The Road to Certification: GFC 500 for the Retractable 182s

The Road to Certification: GFC 500 for the Retractable 182s

Garmin’s GFC 500 autopilot is still very new, and supported aircraft are being added all the time. TROY WHISTMAN, a Cessna Flyer Association member, was selected and agreed to loan his 1980 Cessna TR182 Turbo Skylane RG as a test aircraft for Garmin’s certification of the autopilot in retractable Skylanes. Here is his story, including tips for how you might improve the chances of your aircraft being selected, too.

The morning of Monday, July 15, 2019, dawned over Olathe, Kansas, with a beautiful sunrise, as I awoke with a burst of excitement. Today, with any luck, I’d be picking up and flying home our 1980 Cessna TR182 Turbo Skylane RG with a newly installed 3-axis GFC 500 digital autopilot. 

Exactly 116 days (almost four months) earlier, on March 21, I had flown our airplane from the Dallas area to New Century AirCenter Airport (KIXD) in Olathe, Kansas, home of Garmin’s Flight Test group. I left the TR182 in their care while they used the airframe as a certification test vehicle to make the autopilot available for all retractable Cessna 182s. This story covers that journey.

A game-changing autopilot 

Like many who fly older Cessna aircraft, my 39-year-old airframe was equipped with a Cessna Nav-o-Matic single-axis autopilot—the 300A, to be exact. While it held course/heading just fine, it couldn’t fly a glideslope, handle vertical descents, or capture (much less hold) an altitude during cruise flight. 

One might argue that Cessna 182s are stable aircraft, and you really don’t need an autopilot to hold altitude when properly trimmed. In fact, I made that same argument to my friends for several years as I flew single-pilot IFR in this excellent airframe.

Then, Garmin announced the GFC 500. 

I looked at the feature list: IAS mode climbs (often called FLC, for “flight level change”); VNAV capabilities to automate step-downs on approaches and arrival procedures; full GPSS capability for holding patterns, entries, and turn anticipation; ILS/LOC/BC/VOR/GPS/RNAV approach capabilities; envelope protection; and the LVL button. This thing had it all. 

While I wanted one installed yesterday, Garmin certifies these units by make and model. When the Cessna 182 was finally announced as ready and available, I tried to make the case with them that the R182 and TR182 were covered by the same 3A13 Type Certificate Data Sheet, and thus the R182/TR182 should be covered by their standard Cessna 182 certification work, to no avail. 

Due to the CG shift with the gear in transit, the Lycoming versus Continental powerplant, and the higher service ceiling, the TR182 would need to be certified under its own program of test flights.

Like many of you who are interested in the GFC 500 program, I religiously followed Garmin’s Supported Aircraft matrix showing which aircraft are currently covered by STC, which aircraft they are currently working on certifying, and which programs they plan to begin in the next 12 months. I also filled out Garmin’s online form expressing my interest in seeing the TR182 certified. (The links to a list of supported aircraft and Garmin’s request form for additional models can be found in Resources. —Ed.) 

Not to be deterred, I emailed way back in July 2017 to log my interest and received the same default message that many of you have received:

 “Thank you for reaching out. Your request has been received and we will use this information to evaluate the marketplace for future certification efforts.”


Improving my odds

I had no idea what criteria Garmin uses to select certification program candidates, but I knew my name was in the hat because I filled out the form. (NOTE: There is a separate form to complete if you’re interested in allowing Garmin to use your aircraft to certify their GFC 500 or 600 autopilot, versus just interested in buying one when it’s ready. See Resources. —Ed.)

However, I did know that the GFC 500 requires a Garmin G5 to be installed, as the G5 has the “brains” of the autopilot. So, in the winter of 2018, I undertook a panel renovation with the help of a good friend, supervision by an A&P/IA, and advice from a booked-through-2020 avionics shop owner. 

We did the work ourselves, gutting the old 1980s plastic panel and associated wiring, and over the course of two months transformed the airplane with a laser-cut flat metal panel with laser-etched labels (see sidebar at right), new glareshield and instrument lighting, dual recessed G5s, flush-mount JPI 830, custom warning LEDs for alert conditions and a new avionics electrical panel with extended main bus. 

Through our avionics contact, we obtained the wiring diagram for the GFC 500 install and pre-wired the airplane for the GMC 507 (that’s the GFC 500’s control head in the radio stack; the box with the blue LVL button), breakers for the autopilot servos, dual inputs to the audio panel for autopilot alert tones, yoke wiring for the trim switches and disconnect switch and more. 

The plan: get the airplane as close to “ready” as it could be, with the goal of improving our odds of being selected by minimizing the work Garmin would have to do if we were selected. 


Getting lucky

From the moment the program had been launched, I used web monitoring tools to let me know the instant any change was made to the “available/working on it/coming soon” list on Garmin’s website. 

When I’d see that a new plane was available, even though it wasn’t the R182/TR182, I’d often post a positive message to the North Texas Aviators Facebook group, alerting others that flew whatever newly certified type was just announced. One member who flew the Grumman Tiger got his Garmin autopilot installed just after it was certified. I drooled over his installation for some time. 

Then, one day in February 2019, after posting yet another “not for my R182/TR182 but YOUR aircraft type is now available” message on Facebook, I received a private message from someone who happened to be a North Texas representative of Garmin that had seen my posts. 

He and I spoke over the phone, and then he made an email introduction to the certification team at Garmin about me and my plane. I never got a reply from that team… it was as if the email went into a black hole.

Later, I discovered they weren’t being rude but rather, were and are laser-focused on the projects they’re currently certifying. They simply file away incoming emails/requests for future reference.

Once I completed the panel upgrade project. I sent the Garmin team another email, with pictures of the work we’d done and all the pre-wiring that was in place. Still, crickets.

In mid-March, out of the blue, while working from my home office, my cellphone rang. The caller ID showed the number was from Olathe, Kansas, and my heart skipped a beat. Could it be?

I answered the phone and on the other end of the line was the lead Garmin Autopilot Certification Program manager. He told me that they’d reviewed the list of eligible aircraft for the R182/TR182 certification program and were ready to begin that project. Would I still be interested in letting them use my airplane?


I answered without hesitation.

I learned it would take about 10 weeks and that the autopilot and installation would be at no cost to me in exchange for allowing them to use the airframe. That sounded like a swell deal to me!

In the two weeks between that call and delivering the airplane to Garmin, there was a flurry of activity: legal forms needed to be reviewed and completed, allowing Garmin to use the airplane and defining what each of us was responsible for, discussion of liability, insurance coverage and so on. 

I utilized my AOPA Legal Services Plan to have the contract reviewed before I signed it. Garmin then sent one of their A&P/IAs to Texas to do a thorough pre-acceptance inspection. He checked logbooks, annuals and conducted a thorough review of the airframe. 

And when I say thorough, I mean thorough. He didn’t find much wrong with this well-maintained airplane but did note, for example, that “the washers on the bolts securing the rear bench seat to the floor might not be the correct ones.” Did I mention he was thorough?  

Garmin expected the certification effort to take about 10 weeks, until about June 6, 2019, so they verify before accepting your airframe that its annual won’t expire in the middle of their work. My annual was set to expire June 30, so we were good to go.

That’s how it came to be that on March 21, 2019, I flew our airplane to Olathe, Kansas, and flew home commercially.

For the next four months, I followed flight test progress on FlightRadar24 and FlightAware; and eagerly consumed email updates with pictures sent by the team at Garmin. 

At one point, test flights were suspended for a couple of weeks due to a servo failure during heavy ground tests. The GFC 500 is so popular in the market, however, that it took Garmin themselves a couple of weeks to get a replacement. 

