Garmin autopilot
8 min read

The answer is, whenever the autopilot maneuvers the airplane in some way you didn’t expect, or don’t fully understand, disengage the autopilot and hand fly.

If you ask yourself “what’s it doing now” the immediate answer should be to punch it off.

Autopilots can and do occasionally fail. When they do, the certification theory is that the human pilot is always in position and fully capable of taking over control of the airplane. That’s why the flight manual supplement limitations for autopilot use all decree that a qualified pilot must be seated at the controls.

But a more frequent failure of automatic flight is the human pilot’s lack of understanding of the autopilot modes and operation. A real hardware/software failure of an autopilot could lead to a dangerous situation, but so can pilot mismanagement of a fully functioning autopilot. The results are essentially the same in either situation—the pilot in command is not fully in control of the airplane.

Garmin autopilot

That level button is there to help the pilot, not the avionics.

Avidyne and Garmin, and some other autopilot makers, are very aware of gaps in pilots’ complete understanding of autopilot operation. That’s a reason they pioneered the “straight and level” mode. If you are not completely aware of what the autopilot is doing, or what the attitude and rate of change of the airplane is, pressing the straight and level mode button levels the wings and holds a safe pitch attitude.

For much of the history of general aviation autopilots were probably much more reliable than the sensors they require. For example, in the bad old days when we relied on vacuum pumps to stay alive while flying in the clouds, a pump failure would disable the autopilot as well as deprive us of essential gyro attitude and heading information. Even when the pump worked, the spinning metal gyro reliability was not good. And even if the gyro continued to spin, the electrical elements that transmitted the gyro data to the autopilot could and did fail.

Reliability—plus low cost—was a selling point for S-TEC rate-based autopilots. Those systems use a turn coordinator gyro to derive a sort of wings level information for the autopilot. Because the system doesn’t really know if the wings are level, only that the airplane is turning or not, they don’t fly with the same precision as a conventional autopilot. But the S-TEC would keep working after the vacuum pump or attitude gyro failed, and that was an important safety advantage.

The better solution that is happily becoming more available and common in all types of airplanes is the attitude heading reference system (AHRS) that uses non-moving electronic sensors to calculate attitude and heading and yaw. With no moving parts the AHRS is much more reliable than the vacuum pump/spinning metal rotor gyro. And because AHRS use so little electrical power it’s easy to install a battery that keeps the AHRS working for 30 minutes, or much more, after a total electrical failure.

With AHRS providing the essential aircraft attitude data, and vastly improved components in the autopilot itself, the reliability of automatic flight control is much better than even a decade ago. But I’m not sure all pilots are receiving the training necessary to safely and fully use the automatic flight control systems. At least that’s true at the piston airplane level.

Think about it. Will you find even a basic autopilot in a training airplane? Almost never, except at universities and major flight academies where the curriculum is designed to create professional pilots. Everybody in aviation knows that when a pilot moves beyond the basic singles used for training they will use an autopilot, but when will they be trained? Sadly, many never are, at least in a comprehensive way.

I believe one of the best ways to learn how your autopilot works is to perform the complete preflight test that is described—often even required—in the flight manual supplement in your POH.

The autopilot test will confirm that the different methods to disengage the autopilot actually work. Most autopilots have several ways to disengage the system. There is a button under your thumb on the control wheel horn that, when pushed, disengages the autopilot. In most systems pushing the button all the way down also removes power from the electric pitch trim. If the autopilot has full pitch functions, moving the electric pitch trim button on the yoke with the autopilot engaged will disengage it. And there is a button or lever on the autopilot mode control to disengage. All of those methods to disengage the autopilot are checked during the preflight test.

Avidyne panel Cirrus

You do test the autopilot before every flight, right?

Any newer full function autopilot will have a “split” electric trim switch. That means that two switches must close to send power to the electric pitch trim servo. In most systems the trim switch under your thumb has two halves, and both halves of the switch must be moved together to activate the pitch trim. In others you push down and then move the switch fore or aft for trim. Either way, you’re checking the circuity, and safety, by moving only half the switch at a time to make sure the trim doesn’t activate.

