The International Cessna^® 170 Association

I talked to the installer at stc group yesterday. He is supposed to be contacting me with some prices and I'll post something after he gets back to me. I'm willing to take my plane there but I'm hoping I can get some folks to help out with the R&D Fees. It should be affordable for everyone that way.

Have there been any updates on this STC?

I haven’t seen anything from stc group that indicates any movement. I did however see on garmin’s website that the 170b is on the list for gfc500 stc for next 12 months as posted below:

GFC 500 Certification Programs Planned to Begin in the Next 12-months:

(Not currently under development and subject to change.)

Cessna 170
Models: 170B

It would be a dream come true considering I already have dual g5s.
I submitted interest to Garmin last year. Seems to have helped.

Looking at the November 2023 issue of "Flyings Aircraft for Sale" issue, looks like Garmin has GFC500 certification program in progress?
New panel in a 170 with just installed A/P.

This plane was at Garmin’s booth at Oshkosh. I spoke to the Garmin autopilot program manager there. That plane has a space waiting for the autopilot panel, but Garmin had not started the 170 certification program yet. This plane owner is just “ready to go”. Garmin plans on getting an STC for the 170B. I requested they add the 170A, but I’m not sure how likely that is since there are flight control cable routing differences and dihedral and flap differences. He told me that those differences would constitute a complete additional approval program.

I thought I'd post a addendum to this thread. As of December, 2024, The STC Group has an STC under which their Trio autopilot can be installed in the C170B. I believe my own C170B was the first one to receive his mod. If coupled with an approach certified GPS, it will track the inbound course, as well as "bend over" and track the glide slope. One should be advised their autopilot is not certified to operate at less than 500 ft AGL, so coupled down to a 200 AGL minimum is not approved. Coupling to a VOR, LOC, or ILS GS is not provided, but the autopilot works great with my Garmin GPS175. Their control head comes in two different configurations: a radio rack installed box around 2" tall, or a round version, which fits in a standard 3-1/8" instrument opening. One item of note is the necessity of converting a 4" inspection hole under the rear of the right wing, to a 6" opening. It will be impossible to install the roll servo without this mod. Expect a significant expense to install, as one must work in some confined spaces, and route wires from the instrument panel into the right wing, and back into the tail. Nothing terribly difficult, but lots of separate tasks, all of which take time to do properly. You may be able to save some money if you can find a shop/mechanic who has done at least one of these installs before. Once installed, you'll really enjoy the performance. Would I do it again - absolutely.

Karl

Just finishing my install of the Trio. At OSH2024 Garmin has no plans to do an autopilot for the 170. As soon as the Trio received certification that may have changed. But for me, the requirements creep of a Garmin AP means many many thousands of dollars. The Trio is plug and play for my panel, looking forward to running it.

redacted wrote: ↑Wed Feb 12, 2025 6:00 am Just finishing my install of the Trio. At OSH2024 Garmin has no plans to do an autopilot for the 170. As soon as the Trio received certification that may have changed. But for me, the requirements creep of a Garmin AP means many many thousands of dollars. The Trio is plug and play for my panel, looking forward to running it.

I spoke to Garmin at Oshkosh 2023 and 2024 and they do have the 170B on their list, but not the straight 170 or the 170A. They even had a 170B behind their tent that has a spot saved in the panel for an autopilot controller. The controller was installed in the panel, but not hooked up to anything. They are not knocking new airframes off the list very fast, though.

I received my Trio last week. Can’t wait to get it installed. Will post back after first flight with a/p.
Thanks to Jeff Odum!
-jim
1954 c170b

Wrapped up my install today and did the first test card to send back to Jeff. Airplane buttoned up from annual too so awaiting nice weather to fly.

Nathaniel ,

When you get a chance, please share some details with us? What GPS are you using? Do you have an EFIS for enhanced control? Did you install a 6" inspection opening in the right wing? How much of the work were you able to do yourself? Pictures are appreciated as well.

Karl

In no particular order--

Number 8 screws are a #16 drill bit. Number 10 or AN3 is a #12.

Had to open up the 4" hole in the wing to 6". Also you're gonna want an inspection hole in the outboard flap well area outboard of the last flap track, outboard as far as possible, to install the one screw into the aileron servo mount that is in the flap well area behind the false spar. We managed without but it was over 4 hours to install that one screw - not kidding. You cannot fit anything in the wing without the bigger inspection hole fyi, so there's no getting around that. The mount and servo won't fit into the wing assembled, you have to assembled them together inside the wing which is also pretty hard. So we installed one screw loose in the servo to servo mount, fished it into the wing and then spun it around inside the wing and origami'd it into place and installed the remaining 3 screws. Only then did we screw the mount to the rib. The inspeciton cover you'll need for the new hole is the S329 part. eBay and BAS have them readily available. When we opened up that hole we upsized the rivets, flush riveted everything and installed nut plates for structual screws for attaching the cover.