As it turned out, they were right and I was wrong—the 3A13 TCDS, which covers all the 182 series, wouldn’t have been enough to certify the TR182, which flies as high as FL200. The air is so thin that high that control pressures become exceedingly light. 

Garmin had to have the G5 team make software changes to the gain range for the GFC 500, which allowed them to configure it to work as it should at the TR182’s service ceiling. That software change happened quickly, demonstrating the responsiveness of this great team of individuals.

I was pleasantly surprised that they equipped my airplane with the yaw damper feature. It’s optional when you buy the autopilot, but they can’t sell it if they haven’t certified it for your make and model. So, when your airplane is selected for the STC program, you end up with a yaw damper. “Oh, well, if you insist!”

My airplane didn’t come with electric elevator trim when it left the factory, but it has it now, thanks to the automatic electric trim installation Garmin added. All told, I received a modern 3-axis digital autopilot with four servos: yaw, pitch, roll, and automatic electric pitch trim.

“It’s done!”

While it seemed like forever being without my airplane, the day finally came when Garmin called to say, 

“It’s DONE!”

And that’s how I came to be standing in the hangars of Garmin’s Test Flight Center doing final inspections and picking up my airplane. I arrived at 8 a.m. and brought doughnuts as a thank you for the team. 

The install was completed, but the final FAA paperwork and STC were still in progress. I wondered if I’d get out Monday or would need to wait until Tuesday. While all the paperwork was being done, Garmin dedicated a shop crew to washing and detailing the plane, even using leather cleaner on the interior. 

The certification team worked hard and got all paperwork finished by early afternoon. I signed paperwork and they turned the airplane back over to me. The plane was returned better than I left it: clean, with a full tank of 100LL, and I enjoyed a relaxing IFR flight home with the new autopilot guiding the way. 

It locked onto altitudes, headings and nav courses like it was on rails, and handled updrafts and downdrafts without wavering or disconnecting. I’m very pleased with the outcome.

Afterward, as I was looking through the logbook entries, I stumbled on another surprise. While Garmin was in possession of the aircraft, they had discovered that the wet wing’s fuel tank top access plates wept a bit when the tanks were completely full. 

I had never discovered this as I typically run the tanks at the tabs (65 gallons) for more useful load. During the time the plane was waiting for the failed servo replacement, Garmin drained the tanks, removed and then resealed those covers. 

The story: Garmin takes care of you. They are great people at a great company. While I felt they were doing me a favor by “giving me a free autopilot,” their behavior was as if I were doing them a favor by letting them use my airplane.

How do YOU get selected?

I don’t have any magic answers. There’s a bit of luck in this. But I will share something the program manager told me as I picked up the plane. 

When it comes time to make the selection, Garmin staffers sift through many online submissions and paper interest forms filled out at Oshkosh and other events. 

On a recent project startup (not mine), while looking through the paper forms submitted at EAA AirVenture Oshkosh, one had “YES! YES! YES!! PLEASE PICK ME!!!” written in big bold capital letters at the bottom where this question was asked: “Would you consider letting us use your airframe to certify the autopilot?” 

Any idea who got picked? 

You guessed it. 

If your airplane is as close to stock (conforming with the original type certificate) as possible, that improves your odds of selection. If you have STCs that alter flying characteristics significantly such as vortex generators, a STOL kit, and/or engine/prop upgrades, that may reduce your chances.

My odds were slightly improved by having the turbo example of the R182/TR182 series, due to filter-down effect. If Garmin certified the autopilot for the TR182, it would also apply to the R182, but the reverse was not true.

So, my advice to those who want to be selected is to show some public enthusiasm for the product, go where you might meet Garmin people, be genuine and shake some hands. As with many things in life, it’s not what you know, it’s who you meet and how you interact with them that influences opportunities that come your way. 


That’s my story! I hope you get to experience flying your plane with the awesome GFC 500 soon, whether you get it for free or pay to have it installed.

In closing, I must say “Thank you!” to the entire team at Garmin for a positive experience and a fantastic autopilot.


Troy Whistman is the father of three grown daughters and has been married 30 years to his lovely redhead bride, affectionately called “Lady Red.” Together, they base their airplane at the Mid-Way Regional Airport (KJWY) south of Dallas, Texas. Whistman holds a commercial airplane SEL certificate with instrument airplane rating. When not flying for fun to catch a sunrise or sunset, he enjoys using his toys as tools to help others: he flies for and is on the board of directors for Angel Flight South Central, and thinks flying kids for Challenge Air is some of the most rewarding flying he does. Send questions or comments to .



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Tips & Tricks for Flying with the Garmin GFC 500 AFCS Autopilot, Part 2

Tips & Tricks for Flying with the Garmin GFC 500 AFCS Autopilot, Part 2

In the last article, we left off with our shiny new Garmin GFC 500 autopilot fully configured for departure. We’d been instructed to “fly heading 180, climb and maintain 3,000 feet.” 

Takeoff: From runway to airborne

With the preflight work done, take off as normal. Saying “Gear up, flaps up, yaw damper on,” will make you feel like one of the big kids, as you press the “YD” key and let the servos handle the rudder. Above 800 feet agl with the airplane cleaned up, reach over and enable the autopilot by pressing the “AP” button whenever you’re ready.

If on a GPS direct track for the first leg, I’ll press Direct To-Enter-Enter on my navigator first, to recenter the course from the present position, then press the “AP” button to engage the autopilot.

George is now flying the airplane…or is he?

Important: Develop this habit! Whenever you select a new mode on the autopilot, always bring your eyes back to the top of your G5 or G3X and verify the modes selected. Every. Single. Time.

I developed this habit shortly after finding myself in the clouds on a departure, thinking George was flying—very poorly, I might add, and my scan was confirming that—and then discovering that nobody had been flying the plane for about 60 seconds. Talk about a wake-up call!

The correct lateral/vertical modes were selected, but the autopilot itself hadn’t enabled. Maybe I didn’t quite push the button while getting bounced around in those Houston afternoon thermals. Verify the modes! Some people call it “checking the scoreboard,” with the “scoreboard” being the autopilot status line at the top of your PFD.

Do you have green AP and YD annunciations on the scoreboard? No? Fix that! Yes? OK, then George is flying. But what is he doing? Verify the annunciated lateral and vertical modes. It’s showing a green IAS 90? Great, George is flying you at an indicated airspeed of 90 knots. Or is he? Cross-check! Is that what your airspeed indicators read? Great.

Where will George level off? Check to see that the cyan altitude bug is set at your expected level off altitude (did tower give you a last-minute change?), and that a white (armed) ALTS mode is annunciated next to the IAS 90. If so, “George” will level off at that ALTitude Selected.

I can’t stress this enough—always “check the scoreboard” after doing anything on the GMC 507. Every. Single. Time.


Fair warning. You’re going to be bored.

The GFC 500 is going to do such a fine job flying the airplane that you’ll find you get bored, unless you fill that time with other activities. Of course, I’m not suggesting you read a book or get caught up on Netflix! You now have time to focus on other important pilot duties.

Check the weather ahead and develop options. Refine your Plan B if needed. Scan those engine instruments more often. Check out the approach plates and arrival airport diagram. Which runway are they using? What taxiway do you think you could make, and what taxi route to your FBO makes sense?

It’s amazing the amount of “thinking time” you get back with a well-running autopilot as your co-pilot. But don’t get so busy you forget to monitor George frequently!