Most newer autopilots also have some level of monitoring. The monitor circuit is primarily concerned with un-commanded pitch trim movement, which is potentially the most hazardous automatic flight control system failure. During the preflight check you will press a button or touch a key to test the continuity of the monitor circuit.

Finally—and I believe the most important aspect of the autopilot check—is that you test the servo clutches. Autopilot servos—the devices that actually move the flight controls—have a system that limits their authority. In other words, the muscle of the autopilot has low limits.

To test the actual strength, as it were, of the autopilot, you engage it on the ground, and then move the flight controls to be sure you can easily overpower the system. You’ll be surprised how little effort is required to overpower the autopilot servo. That teaches you that if the autopilot goes crazy and for whatever extremely rare circumstance won’t disengage, you can take command manually.

The even more critical preflight test is to engage the autopilot in a pitch mode and pull and then push on the controls. When you pull back with the autopilot engaged, after a brief delay, the pitch trim should start to run nose-down. Push on the controls and the pitch trim should run nose-up.

Because the autopilot servos are comparatively weak, they must keep the airplane in trim in order to hold the desired pitch attitude and/or altitude. When the autopilot senses an out-of-trim force it commands the pitch trim servo to move until the force is relieved. That is exactly the same logic we use when hand flying.

What is so vital—and has undoubtedly been the cause of many accidents—is to learn from the preflight test that if you push or pull on the wheel while the autopilot is engaged, you will cause the system to mis-trim the airplane. If you continue to push or pull long enough with the autopilot engaged, the trim force could become so great it overpowers you. It’s easy to overpower the autopilot, but may be impossible to overpower the trim.

I remember years ago being at the old King Radio hangar in Olathe, Kansas, the day after a lucky, but untrained, pair of A36 Bonanza pilots arrived hopping mad because the King autopilot had tried to kill them. The autopilot was doing something the pilot didn’t like, or understand, so he grabbed the controls to take over. Before long, according to him, the autopilot gained superhuman strength. If he hadn’t had a pilot friend in the right seat joining him to pull on the control wheel the damn autopilot would have flown them both to their death.

What happened, of course, is the pilot didn’t disengage the autopilot before pulling on the wheel. For several seconds he easily overpowered the autopilot pitch servo, but his pulling caused the pitch trim servo to run nose down. Before long the trim tabs were full nose down and the stick force was incredible. He and his buddy wrestled the Bonanza to the runway without ever thinking of reaching over to the trim wheel to retrim the airplane, all the while convinced the autopilot was trying to kill them.

And that’s the last step in the preflight test. Run the electric trim and grab the trim wheel to be sure you can physically stop its motion. If you can’t keep the trim wheel from moving with your thumb on the electric pitch trim, don’t fly the airplane until it’s repaired.

Finally, when you’re reading the POH autopilot supplement check the altitude limitations for autopilot engagement. On a precision approach the minimum engaged altitude is most likely 200 feet, the standard DH for an ILS or LPV approach. But there may be restrictions on how much flap can be extended during the coupled approach. On departure there will be a limitation on minimum AGL altitude for autopilot engagement, usually 400 feet in larger airplanes, but often 1,000 feet in lighter airplanes. There are also typically minimum altitudes for autopilot use in cruise. These minimums are the result of worst case failure flight testing and demonstrated that a trained and competent pilot had sufficient altitude to recover after the failure.

Autopilots offer huge potential safety. That’s why you can’t fly single-pilot for hire, or in a jet, without them. But all of that safety potential can turn into a hazard if the human pilot isn’t fully trained on autopilot use.

Mac McClellan
13 replies
  1. Enderson Rafael
    Enderson Rafael says:

    Although focused on light aircraft systems, is impossible to not relate this article to airliners, specially when both Lion Air and Ethiopian MAX crews never disengaged the Auto Throttle, which was one the first items of any possible failure they should have been able to recognize (unreliable airspeed or runaway stabilizer), and this, way more than the MCAS itself, led to the unfortunate fate of those flights. The modes and inhibits of autopilot systems have an overwhelming complexity, and it is not that rare to catch yourself having second thoughts about “what is it doing”. There is even that joke that says that the difference between a junior pilot and an old one is that the junior says “what is it doing?” while the old pilot says “I does it once in a while”. But, jokes apart, I imagine how many pilots learned that A330s stall when on Alternate Law, or that Boeing 777s inhibit the TOGA button after touchdown thanks to AF447 and EK521. Thanks for sharing this article, Mac!