The servo arms, screws, washer and safety cotter pin come in the parts bags. Just install them right away on the servo cuz once the roll servo is in the wing it's can't be done then. Pitch servo, doesn't matter. The roll servo mount only has two of the four nutplates installed. Not sure why they don't do all four for you. Get some ground down dies because they are floating nutplates and a regular die is too big and when you squeese the rivet you'll squish the ears on the floating nutplate and turn it into a regular nutplate. The holes in the roll servo mount to clear rivets in the rib are just reference holes to help you see where you need to drill out even more for clearing those rib rivets. Step bit for the win.

I installed all the electrical and pitot static stuff, all the wiring and panel work, ran the wires to the wing and tail. I also did the final installation in the tail after the mounting holes were drilled, just cleanup and nuts and bolts and the pushrod and cable clamp rigging. The aileron wiring I ran inside heat shrink tubing (the kind with some adhesive on the inside) and that added some rigidity to the wiring and some weather protection since it had to run in the flap well area with the flap/aileron cables. You can get a 50' roll of the stuff on Amazon. It was literally pushing rope to feed it down the heat shrink tubing, took about and hour. This area is where the wiring for the 172 flap motor runs so it's not unheard of to run wires there, but watch out for flap pushrod interference and cable interference. The heat shrink adds some weather protection too since it's open to the flap tracks. Make sure you secure the wiring every place you can reach and those places you can't reach either---Amazon has great deals on surgical tools, I recommend forceps of many sizes and angles. My mechanic did the heavy lifting, the sheetmetal work and setting up the mounts for the pitch servo in the tail. I have extended baggage and it clears the pitch arm but an inch, tons of room.

Installing the control head in the panel was easy, I went with the instrument mount because I had an open hole. Radio stack mount just a different shape. It gets pitot/static plumbing. Stein Air has lots of those shark bit fitting and tygon tubing.

You will have to get with the avionics shop for a leak check of the static system when your install is complete.

You have to send Jeff Odum your completed test card 1 to get the authorization to use the STC. I filled it out, snapped pics and emailed it.

The autopilt is hungry for volts. If you power it up without the servos plugged may not work at all. All powered up you are going to have to have a ground power source hooked up. This is an absolute must. Anything under 12.5 volts or so and the AP stops working right. If you see its lights flickering a ton or it just keeps rebooting after the field elevation screen then it's low voltage. And you will need power on to help set up the servos to a zero position for installing control linkages and for the first 3 test cards.

I bought an Aera 760 to feed the autopilot. The bare wire harness from Garmin has RS232 to talk to stuff and the Trio uses that or ARINC 429, depending on your install. The RS232 has a high and low wire. The low just goes to ground to complete the circuit. I have nothing in the panel so I had to get something to drive the AP. It has no internal magnetometer or heading input. All flying that is not a waypoint, i.e. heading bug flying, uses GPS track info so you have to feed the autopilot some data or it's just a wing leveler.

Roll servo install pretty straight forward except for that last screw in the servo mount that's hidden in the flap well. Pushrod measuring and cutting and riveting on the end piece for the rod end was straightforward and easy. Get a cable clamp for the aileron cable! Then you don't have to re-rig the whole thing. You have to disconnect the aileron cable from the bell crank because the pushrod installs with it and there's a new hardware stack. Clamp the cable, mark your turnbuckle, loosen it a couple turns to relieve the tension, then it's easy to remove the cable from the bellcrank and install the pushrod and new hardware. Then tighten back to your mark, remove clamp, safety the turnbuckle. It's .041" safety wire if you don't have the newer turnbuckles that use the clips so have fun. I bled a little.

Pitch servo installation, we ran into some problems on mine I'll tell you about at the end. But once that was sorted it seems the best way to install it is to roughly install (clamp) all the pieces in place to check clearances and then attach the servo to the longitudinal pieces and those to the bulkhead cross pieces so you have one giant piece to install back there and match drill holes into the bulkheads. Then it''s only one moving part compared to 5 (servo plus 4 piece of extruded aluminum) that you have to get to all align at the same time. We had to lower the forward bulkhead cross piece because it was interfering with an elevator cable, then had to lower the rear cross piece accordingly because it has to be level and square to itself - it has to be because it's all extruded aluminum and that can only bolt together flat and square, you can't soak up any angular missteps. When we lowered the rear piece to match it was interfering with a pulley so we had to rotate the piece to create some space, this changed how the longitudinal pieces mounted to it. Kind of snowballed but we got it.