Tip/trick: While in NAV mode, tracking that GPS course, reach over every few minutes and push the “HDG/TRK” knob on the GMC 507 to sync the heading bug to your current heading. The difference between the heading bug and your actual track, as shown on the HSI, will give you a good idea of the winds aloft, and how they are changing over time.

If you have a GAD 13 installed and the recent G5/G3X firmware, you’ll see winds aloft and OAT displayed on the instruments. Having that heading bug already synced up is especially helpful when ATC says, “Turn 10 degrees right for traffic.” Note the current bugged heading, spin it right 10, then reach over and press “HDG” on the GMC 507. What’s next? You know: Check the scoreboard. Every. Single. Time.


What’s this VNAV thing?

Vertical navigation can be thought of as a magenta path, not over the ground, but rather through the vertical profile. It is a GPS-computed path to help you meet the step-downs in a “descend via” clearance on an arrival procedure, to help you with that crossing restriction ATC just gave you, or to ensure you get from your cruising altitude to pattern altitude 3 miles before you arrive at your destination airport.

The “VNAV” button on your GMC 507 enables the autopilot to fly that vertical path precisely—all you must do once it’s engaged is manage the throttle to keep your airspeed where you want it. But there’s the catch: “once it’s engaged.” There are some gotchas that trip people up and keep VNAV from working when expected.

First, you need your GFC 500 system to be coupled to an IFR navigator capable of VNAV. The GTN 650 and 750 are two such navigators. If you’re flying a GNS 430/530 series, even if it’s WAAS enabled, you’re out of luck. The 430/530 units do have a vertical descent planner that can tell you the VSR (vertical speed required) to meet a defined crossing restriction, and I use that feature—but it won’t couple to that fancy “VNAV” button on the GFC 500 autopilot.

If you’re flying a GTN series navigator, though, and load a procedure that has crossing restrictions or add your altitude constraints to your flight plan, then VNAV should be available on the GFC 500.

Second, to activate VNAV, you must perform certain steps in order. The most often missed step is that you must set your selected altitude (ALT SEL) to a lower altitude than you’re currently flying.

Here’s a scenario. You’re at 9,000 feet msl, and your flight plan says you want to be at 3,000 feet msl at GOTHI intersection, and your MFD properly shows a TOD (top of descent) and BOD (bottom of descent), and the “VNAV” button is lit up on the GMC 507, and you hear that voice that announces, “VERTICAL TRACK,” one minute before the descent begins. All of this looks good!

But the airplane will remain at 9,000 feet and never actually descend, flying right past your TOD marker and remaining at your current altitude, if you leave that selected altitude bug on the G5/G3X set to your current 9,000 feet.

Why? Think of it this way: the autopilot will always err on the high side to avoid flying you lower than you want. If you want to descend lower, you must set the selected altitude lower than the altitude you’re currently flying.

Tip/trick: If you want to use VNAV, set up the altitude profile in your navigator’s flight plan. Verify you see the TOD/BOD markers on your moving map. One minute before descent, you’ll hear that nice lady’s voice in your headset announcing “VERTICAL TRACK.” That’s your cue!

You’re going to do three things after that audible prompt:

Set the ALT SEL bug to the lowest altitude you’ve been cleared to by ATC, or the lowest altitude in the published arrival procedure.

Press the “VNAV” button on the autopilot.

Check the scoreboard! Every. Single. Time.

Several other factors determine if VNAV will engage or not, such as the requirement to be navigating on a course (i.e., direct to a fix, or between two waypoints). If you’re just wandering around on your own, VNAV isn’t an option.

Garmin recently published a 20-minute training video that thoroughly covers all aspects of VNAV training. If you fly with a navigator capable of doing VNAV, I strongly recommend you take the time to view it.


Some final tips

  • Pressing the “AP” button before selecting lateral or vertical modes will activate the Flight Director (FD) and engage the autopilot in the default PIT (pitch) and ROL (roll wings level) modes. That’s probably not a mode set that you will ever really want, so always select your desired lateral/vertical modes before pressing the “AP” button.
  • If lateral/vertical modes are selected, but the autopilot and flight director are not engaged, you can engage the FD separately from the autopilot by pressing the “FD” button, and then hand-fly to match the flight director’s cues. Pressing the “AP” button enables both the autopilot and FD since the autopilot works by following the flight director’s commands.
  • If, while climbing or descending, ATC tells you to level off immediately, simply reach over and press the “ALT” button twice. It will grab your current altitude and lock onto it.
  • If you find your ALT hold is off by 20-40 feet, using the pitch trim wheel on the GMC 507 while ALT mode is engaged, will change the selected altitude reference UP or DN in 10-foot increments. Utilizing this method, you can tweak your altitude by up to 200 feet. If you need to “tweak” your altitude by more than 200 feet, you’re really “changing” your altitude, so just dial in a new selected altitude and use another vertical mode (VS, IAS, or PIT) to make that change. If ATC gives you a new barometer setting, setting the new value on the G5/G3X is all you need to do. The autopilot will seek up or down a few feet to capture your selected altitude at the new pressure level.
  • GA (Go Around) mode is great! You’re allowed to use the autopilot down to 200 feet on an approach (which is the only exception to the STC’s “don’t use the autopilot below 800 feet AGL” limitation). When you get to your missed approach point or decision height and don’t have the runway environment in sight, push the throttle forward and let that index finger reach forward to touch the “GO AROUND” button as you do the rest of your gear/flaps cleanup. Your PFD will command a pitch up and straight-ahead climb, and the autopilot will immediately follow that guidance. Once the initial part of the go-around procedure is completed (which may be a straight-ahead climb to a specific altitude before making any turns), unsuspend your IFR navigator to resume waypoint sequencing, and select the appropriate lateral/vertical modes on the GFC 500. The autopilot will fly the entire missed procedure, including holds. Practice this in VMC first!
  • If you change navigation sources (i.e., toggle your navigator from GPS to VLOC), the autopilot will sense this, and the lateral mode will revert to ROL (wings level, it doesn’t even hold a heading). Thus, anytime you do any work on the navigator, do your scoreboard check afterward. Switching from GPS to a LOC or ILS course will require you to reselect the appropriate lateral mode. Speaking of which…
  • On an approach, should you use APR or NAV mode? The answer is easy: If the approach you’re flying has vertical guidance (think ILS, LPV, LNAV+V, or LNAV/VNAV), then use APR mode. If the approach you are flying doesn’t have vertical guidance (think LOC, VOR, or a BC approach), then use NAV mode. You can select either mode (NAV or APR) as soon as ATC has cleared you for the approach. You’ll see a green HDG for active mode, and a white annunciation for the armed mode.
  • Related to the last tip: one time, ATC had me on a vector (HDG mode) to intercept the GPS LPV approach, and I had APR mode armed. Shortly before intercept, ATC told me they were going to fly me through the course and then bring me back for sequencing purposes. If this happens to you, you’ll need to reach over and disable the armed approach by pressing the “APR” button to deselect the mode. This will leave HDG mode as the only active lateral mode and allow you to follow the controller’s vectors. Once inbound to the approach course again, reactive the approach by pressing “APR.” If you fail to deselect/deactivate the approach mode once it’s armed, the autopilot will turn you inbound, which is not what the controller told you to expect or do.
  • GPS approaches (especially LPV) are the simplest approach to fly with the GFC 500. If you have the option, choose it. There’s no mode to change when inbound, simply press “APR” when you get close, outside the FAF, and have been cleared for the approach. That’s it. Other approach types involve more button-pushing. Get out the flight manual supplement, grab a buddy in VMC, and go practice the detailed step-by-step in the manual until it is ingrained in your subconscious. IMC is not the place to figure things out.


