting landings." And if you want to listen to the ballgame, just stay home.
Finally, I'll throw a few ideas out there:
GPS: I wholeheartedly agree with those recommending the GNX375. It gives you a color moving map GPS, ADS-B In/Out transponder, traffic and weather both on its own screen and on your iPad in ForeFlight or Garmin Pilot. It's an awful lot of the modernization of aviation since your plane was built all wrapped up into one box.
Nav/Com radios: If you're going the low-budget route, you can keep what you've got if you like them and you're happy with how they're working right now. Another possibility would be to upgrade to a GPS/Nav/Com like the GTN 650Xi, keep one of your radios as the #2, and keep the other one on the shelf in case the one you leave in the plane goes bad. Keep in mind that doing this instead of the GNX 375 will mean you'll want a separate ADS-B transponder such as the GTX 345, so the extra cost of the 650Xi isn't the only cost changing. Also, whatever you do, if you're going to keep either or both of the old radios, ensure that a good component-level avionics tech gets in there and replaces any electrolytic capacitors, otherwise they can leak acid and wreck other components and make the radio unserviceable.
DG: The GI 275 is very nice and fits in the existing hole and keeps the classic look for the panel. It also works well with the other Garmin stuff, and you'll be able to use it as an HSI and have navigation information from the GPS. Unfortunately it is not compatible with those nav radios (presuming those are the ones that are Cessna ARC guts with the flip-flop digital faceplate) - The 275 *can* do nav from a few Collins and Bendix/King radios so if I'm mistaken on what you have there maybe you can do both.
AI: I have a GI275 AI, and after flying with it the first time my only regret was that I didn't get two of them! If you do get two, you can ditch your vacuum system. Also, if you have the AI, it legally functions as primary for all of your instruments, so if any of the four remaining ones (airspeed, altimeter, VSI, turn coordinator) fail away from home you can still get home after disabling and placarding the inop instrument. This is one of the big differences between the GI275 and the G5 (which still requires all of your existing instruments to be installed and functioning).
Engine monitor: You may want to consider an engine monitor that legally replaces your existing gauges (GI 275, JPI EDM-900 series, EI CGR-30P). Some Cessnas of roughly that vintage used gauges that did their measuring right there behind the panel - Meaning that you may have an oil line coming from the engine into the cockpit. If that is the case on your plane, you may want to get rid of it. I personally don't want any more fuel or oil lines in or near the cockpit than absolutely necessary!
Audio Panel: I see you have the ancient KMA20 and then a separate intercom. OK if they work, but "while we're in there" it might be a good time to consider a modern replacement.
1973 Cessna 172M. My plan is adding a Garmin 175 to replace the ADF and installing a Garmin 275 to replace my DG? Thoughts and feedback are greatly appreciated.