Memorize how to get into the maintenance settings and get to the LNAV and VNAV servo screens that show the throw of the arm. You'll need these at zero when measuirng the aileron pushrod to cut it and when installing the cable clamp for the pitch servo. It's best to have this screen up so you can verify it's at zero and turn the servo on so the arm will fight to stay in the zero position while you mess with moving things around the measure them. Quick look that it's still at zero will save you a missed measurement. Your ailerons and elevator should be clamped neutral for the whole install. They say to level the aircraft but there's no need to do that. That's for the pitch servo install - just make sure it's square to itself and the plane. They don't want any torsional loads on the bulkheads. We ran a piece of angle iron across the cockpit door sills with a digital level on it and had a second digital level in the tail on the bulkhead cross pieces and just made sure those angles matched so we knew it was matched to the aircraft.

Overall the kit is good. The STC covers over a dozen airplanes so the install instructions are pretty generic, even for the 170. Get ready to do some problem solving. Kit is complete with hardware but we upgraded everything where we could to better hardware. Some of the hardware stacks just don't work out to well so have some thick and thin washers handy, some extra screws or bolts of varying lengths, and get ready to create a thin large area washer if you need. The hardware stacks for the servo connections are very tight and it's super easy to have a cotter pin interfere with the servo or the pushrod. Or just have the drilled hole in the bolt be off by the same thickness as the anodizing they used. It's not just a setup job, definitely have to do some problem solving. Keep the parts catalog handy too. You will easily see hardware stack callouts and parts callouts there where that info doesn't exist in the installation instructions. Neither servo will need to run to its mechanical limit in this operation so as you install stuff and are checking for clearances don't worry if something interferes at full mechanical travel. In operation it's impossible for the servo to move that far so if you can check by unclamping your neutral controls and going through full control movement.

That said, make sure you can clamp and unclamp your neutral controls easily and it's repeatable. It will make things easier.

Don't forget to update your weight and balance. They have an example on their website to guide you. My install added a little over 7#.

Add the aircraft flight supplement to your aircraft's docs.

Adds the instructions for continued airworthiness to your annual inspection folder.

I've completed all the ground checks. Waiting for weather for the flight tests.

This is a one off and not applicable to the rest of you ---- The big hurdle on my plane was there was a repair some time in the late 50's or early 60's. Not a lot of info but I have 1953+ empannage skins on the plane so I know some work was done. Turns out when they made the repair they didn't rivet one bulkhead back into place....the bottom of it is shifted aft an inch. This created an angle that meant the bulkheads weren't square to each other or the world and nothing would mount square and flush. We machined 3"x1" aluminum shims of the exact angle needed to soak up that angular difference and let the heads and tails of the bolts be perpendicular to the mounting surface and then could install everything back there squared up. Upgraded it all the AN3 bolts and hardware. Much happier with the installation like that than just running some screws through the .032" bulkhead aluminum. It took some head scratching and a call to the local machinist to see if it could be done but 24 hours later he walks in with 8 precisiely machines shims, all within .2º of the angle we needed.

Yes this is a little rambling, it's been 8 weeks on and off of this install but it's done. Every install will be different. Please don't take anything as the gospel here. Just maybe keep in mind as you have to troubleshoot your own installation. This worked of us, your results may vary.

What a great detailed summary of the installation. I'm sure it will help many people in the future.

Thanks!!

Nathaniel,
Thanks for taking the time to post the detailed narrative on your Trio AP install! It sounds like you encountered many similar challenges to my own. I noted a couple of topics where I would like to offer alternatives. DISCLAIMER: Sorry, I have to say it; the reader assumes complete and total responsibility for the use of the following information, and any consequences that result.

1)

Also you're gonna want an inspection hole in the outboard flap well area outboard of the last flap track, outboard as far as possible, to install the one screw into the aileron servo mount that is in the flap well area behind the false spar. We managed without but it was over 4 hours to install that one screw - not kidding.

PLEASE DON"T cut any more inspection holes in your airplane than absolutely necessary! As you mentioned, only 2 nutplates are installed on the roll servo bracket. So, install 1, but leave the top forward nutplate uninstalled. With your flaps down, install the three screws into the three nutplates you have open access to. Now, using a long drill bit, drill the fourth hole for the bracket, from the outboard side, operating through the 6 inch inspection hole (Actually, I think I used a power screw driver, turning a hex adapted drill bit). Next, get a #8 hex head machine screw of the same dimensions as the fourth mounting screw. tightly coil some .031 safety wire around the screw, just under the hex head. Reaching into the wing, insert the bolt pointing outboard toward you. The safety wire is to retrieve the bolt if dropped, and to hold it during installation. Once you have the bolt through the hole, hold the safety wire, and install a flat washer and locking nut on it. Now, pull the safety wire to uncoil it from the bolt, and remove. Finally, using a small "ignition wrench to hold the hex head, tighten with a 1/4" drive ratchet, long extension, and a universal (wobbly) socket. This combination works, and should cut down on the time required by an appreciable amount.