I hope you found these tips helpful. The GFC 500 is an amazing piece of equipment, but as with all things in aviation, it demands respect for its capabilities and its limitations. Learn both well, practice before doing it for real, and leverage its ability to make your flying safer.


Taking Off and Going Around

The first phase of flight is just getting airborne and away from the ground on runway heading to 400 feet agl before making any turns, especially when IFR. The GFC 500 can help you there, too. Simply press the GO AROUND button, probably installed near your throttle, and the GFC 500’s flight director command bars will command a straight out wings-level climb at an appropriate pitch attitude, as specified by your aircraft make and model’s STC, and the lateral/vertical modes annunciated on your G5 or G3X Touch will both indicate “TO” for “takeoff.” 

Tip/trick: The same GO AROUND button also commands the pitch bars for a go-around at the start of a missed approach, and you’ll see the lateral and vertical modes annunciated as “GA.” How does the same button handle both “TO” and “GA”? If you’re not flying (i.e., your groundspeed is slower than 30 knots), the system knows you’re setting up for TO (takeoff); if you are flying, then you must want to GA (go around). Boy, this system is smart!

Tip/trick: Use of the “TO” mode is optional. I might use it for positive flight director guidance when departing into a very low overcast, but if the ceiling is at least 800 feet agl (the minimum autopilot engagement altitude), I actually find it easier to preselect my NAV/HDG and IAS values and modes, and just fly straight ahead on runway heading, ignoring any turn the flight director command bars are providing cues for, during that first phase of flight. It reduces workload a little, as my only task then becomes pressing the AP button at 800 feet agl—lateral and vertical modes/values were already set on the ground.


Troy Whistman is the father of three grown daughters and has been married 30 years to his lovely redhead bride, affectionately called “Lady Red.” Together, they base their airplane at the Mid-Way Regional Airport (KJWY) south of Dallas, Texas. Whistman holds a commercial airplane SEL certificate with instrument airplane rating. When not flying for fun to catch a sunrise or sunset, he enjoys using his toys as tools to help others: he flies for and is on the board of directors for Angel Flight South Central, and thinks flying kids for Challenge Air is some of the most rewarding flying he does. Send questions or comments to .

Part 1 of this series can be found here: Tips & Tricks for Flying with the Garmin GFC 500 AFCS Autopilot, Part 1



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Tips & Tricks for Flying with the Garmin GFC 500 AFCS Autopilot, Part 1

Tips & Tricks for Flying with the Garmin GFC 500 AFCS Autopilot, Part 1

Author TROY WHISTMAN walks you through preparation for a flight with the state-of-the-art Garmin GFC 500 digital autopilot.

With Garmin’s introduction of the GFC 500 digital autopilot, more accurately an Automatic Flight Control System (AFCS), capabilities previously absent in small General Aviation aircraft are now available for about $25,000 installed.

Your vintage Cessna may have come from the factory with a single-axis Cessna Nav-o-Matic that could “kind of” hold a heading or scallop along a VOR course utilizing analog inputs, while you managed altitude and trim settings. Newer digital systems like the GFC 500 from Garmin, the DFC90 from Avidyne, or the S-TEC 3100 from Genesys Aerosystems provide full two-axis pitch and roll control with amazing levels of accuracy and stability.

Full installations with all options can include automatic electric trim, allowing the autopilot to control your pitch trim; as well as yaw damper capability. The latter is an advanced feature previously reserved for cabin-class twins, single-engine turboprops, and jet aircraft.

A yaw damper (often abbreviated as YD) uses a dedicated servo to automatically provide just the right amount of rudder to keep the ball centered in all phases of flight. Passengers in the back seat enjoy a smoother ride without the side-to-side yaw or Dutch roll that can occur without a yaw damper, and it makes the pilot’s job easier—and the autopilot’s too.

This article will focus on how you can effectively utilize the features of the Garmin GFC 500 autopilot in VFR and IFR flight, including some tips and tricks learned after flying behind the unit in both VFR and IFR conditions since last summer.

My installation was covered in the October 2019 issue of Cessna Flyer and utilizes dual G5s and a Garmin GNS 430W. I’ve also flown from Oregon to Texas in a friend’s dual G5/GFC 500/GTN 650 setup. Where there are differences with the Garmin G3X Touch, or GTN navigators, I’ll try to point them out.  

First things first: RTFM (read the Flight Manuals)!

While it’s tempting to just “hop in and go” when new toys are installed in our airplanes, it’s critical that you have a solid understanding of the equipment before your feet leave the ground. For the GFC 500, that means reading not only the generic Pilot’s Guide but also the airframe-specific Airplane Flight Manual Supplement (AFMS), cover to cover.

Get to know all the symbology, failure modes, and how these are annunciated (as well as how to respond to them). Both referenced documents are available for free download from Garmin’s website. (“Annunciate” is used in this article to indicate a lighted display. This follows the usage in Garmin manuals. —Ed.)

Also consider watching training/familiarization videos, such as Sporty’s excellent in-depth one-hour webinar featuring a Garmin training specialist, which is available for free on YouTube. (Find a link below in Resources. —Ed.) 

Not much is more disconcerting than being in the clouds, wondering, “Why is the autopilot doing that?!” Which leads us to our first, and arguably the most important, topic… 

Know how to kill the autopilot

Whether the autopilot is misbehaving, or you need to make a course change right now to avoid traffic, or it’s time for you to hand-fly at the end of an approach, you need to know every means at your disposal to disconnect the autopilot and return control to you, the human pilot.

With the GFC 500, there are four ways to cancel autopilot operation:

1.) The most common way to disconnect the autopilot is to momentarily press and release the red “AP DISC/TRIM INT” button (“Autopilot Disconnect/Trim Interrupt”), typically installed on the left horn of the pilot’s yoke. A disconnect tone will sound, and an amber AP will be annunciated on the G5 or G3X autopilot status box.

If the optional yaw damper is installed, pressing this red button will also disconnect the yaw damper, and the amber YD will also annunciate to indicate this function is disabled. Since it disables all modes, including the yaw damper, this is the preferred method to quickly disconnect all autopilot modes before landing. 


Tip/trick: The disconnect tone is a series of three loud DOO-dah tones; pressing and releasing the button quickly more than once will cancel/mute additional tones, which can be nice for passengers (and you). 

2.) Use the manual electric pitch trim switches. This makes sense! If you have the optional automatic electric pitch trim, then the autopilot is managing trim while it is engaged. If you subsequently use the trim switches, the autopilot is going to (metaphorically) say, “Fine buddy, you wanna fly? She’s yours,” then play the disconnect tone and relinquish the flight controls.

When disconnecting the autopilot using this method, the yaw damper will stay engaged, so you can keep your feet flat on the floor while managing pitch and roll yourself. Do remember to disconnect the yaw damper before landing. You’ll want full control of the rudders! 

Tip/trick: I’ve modified my GUMPS checklist to be GUMPSY, a reminder to verify the YD is disabled before landing. 

3.) Press the “AP” key on the GMC 507 mode controller. This method will also leave the yaw damper enabled, until the “YD” key on the mode controller is pressed, or the red “AP DISC/TRIM INT” button is pressed.

4.) Pull the autopilot circuit breaker. Obviously, this is a final resort, but you should know in advance where the breaker that provides power to the system is located—commit it to memory. Pulling the breaker will remove power from all servos and the GMC 507 control head. I had my breaker installed at one end of the bottom left row on my avionics subpanel so I could find and pull it without looking. In the Cessna TR182 Turbo Skylane RG I fly, that’s down by my left leg on the sidewall. 