    • James Macklin
      James Macklin says:

      Disengaging the autopilot and assuming hand flying is a skill. Pushing the RED BUTTON and inputting a control such as a descent can introduce an unexpected turn. Pushing your hand forward often includes a roll that can lead to spacial disorientation.
      Think JFK Jr probably died this way.

  2. Michael
    Michael says:

    Good advice. However, “runaway trim” happened to me twice in my Mooney Ovation using AFC-700.
    In The first time, using my FLC to a defined altitude, the AP did not level at altitude but instead, trimmed the plane all the way aft. After disengaging the AP, and pullOmg the circuit breaker It was almost impossible to overcome the trim by hand. I then, reduced the spread the deprive the stabilizer of access control and manually used the wheel trim forward. It worked and I was able to stabilize the flight. In the second incident, it happened to me at take off, the limb looked like an F-16 going straight up. Learning from the first incident, I reduced the spread and trimmed manually down, didn’t help the sweating, by stabilized the flight. Put all the aero optic skills to work, act cool and logically and you will prevail. Fly the plane or it will kill you if you don’t.
    Fly safe.

  3. Bob Yarmey
    Bob Yarmey says:

    Capt Mac,
    Fantastic article! During frequent BFR’s, IPCs and initial instrument training I have seen first hand the dire misactions you have so astutely articulated. Of great concern to me are the low experienced instrument pilots flying TAA aircraft who have wrongfully allowed themselves to turn into “push button” flyers. Perfect case in point is the CJ4 owner operator who needlessly augered in with several trusting and unwitting passengers into Lake Erie minutes after a night departure out if Burke Lakefront a few years back.
    I sincerely hope many pilots read your article and take it to heart. Thank you for sharing your invaluable wisdom. For many years, you have been a true apostle for aviation safety.

    Senior instructor,
    Professional Instrument Courses.

  4. Pat
    Pat says:

    First, let me say, I love my auto-pilot, second only to my wife and dogs. I fly a Premier Jet single pilot and as Mac stated the AP is a required item for that. The AP almost universally does a more precise and smoother job of every task of directing the airplane than do I. But I wholeheartedly agree that a thorough understanding of what your AP is capable of doing and even how it does it is important. When I first started flying the Premier, six years ago, almost every “what’s it doing?” was pilot induced and not the fault of the AP. When I am hand flying the airplane I feel like a “worker bee”. When the AP is flying the airplane I feel like I am in “management” and responsible to make sure that my electronic employees are performing their assigned tasks as directed. It’s kind of like working your way up to management having started on the production floor. You know what you expect of your electronic employees because you have done it yourself for years. And, I hope that I don’t lose my medical for admitting that, I occasionally converse with my electronic employee as if it was my co-pilot, almost always offering praise and only rarely offering a reprimand by pushing the LRB and taking control.

      Mac MCCLELLAN says:

      Hi Pat,
      Your analogy of being an airplane manager of the autopilot is right on. That’s exactly what you are doing.
      When I first started flying jets 40 years ago the standard was that the pilot flying didn’t do anything but fly. He was a human autopilot. You couldn’t review a chart, look at a checklist, or even reach around for switches if you were the one flying. To do any of those tasks required you to “hand the controls” to the pilot not flying.
      We can do the same with two human pilots, but when flying solo it’s the autopilot we need to manage and monitor exactly as you would a second human at the controls.
      Mac Mc

  5. Stephen Shore
    Stephen Shore says:

    Great article and spot on. I flew for years with no AP. I got my IFR rating and never flew with an AP because I did not have one. Consequently, even with alot of experience flying airplanes, I had zero knowledge of AP’s because I never had one.

    Fast forward to a few years ago when I bought an airplane with an AP – King KFC200. I subsequently upgraded the panel including an Aspen ProMax with an AHRS adapter and even an altitude pre-select add on.