2)

The aileron wiring I ran inside heat shrink tubing (the kind with some adhesive on the inside) and that added some rigidity to the wiring and some weather protection since it had to run in the flap well area with the flap/aileron cables. You can get a 50' roll of the stuff on Amazon. It was literally pushing rope to feed it down the heat shrink tubing, took about and hour.

Tip for pushing wires down a length of heat-shrink, or any other type of tubing; run a small, single conductor fish wire first. Then strip a generous length of insulation and attach to the wire(s) to be pulled through. Attaching to the wire(s) to be pulled can vary. Soldering is preferred as it greatly lessens the possibility of the fish pulling away from the wire(s) being pulled. You can even "lube" the wires just behind the joint with dish soap if you want, then gently pull your wires through. If you can't reach both ends yourself, a helper is a necessity.

For this specific project, a great routing for the roll servo wire is to go forward from your 6" inspection hole, through lightening holes in the spars, then a gentle left (or inboard) turn to the existing set of holes in the nose ribs through which the nav light wiring passes. To protect from chafing where the wire passes through the lightning holes, one can use an approximate 4 ft piece of 1/4" OD Poly-Flo Polyethylene tubing as a conduit. Since there is lots of extra servo wire, here's a suggested method:
1) Route the roll servo wire up the right front door post, and out into the wing route area.
2) Remove the inspection panel in front of the strut to wing spar junction.
3) Remove the right wing tip, or at least the right nav light.
4) Disconnect the wire from the nav light, and attach at least 10 ft of "fish" wire to the nav light wire.
5) Pull about half of the nav light wire back into the wing route area.
6) Tightly tape the roll servo wire to the nav light wire.
7) Gently pull the nav light wire back toward the right wing tip. It really helps to have an assistant during this.
8) When the two wires appear in the inspection opening in #2 above, remove the tape to separate the wires.
9) Pull the extra roll servo wire out of the inspection hole, and pull any remaining nav light wire back out to the wing tip and secure there.
10) Using a stiff wire, (12 or 10 gauge solid copper house wiring works) or other suitable item, insert it through the roll servo inspection hole (the one that was increased to 6" diameter). Work it forward, perpendicular to the wing spars, until it emerges into the leading edge of the wing, outboard of the inspection opening in #2 above.
11) Hook this wire with a loop or hook and while a helper keeps feeding it forward from the 6" opening, pull it inboard toward the 4" opening (#2 above).
12) Tape the roll servo wire to this stiff wire. Push and pull the stiff wire back to the 6" opening until you have all the extra wire fed through.
13) Now we need to protect the servo wire with a length of Poly-Flo Polyethylene as a conduit back through the wing. Since the servo wire has 4 sockets already crimped on, this will be a bit of a challenge, but it can be done rather easily.
14) Carefully slice and remove the heat shrink at the terminal end of the servo wire. Do NOT hurt the wires or insulation!
15) Use a length of 22 gauge as a fish wire. Insert it through the 4 ft length of 1/4" Poly-Flo Polyethylene tubing.
16) Strip at least 1.5" of insulation from the end of the fish wire.
17) Set up your servo wire for pulling through the Poly-Flo Polyethylene tubing in the manner depicted below. The four sockets will NOT pass through side by side. A staggered configuration works well. Recalling the knot craft you learned as a Scout, tie the bare conductor of the fish wire to the servo wire using a clove hitch knot. This knot is exceedingly useful for this type of application; it tightens as pulled, and causes only a slight increase in diameter to the wires being pulled. 18) After pulling the servo wire through to the tubing, slide the tubing to the front of the wing and inboard to where the wires emerge from the hole in the nose-rib. Secure the Poly-Flo Polyethylene tube with wire ties, and trim its length if desired, again using extreme care to not damage the wire inside.
19) Remove the fish wire and straighten the four conductors and sockets.
20) Replace the piece of heat shrink over the terminal end of the servo wire in the same manner, length and color as the one removed in #14.
21) Reconnect nav light and reinstall. Test for proper operation.
22) Proceed with installing the servo wires into the Trio provided DB9 connector, and the balance of their installation procedure.

This sounded complicated to me as I proof read it. However, it really works, simplifies the routing of this wire, and keeps it completely clear of moving aileron and flap cables. Hope this helps!
Karl

Excellent hack for that 4th roll servo mount screw in the dead area. Impossible task really.

I should add I don’t have 170 wings on the plane. They’re 175 wings.

Flew the plane today and tuned all the gains on the A/P and finished the flight test cards. Only 144 more days until Oshkosh 2025!!