Tip/trick: Talk with your avionics installer about selecting a location that is easily identifiable by touch and feel. Being able to do this quickly (and “blind”) could save your life if you ever need it. 

One more note on regaining control from a misbehaving autopilot: If the autopilot is engaged, don’t try to overpower the GFC 500 without disengaging it first. The automatic electric trim will operate in the direction opposite your overpowering force, which could result in large out-of-trim forces when you finally do get smart and turn the autopilot off.

Always disengage the GFC 500 first, then hand fly.

In an extreme errant autopilot behavior scenario, rather than pressing and releasing the red “AP DISC/TRIM INT” button as described above, press and hold the button instead, while you hand fly.

Pressing and holding the button will both disconnect the autopilot and remove all power from the servos as long as the button is depressed, so they can’t fight against you. Then, reach down and pull that breaker (you know right where it is, remember?) before you release the red button. 

A quick tour: lateral on the left, status in the middle, vertical on the right

A quick look at the G5’s autopilot status line and the GMC 507 control head reveals some conscious thought about functional layout; both are divided into three sections.

The leftmost section displays (on the G5) and selects (on the GMC 507) your lateral navigation modes. You can track your heading bug (HDG) if you have a GMU 11 magnetometer installed, a specific GPS track (TRK), a NAV course (GPS, VOR, or LOC), or an approach (APR) with vertical guidance, such as an ILS or GPS LPV approach.

You’ll find the dedicated “HDG/TRK” knob is easier to use for setting the heading bug on the G5 than using the knob on the G5 itself. Train yourself to make all heading (and altitude) selections on the GMC 507.

The center section displays and selects autopilot status/modes. Enabling the autopilot (AP) will also enable the flight director (FD). The FD can also be enabled with the AP disabled, which is a cool mode where the GFC 500 will tell you how it would fly, but leave the flying up to you (see the “What About the Flight Director?” sidebar for more information).

The yaw damper is enabled/disabled with the “YD” button, and then, of course, there’s the famous blue “LVL” (Level) button, which will recover you to straight and level flight.

Important: Read the Limitations section and the Electronic Stability and Protection (ESP) details in the manuals to avoid surprises. The blue “LVL” button, and basic autopilot engagement, is predicated on not being outside certain limits of roll, pitch, and airspeed. Fly outside those specified limits, and none of the modes will engage.

Finally, the rightmost section covers your vertical navigation modes. A dedicated pitch trim/vertical speed wheel and “Altitude Select” knob provide for easy entry of desired values (see “The GMC 507 Vertical Mode Wheel” sidebar for more information on intuitive use of the wheel).

You can climb/descend in any of three modes: IAS (hold a specific airspeed), VS (hold a specific vertical speed), VNAV (track a GPS-computed descent profile; more on that next month). The “ALT” button enables altitude hold. Pushing the “ALT SEL” knob syncs the altitude bug to your current altitude.

White triangles illuminate above any selected mode to positively identify active selections, which you should cross-check on the status line of your PFD to ensure the desired modes are activated.

Modes that are active are depicted by green text on the PFD autopilot status line. Armed modes are depicted by white text. In the example depicted at left above, where we see VS +500 in green, and ALTS in white, it’s telling us the autopilot, and flight director are “actively” (green) set to climb the airplane at a vertical speed of 500 fpm up (+), and it’s “armed” (waiting) to capture the ALTS: the ALTitude you have pre-Selected with the altitude bug.

Gotcha: While Garmin follows this “lateral on the left, status in the middle, vertical on the right” layout convention consistently across the G500, G600, and G5 product lines, the G3X Touch swaps two of these, breaking the convention. The AP mode is on the left, the lateral mode in the center, and the vertical mode on the right. It’s not a deal-breaker, by any means, but something to be aware of for those flying the GFC 500 behind the G3X Touch.


Now that you know every possible way to disable the thing if you need to, and how the interface is laid out, let’s go fly! (Before disconnecting the autopilot, firmly grasp the controls and be prepared for the worst. It's also a good idea to glance at the aircraft's trim indicators to verify that the controls are trimmed for that flight condition. -Ed.) 

Setting things up before departure

OK, you’re sitting in the runup area, ready to depart. The autopilot is disabled and will stay that way for a while because you’ve RTFM and know that Section 2 (Limitations) of your AFMS specifically calls out 800 feet agl as the minimum altitude for autopilot engagement.

Legally, you can’t use the autopilot below 800 feet agl (except while on an approach, then you can use it down to 200 feet agl). Note your field elevation, add 800 feet: that’s the altitude at which you can enable the autopilot. However, you can preset some things to reduce your workload after departure, and you can use the Flight Director to guide you, even below 800 feet.

Using the GMC 507, let’s configure the autopilot for the lateral and vertical modes we’ll want to engage after departure, and assume that you’ve received the following departure instructions: 

“After departure, fly heading 180, climb and maintain 3,000 feet.”

Step-by-step, here’s how you’d set that up:

Set the heading bug on your PFD and HSI to 180 degrees by turning the “HDG/TRK” knob on the GMC 507 mode controller.

Press the “HDG” button to enable that mode. A white triangle will appear above the button, confirming it was selected. At the same time, you’ll see HDG appear in green as the active armed lateral mode on your PFD.

Astute pilots will notice that the FD (Flight Director) mode has now become active, even though we didn’t press any buttons. Magenta-colored Flight Director command bars will appear on your PFD, indicating the bank angle required (left or right) to turn from your current heading to the selected heading. You’ll also see a green PIT (pitch) vertical mode armed, even though you haven’t selected a vertical mode, because that’s the default vertical mode.

Set the altitude bug on your PFD to 3,000 feet msl by turning the “ALT SEL” knob on the GMC 507 mode controller while viewing the cyan altitude bug at the top of your PFD’s altitude tape. When you stop turning the knob, you’ll see the GFC 500 has armed ALTS mode, as indicated by white text, and it still plans to get there by flying a specific pitch (PIT is still in green).

We want to climb at an indicated airspeed to 3,000 feet msl, so press the “IAS” button. PIT will be replaced by green IAS 90 (or whatever Vy speed is defined in your airplane’s STC), indicating the aircraft will climb at 90 knots, and ALTS will be in white text, indicating that it’s “armed” (waiting) to capture the ALTitude you have pre-selected with the altitude bug.

A link to a video demonstration of this specific scenario is available in the resources for this article, if a picture is worth a thousand words to you.

We covered a specific scenario above, but other options are available to you as well to help you fly any departure clearance or route you might receive. Let’s talk about those while we’re here getting ready to depart. 

Lateral modes

If you’re going to track a course, make sure the correct flight plan is programmed in your GPS, or tuned and course selected if flying a VOR radial. Then, press the “NAV” mode button.

You might be flying a specific heading to intercept that course, so it’s totally legitimate to press “HDG” and then press “NAV,” in which case HDG mode will be active (green), and NAV mode will be armed (white). The autopilot will fly the heading until the NAV course starts to center and will smoothly intercept and track it. 

Vertical modes

I like to climb in indicated airspeed (IAS) mode and descend in vertical speed (VS) mode. If you’re climbing out of an area with terrain concerns, perhaps you’ll set Vx as your initial speed instead of the default Vy speed. You can adjust that speed using the little wheel in the vertical modes section of the GMC 507 (again, see “The GMC 507 Vertical Mode Wheel” sidebar for how to effectively use the wheel). 

Tip/Trick:  You can’t stall the airplane if you climb in IAS mode! If you have a long climb at a fixed vertical speed, and engine horsepower decreases as you climb, pitch must increase to maintain that vertical speed. Result? Airspeed drops off as you climb, threatening a stall. To prevent this possibility from ever occurring, just set an IAS mode for all climbs, and reserve VS mode for descents. 