    As is usual, the avionics shop was zero help in getting me educated in how to use my legacy KFC 200 with the newer glass panel items I installed (GTN 750 with the Aspen ProMax). My CFI was of no help either.

    If it were not for eBay (purchase of old King manuals) and YouTube (instructional videos on how to fly my AP) I never would have been able to learn how to use the AP with the new equipment. Even with the above tools, it took me many months before I became confident enough to use the AP on coupled approaches in actual IMC. I tried a few early on and things went “hay-wire” and it was a long time before I had the courage to fly an approach without me actually doing the flying!

    BTW – now that I know how to use the darn thing, I definitely see how pilots can get overly addicted to having one. I can see how hand flying skills can quickly erode from over reliance on this great tool. I tend to hand fly about half of my approaches just because I am concerned about losing my ability to do so if I don’t do it! But on the other hand, if I go too long without using the darn AP I get a little concerned about losing the memory of the routine of using the AP on an approach!

    Kind of crazy how this new technology can be a great help and a hindrance at the same time. Of course it does not help that I am 57 years old!

  6. Dan Schwartz
    Dan Schwartz says:

    Excellent article. I always have my students demonstrate multiple ways to “kill” the autopilot & electric trim (5 on the Da-42). It surprises me how many pilots during a Flight Review or IPC (in their own familiar airplane) skip this step during run up.

  7. Karrpilot
    Karrpilot says:

    I fly a 1978 Cessna 182 RG. It has an auto pilot, but I never use it. I have engaged it from time to time, just to make sure it’s still functional. But it’s original to the airplane, and quite sloppy. I can definitely fly said airplane better and more precise myself.

      Mac MCCLELLAN says:

      You’re right, Karrpilot, the ARC autopilot in your Skylane probably doesn’t fly the airplane very well.
      ARC autopilots use “torquers” instead of conventional servos. In a servo a small electric motor uses a gear train to produce the power to move the flight controls as commanded by the autopilot. In the ARC system the “torquer” responds by moving in proportion to the amount of electrical power the autopilot sends to it. So the reaction of the “torquer” is somewhat slower, and lacks precision depending on many factors, not the least of which is the air loads fed back by the control surfaces.
      ARC autopilots can do a decent job of flying the airplane but only if they are carefully adjusted by an experienced technician, a person who is increasingly difficult to find. Among the most critical adjustments are control cable tension. Any extra slop in the controls, or the bridle cable that links the “torquer” to the primary control cable, will cause the autopilot to slop around as you describe.
      If you ever get the chance to fly a modern autopilot you will be amazed. A good autopilot flies the airplane much better than a human ever can because it is never distracted. We must scan and comprehend a variety of sensory inputs to then know how to move the controls to fly the airplane with precision. An autopilot never scans. Its channels are each focused on one input and it monitors those data 100 percent of the time.
      We human pilots may come close to matching the precision of a good autopilot for brief periods, but our attention span and ability to concentrate lasts only so long, but the autopilot won’t waiver no matter how long the flight.
      Mac Mc

  8. Doyle Frost
    Doyle Frost says:

    Never forget what we went through with a brand new KC-135, fresh from Seattle, at the base I was stationed at as a young autopilot tech. At Debriefing, the entire cabin crew, was ready to kill me. Every time the autopilot was engaged, or disengaged, it immediately started a 1,000 foot porpoising maneuver. None of the crew were ready for that. They were extremely upset, having to “hand fly” that plane for six hours when flying from the west coast all the way to the east coast, where we were located. (Six months later, we discovered a “cold solder joint” on the A/P man amplifier cable bundle, to the elevator servo. Any inclement weather could cause a similar action.)

  9. Jared Cox
    Jared Cox says:

    I would love to talk to Mac about this article, because it sounds like a situation I had 3 years ago in my 2000 A36 that nearly killed me. Until recently, I have accepted the answers my local mechanic gave me about the issue. However, the answers I was given are a little vague, and I don’t think the mechanics were sure. I would greatly appreciate the opportunity to connect with Mac and see if he can help me better understand what happened that day.


Leave a Reply

Want to join the discussion?
Feel free to contribute!

Leave a Reply

Your email address will not be published. Required fields are marked *