OK, now that we have our GFC 500 configured to “fly heading 180, climb and maintain 3,000 feet,” we’re ready for departure. In the next installment, I’ll cover the use of the GFC 500 from takeoff to touchdown, as well as more tips and tricks for getting the most out of this incredible digital autopilot. 




The GMC 507 Vertical Mode Wheel

My first few flights, I found myself struggling with that little wheel in the vertical mode section on the GMC 507 autopilot mode controller, because it seemed to behave differently in VS versus IAS mode, even though it’s labeled “DN” at the top, and “UP” at the bottom. 

Scrolling the wheel up would increase the value when in IAS mode, but decrease the value when in VS mode. Exactly opposite each other! I never seemed to know in advance which direction to roll the wheel to get the value I wanted.

But it all made perfect sense when I started thinking of the little wheel as a trim wheel.

If you’re trying to set a higher vertical speed (climb rate) with the standard manual trim wheel in your airplane, you’d trim the wheel toward nose up (wheel down), while that same movement of the aircraft’s trim wheel would create a lower indicated airspeed (fly slower). 

The opposite is also true. Rolling the wheel toward nose down (wheel up) would give you a lower vertical speed/higher indicated airspeed. 

Thus, if you think of the wheel on the GMC 507 as a trim wheel, you’ll always turn it the right direction, regardless of which mode (IAS or VS) you have selected.


Troy Whistman is the father of three grown daughters and has been married 30 years to his lovely redhead bride, affectionately called “Lady Red.” Together, they base their airplane at the Mid-Way Regional Airport (KJWY) south of Dallas, Texas. Whistman holds a commercial airplane SEL certificate with instrument airplane rating. When not flying for fun, he enjoys using his toys as tools to help others: he flies for and is on the board of directors for Angel Flight South Central, and thinks flying kids for Challenge Air is some of the most rewarding flying he does. Send questions or comments to .

Part 2 of this series can be found here: Tips & Tricks for Flying with the Garmin GFC 500 AFCS Autopilot, Part 2



CFA Supporters

Further Reading/useful videos
Garmin GFC 500 manuals
Garmin VNAV tutorial
Sporty’s GFC 500 webinar
Configuring the GFC 500 for Departure 
 Garmin receives approval for the GFC 500 autopilot in the Cessna 180/185

 Garmin receives approval for the GFC 500 autopilot in the Cessna 180/185

April 24, 2019 – Garmin is pleased to announce it has received FAA Supplemental Type Certification (STC) for the GFC 500 autopilot in the Cessna 180/185.

 Intended for piston single-engine aircraft, the GFC 500 delivers superior in-flight characteristics, self-monitoring capabilities and minimal maintenance needs when compared to older generation autopilot systems.

Specific aircraft models approved for the GFC 500 autopilot include:

• Cessna 180 – Models 180, A, B, C, D, E, F, G, H, J, K

• Cessna 185 – Models 185, A, B, C, D, E, A185E, A185F

The GFC 500 autopilot uniquely integrates with the G5 electronic flight instrument or a combination of both the G5 and G500 TXi flight display to provide pilots with an economical and modern autopilot solution.

The autopilot mode controller contains large dedicated keys and knobs, a control wheel that allows for easy adjustments to aircraft pitch, airspeed and vertical speed and a level button that returns the aircraft to straight-and-level flight.

As a standard feature, pilots receive Garmin ESP with the GFC 500 autopilot, which works to assist the pilot in maintaining the aircraft in a stable flight condition. ESP functions independently of the autopilot and works in the background to help pilots avoid inadvertent flight attitudes or bank angles and provide airspeed protection while the pilot is hand-flying the aircraft.

In addition to traditional autopilot capabilities such as altitude hold, vertical speed and heading modes, the GFC 500 also includes:

• Premium functions and advanced capabilities such as altitude pre-select and indicated airspeed hold mode.

• Pilots can fly fully coupled descent Vertical Navigation (VNAV) profiles throughout the enroute and terminal phases of flight with a GTN 750/650 navigator.

• Pilots can select, couple and fly various instrument approaches, including GPS, ILS, VOR, LOC and back course approaches.

• Built-in GPS roll steering capability eliminates the need for external roll steering converters, allowing for smoother navigation tracking when installed with a compatible navigator.

• Level Mode, which automatically engages the autopilot to restore the aircraft to straight and level flight.

• Underspeed protection helps prevent the pilot from stalling the aircraft.

• Overspeed protection helps prevent the pilot from exceeding aircraft maximum speed (VNE).

• Pilots can fly coupled “go-arounds” during missed approach sequencing. A remotely-installed go-around button commands the Flight Director to display the appropriate pitch attitude required for the missed approach procedure and activates a loaded missed approach when paired with a GTN 650/750 navigator.

• An optional pitch-trim servo adds automatic trim and manual electric trim.

• With the addition of an optional yaw damper servo, Yaw Damping (YD) mode minimizes yawing oscillations while also helping to maintain coordinated flight by keeping the slip/skid indicator centered.

For customers who already have a G5 electronic flight instrument, the GFC 500 starts at a suggested retail price of $6,9951 for a 2-axis autopilot.

The GFC 500 can be purchased with the G5 electronic flight instrument for a suggested retail price of less than $10,0001.

Garmin continues to broaden its aircraft approval list for the GFC 600 and GFC 500 autopilots.

To view the most up-to-date aircraft STC list, to view certifications that are expected to begin in the next 12 months or to express interest in a specific aircraft make/model, visit or

For more information about Garmin’s full line of avionics, go to



1 Installation not included.


While this Cessna T182 Turbo Skylane has a Garmin 1000 glass display, it still has the same basic pitot and static systems as older aircraft.

Aircraft Instrument Systems: A Brief Guide

Do you know what instruments you can rely on to provide accurate information when the unexpected happens? A&P Mike Berry discloses what you absolutely need to know about your aircraft instruments. 

Aircraft instruments have been a part of aviation since the first flight of the Wright Flyer, which was equipped with a stopwatch, an anemometer (to measure wind speed) and a tachometer. 

With the increase of flight activity in the early years of aviation, aircraft instruments were invented to provide necessary information to pilots for precise control and navigation of their aircraft. 

As a pilot and aircraft owner, it is important to understand not only how aircraft instruments work, but also to be knowledgeable of the systems that they interface with. 

The maintenance and care of an aircraft, including its systems and required inspections, are tasks that the aircraft owner is responsible for—and they are not easy. 

In this article I will give some insight into instrument repair and replacement options as well as the maintenance and repair of systems that drive these systems. 

The basics, and some important questions

All modern aircraft, whether the aircraft has digital or analog instruments, share the same basic pitot and static systems. These systems deliver a very slight pressure to the instruments that they serve, and instrument accuracy is impacted by even slight variations. Leaks, disturbed air or even partial blockage in the lines serving instruments such as the altimeter, airspeed, and vertical speed indicators will certainly affect accuracy. 

There are other systems that are electrical or mechanical in nature and for the most part are self-energized such as the tachometer, oil pressure and oil temperature gauges. While the latest models of aircraft have electrically powered instrumentation, the majority of General Aviation aircraft still retain the self-powered instruments as a matter of reliability and economics. 

It is important as an aircraft owner and pilot to know the basics. In case of a total electrical failure, what instruments can you rely on to continue to provide you with accurate information? For example, fuel quantity gauges on most aircraft require electrical power and will not be reliable with the electrical system shut down. 

Consider the vacuum system that powers most General Aviation gyroscopic instruments such as an artificial horizon (AH) and gyroscopic heading indicator (DG). When a vacuum pump fails, what instruments can you rely on? 

Will your autopilot work? Will a failure of one vacuum instrument cause the other vacuum instruments to fail shortly thereafter? How about the old turn-and-bank or more modern turn coordinator instrument; how are they powered? 

Turn coordinators are electrically powered; a turn-and-bank is powered by vacuum from the engine-driven vacuum pump. 

The most important aspect of any gyroscopic instrument is that a failure may not be immediately noticeable unless the aircraft is equipped with a warning system. 

In the case of a failed pump supplying vacuum pressure to gyroscopic instruments, the instruments will decelerate and become inaccurate over a minute or two, not in mere seconds. This inaccuracy over time can cause a pilot to lose control of the aircraft by following a slowly dying gyro into the ground. Several fatal accidents have occurred over the years for just this reason, and a low vacuum warning can be a lifesaver. 

This panel of this Skylane is equipped with a low vacuum warning and voltmeter.
The rules concerning aircraft instruments

FAR 91.205 specifies required instruments for VFR flight for the most basic aircraft. These consist of an airspeed indicator, altimeter, compass, fuel quantity, oil temperature and pressure, and tachometer. These instruments must be operational for an aircraft to be considered airworthy. 

There may be additional required instruments associated with the specific operations of the aircraft (such as instrument flight rules) and even some instrument requirements specified by ADs, Type Certificate Data Sheets, flight manuals or supplements and STCs. 

It is up to the pilot in command to determine that the required instruments are operational before flight, and that the instruments are certified for the operation intended. While some instruments may legally be inoperative, consideration must be given as to how an inoperative instrument will affect the operation of the aircraft. 

Additional rules concerning aircraft instruments according to 14CFR 65.81, General Privileges and Limitations, are that “… a certificated mechanic… is not permitted to… accomplish any repair to or alteration of instruments. These activities are reserved for certificated repairmen at an authorized repair station.” 

This means that anything other than an external adjustment of an instrument—including installing a compass repair kit—is not authorized. 

Static systems test and inspection for IFR flight is required by FAR 91.411 and must be accomplished every 24 months or “Except for the use of system drain and alternate static pressure valves, following any opening and closing of the static pressure system, that system has been tested and inspected and found to comply with paragraph (a), appendix E, of part 43 of this chapter; and (3) Following installation or maintenance on the automatic pressure altitude reporting system of the ATC transponder where data correspondence error could be introduced, the integrated system has been tested, inspected, and found to comply with paragraph (c), appendix E, of part 43 of this chapter.” 

This means a certificated mechanic with the proper test equipment can certify only the static system (checking for leaks) and not the altimeter or transponder portion which is referenced in FAR 43 appendix E.

EGT instrumentation must use the correct color-coded wiring and may not be spliced, repaired, or in any way modified from the original configuration, including length.
How instruments operate, and why they fail

Traditional (steam gauge) aircraft instruments can be grouped according 
to their operating systems. 

Instruments such as these are often sold on eBay in unknown condition, and may not operate or cannot be repaired. If you are buying an instrument at a flea market or on eBay, you should not only ensure that it can be repaired and certified, but make certain that it is appropriate to your aircraft.
Pressure instruments

Pressure flight instruments operate off of the static and pitot system, are self-powered and extremely sensitive diaphragm-type instruments relying only on variations in pressure to operate. These pressure variations are transmitted mechanically by gears and a jeweled movement as a result of the extension and retractions of the diaphragm. 

As with anything mechanical, age takes its toll on the accuracy of pressure instruments such as the airspeed, altimeter and vertical speed indicator (VSI). These instruments are affected by moisture as well as dust and dirt, and should be kept clean. 

Cloudy or dusty-looking instruments may mean that the system is contaminated and the static system must be purged of moisture or dust and the instruments promptly repaired or replaced. Leakage sometimes occurs between the instrument glass and outer case as well as inside system fittings. Sealants become inflexible over time and lose their ability to keep the system closed. Leakage must not be tolerated, as the accuracy of all the instruments in that system is compromised. 

Aircraft instruments are delicate and require special equipment and training to be successfully repaired.

Instrument repair shops operate as FAA approved repair stations and while all instrument shops adhere to the same FAA rules, some shops may be authorized to do specific repairs while others may not.
Vacuum instruments

Vacuum operated (gyroscopic) instruments have been very reliable over the years, with very few actual failures of the instruments themselves; however, these instruments are subject to malfunction when an aircraft vacuum system fails. 

Vacuum system failures can be prevented with proper care and maintenance (or replacement of components) as specified by the aircraft manufacturer. 

One often-overlooked procedure is to check the vacuum gauge reading in your aircraft against a calibrated gauge. This ensures that the actual vacuum/pressure is set correctly, as over-pressure or under-pressure compromises accuracy, increases wear and creates an opportunity for failure of instruments or the entire system. Another often-overlooked but recommended procedure is to replace both pressure and vacuum filters on an annual basis. 

When replacing a vacuum pump due to a failure, ensure that all hoses, filters and fittings are checked for contamination from foreign material as not only is the newly-installed pump at risk of failure, the instruments may also fail due to foreign material contamination. 

Vacuum instruments are mechanical devices that operate with a gyro spinning at high speed powered by jets of vacuum or pressure impacting on small cups machined into the gyro rotor. The precision-balanced rotor is suspended by a shaft and supported by tiny bearings which are lubricated when the instrument is assembled. There is no provision for lubrication other than when the unit is disassembled during maintenance or repair. 

Gyros rarely fail without some type of warning which may be indicated by excessive drift or precession, noisy or erratic operation. Inactivity really takes its toll on these instruments as the lubrication that is on the tiny bearings tends to drip or wick away from the actual bearing surfaces when the instrument is at rest for long periods of time. 

Inaccurate readings of an airspeed indicator can have a definite impact on performance and overall safety. This example is unairworthy.
Electric instruments

Electrically powered instruments can be of several different configurations, from a simple fuel quantity sender or flap position sender (variable resistor) and indicator, to an electrical tachometer powered by a small generator (though a flexible mechanical cable between the engine and the gauge in the instrument panel is more common). 

EGT and CHT gauges are usually self-powered relying on dissimilar metals in the sender or sensor to generate an electrical signal directly to the gauge on the instrument panel. The color coding of the wires is important as senders with different color coding than the instrument will not be compatible. 

Sending units and wiring for CHT/EGT gauges must not be repaired, spliced, or in any way modified from the original configuration—including length. If it’s broken, replace it. 

Anything electrical is subject to the effects of vibration, corrosion and broken (open) connections; remember this in your troubleshooting routine. 

Also significant in any electrical instrument installation is that individual components of a system are in most cases not interchangeable. For example, a Rochester brand gauge must be connected to a specific type of sender unit intended for use with the Rochester gauge; a Stewart Warner brand sender may not work properly with a Rochester gauge. 

Mistakes can be costly; check the schematic diagrams for the proper wiring, refer to the parts manual for the compatible component, and physically check that the item is what is actually installed in the aircraft you are working on. 

Electrical components do wear out and/or deteriorate over time and malfunction, even if the item is rarely used. Good preventive maintenance practices—such as keeping moisture off of connections, proper routing and attachment of wiring, and reducing airframe vibration—can go a long way in avoiding premature instrument and electrical failures. 

The compass is a required instrument and must have a correction card. According to 14CFR 65.81, General Privileges and Limitations, repair to or alteration of instruments are reserved for certificated repairmen at an authorized repair station.
Repair options

Finding a shop that will work on older instruments is becoming difficult if not impossible, and often owner-pilots are left with no option but to replace an instrument. 

The rules of requiring approved technical data covering repairs and overhauls, approved parts sourcing and proper repair and test equipment are alive and well in the aircraft instrument arena. For this reason, many instruments that were original equipment on General Aviation aircraft 30 to 50 years ago are no longer supported and are not repairable. 

Fuel gauges must be operational, and most can be repaired.
Authorized shops

Instrument repair shops operate as FAA approved repair stations and while all instrument shops adhere to the same FAA rules, some shops may be authorized to do repairs while others may not. 

Do some checking around to see if 
you can find a shop that does repair older instruments. There are some, such as Air Parts of Lock Haven, that specialize in older aircraft instruments and in fact have repair station authority to do extensive repairs. 

Air Parts of Lock Haven also has access to repair parts sources that other shops may not have. Air Parts of Lock Haven repairs older instruments and can also duplicate original instrument dials and faces. 

Many airspeed indicators are aircraft-specific; know your requirements before purchase.
Radioactive components

Many instruments that were supplied as original equipment in the 1940s and 1950s and even into the 1960s came with luminous dials and markings which happen to be radioactive and are now considered hazardous material. 

If you have one of these instruments, it must be shipped as hazardous material with all the markings, shipping labels and details that pertain to hazardous material. Few shops are equipped to handle this material and will refuse the shipment. 

At last check, Air Parts of Lock Haven can receive these instruments and has authority to handle the material, but the instrument will not be returned with the radioactive dials.

Fuel quantity senders wear out and need to be serviced by an approved facility. 
General shipping information

Any instrument that requires shipment to a repair shop must be packaged properly—as if you were shipping eggs—and the package should be marked as fragile and insured. 

It would be prudent to call the instrument shop you are shipping to and ask for a carton to ship an instrument in and wait a few days for the container to arrive rather than risk damage to the instrument in shipping. 

Unfortunately, shipping companies can and do damage aviation material—and an insurance adjuster’s value of the instrument may be much less than what a functional instrument may actually cost. 

Post World War II instruments such as the ones installed in this early model Cessna 172 may have luminous dials that contain radium. Few repair shops are equipped to handle this material and may refuse the shipment.
Buyer beware

A word to the wise: if you are buying an instrument at a flea market or on eBay, not only ensure that it can be repaired and certified, but make certain that it is appropriate to your aircraft.

Markings on a replacement airspeed indicator, for example, must be specific to the make and model of aircraft, and the details may be found in an official flight manual, TCDS, STC-related flight manual supplement, or even AD notes or Service Bulletins. 

If you send in an airspeed indicator with a specific aircraft manufacturers’ part number, what you will get back is a repaired or a replacement airspeed indicator that will have the markings appropriate to that particular part number—which may or may not have the correct markings for your aircraft. There is no choice here as to changing the markings, and adding or deleting marks is not permitted by the FAA. 

The importance of accuracy for performance

The performance listed for your aircraft was obtained when the aircraft, engine and propeller were new, and the aircraft was rigged properly, loaded to the most favorable center of gravity location and flown by a test pilot under optimum atmospheric conditions with accurate instrumentation. While it is possible to duplicate the published performance numbers with an older aircraft, everything must be nearly perfect to do so, and accurate instrumentation plays a big role. 

Although digital instrumentation is replacing analog instruments and equipment, much of the instrumentation still relies on precise pitot or static system pressure which is then delivered to the computer or other device to indicate airspeed, altitude or vertical speed. 

So, unless you have precise pressure, the 78 knots indicated you are using to achieve best rate of climb may not be exactly 78 knots. In addition, mechanical tachometers, whether due to age or inactivity, have a history of being inaccurate. 

Inaccurate readings from just these two instruments—airspeed and tachometer—can have a very definite impact on performance and overall safety, as the aircraft will not achieve published performance numbers. 

Most, if not all, aircraft maintenance shops have tachometer checking equipment and the calibrated tachometer checker should be used to compare required static rpm listed on the TCDS to the aircraft’s actual full-throttle revolutions per minute. An aircraft tachometer can easily differ from the published requirements by 100 rpm or more and some aircraft are rejected during annual inspection because of this. 
Practical application

Any aircraft owner knows that aircraft are expensive to maintain and there is no indication that costs will come down. Aircraft instruments are no exception; however, there are some economical ways to determine if you do have instruments that are in need of repair or replacement. 

System leaks

Static system leaks, for example, can often be discovered by some simple tests. Does the VSI, airspeed or altimeter needle move when a door or window is closed or opened while on the ground with the engine not running? When you open the cabin heat valve or a window in flight, do any of the three instruments just mentioned move abruptly? 

Unusual temporary indications may indicate a leaking system component such as an alternate static port, leaking instrument glass or a broken or cracked moisture trap. 

Altitude discrepancies

Also consider the effect of modifications to your aircraft, as these may impact the static system and overall instrument accuracy. An example of this was an aircraft that was modified with a cargo pod and several electronic sensors for aerial survey operations. 

When the modifications were completed, the aircraft was test flown and at higher altitudes (in the teens). An instrument accuracy check revealed a 900-foot error in the actual altitude versus indicated altitude. 

Errors such as this are rare, but can happen, so be especially vigilant when multiple modifications are made to an aircraft. The possible combined effect these may have on actual versus indicated altitude is worth examining. 

An unofficial altitude comparison can be made between a GPS unit’s derived altitude and the indicated altitude while in flight. Large errors—such as a difference of a few hundred feet or more—should be cause for further investigation into pressure instrument (altimeter) and static system accuracy. 

Electrical fuel quantity

Fuel gauges are another set of instruments that are known to be inaccurate, yet pilots rely on them. A typical electrical fuel quantity system on General Aviation aircraft consists of three parts: the sending unit (using a variable resistor attached to a mechanical arm/float), electrical wiring, and an indicator in the cockpit. 

The sending unit attached to the fuel tank can fail mechanically or electrically, or provide inaccurate readings as both parts can wear or age. The float can absorb fuel and partially sink, providing an erroneous indication. Electrical wiring can become corroded or disconnected, and if a complete circuit is not maintained, may indicate full all the time (or empty all the time). 

Some basic troubleshooting by a technician with a voltmeter and schematic can determine the offending component fairly quickly—especially when the plane is opened up for annual inspection. 

A fuel gauge indicating the quantity of fuel in each tank is one of the required instruments according to FAR 91.205, and most (if not all) components—even on the most ancient aircraft—can be repaired or replaced to make the system work properly. 

Be proactive

As a pilot or aircraft owner/operator it is very important that you properly maintain aircraft instruments and associated systems as well as seek repairs or replacement at the first sign of any deficiency. Operating an aircraft with a faulty or inoperative instrument can have serious consequences. 

Maintenance personnel conducting an annual or 100-hour inspection should not return an aircraft to service, and pilots should not conduct flights with inoperative instrumentation or equipment required by FAR 91.205. 

Michael Berry, a former aircraft repair shop owner, is a multi-engine rated ATP (757/727). In addition, he’s a turbo jet flight engineer, an A&P/IA mechanic, airplane owner and 121 air carrier captain. Berry has 15,000-plus pilot hours. Send questions or comments to .


Air Parts of Lock Haven

Further reading
FAR 91.205
“Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements”

FAR 91.411
“Altimeter system and altitude reporting equipment tests and inspections”

Appendix E to FAR Part 43 
“Altimeter System Test and Inspection”

14CFR 65.81 
“General Privileges and Limitations”

All of the above documents are available at the FAA website: