SR20 G2 & STEC55x autopilot

Has anyone had any problems with the autopilot in SR20 porpoising?

Whenever the autopilots tries to level off from a climb, it over shoots the intended altitude, then it tries to correct itself and puts the aircraft into a descend, again it passes through the intended altitude and climb again, this cycle repeats a number of times generally within + -100 ft. So I help the autopilot with levelling off !!!

I was told it may be the servo motor and if not it will be the autopilot itself.

To test it, need a new motor fitted (needs to be ordered), followed by a flight test, and if it still porpoises, then test with a new autopilot! This isn’t going to be cheap I guess, so has anyone had a similar experience?

Check your bridle cable tension coming from the auto pilot servo.


Before you diagnose it by throwing new parts at it (never the best strategy), have you had them check the rigging? Also, it may just need need new brushes in the servo. There are many, many threads about porpoising in our Forums, so I suggest you join so you can access those. The $65 could save you literally thousands.

Thanks Kelly and Gordon for your quick responses.

Firstly I will be joining the COPA, it is definitely worth the money.

Replacing the motor etc was an Avionics engineers’ advise, however I hesitated on that, because it didn’t sound right to keep changing things in order to fix a fault with no certainty at any stage!!!

Surely there has to be a way of checking the autopilot’s components without having to remove them totally!

I guess I now need to learn as much as I can before I go back for a fix or I end up with a new installation!!!

Smart approach. Plenty to learn from many threads on the subject.

What is your Serial number?

Altitude transducer common problem causing this.


Andy, thanks for your response.

I did test the VS option and interestingly it could manage the height better than the ALT hold or at least it didn’t porpoise.

Do you know of any flight or ground testing that I can do to diagnose without removing/disturbing any of the components.


No, that’s beyond my capabilities.

BTW, I noticed a typo…I meant “altitude” tranducer. I corrected it.

Autopilot Central in Tulsa, an Cirrus SC, are extremely capable on autopilot issues. You might give them a call.


Thanks Andy. I knew what you meant by the transducer.

I will call them tomorrow, hopefully they will be able to help, although I am actually in UK!

I have a 2000 SR20. I found if all is normal and the lubrication on the trim piston is good then occasionally when the winds aloft are at an unusual angle and velocity, I get a funny oscillation. Sometimes I have to monitor it. Otherwise it holds well and I have never had to do anything but make sure the motor was responding and is tight and there was no unnecessary drag on the trim due to poor lubrication. Lubricate the trim rod on the ground and exercise it to see if there is hesitation in movement etc. Check cable tension as well. Another words look at the basics first before piling money into the avionics for no reason.


Thank you Andy,

I contacted “Autopilot Central”, and got some ideas as how to go about it. Very helpful company specifically the person I spoke with (Barry).

I think it will be the Pitch Trim Motor, however I have to get the Altitude Transducer tested first.

AL, thank you for the detailed advise. I will do the lubrication. Although, it seems the cause of the problem is going to be the pitch trim motor if the Altitude transducer checks OK.

Mo I owned a 20 within 10 numbers of yours. Bought it new in 05. The airplane had porpoising issues from day one. I replaced everything from autopilot, servos, etc etc. Cirrus paid most of the bills even after the warranty expired. After five years the factory took back the airplane. The issue with my 20 turned out to be that it was originally manufactured as an SRV. When I went to the factory in 2005 to pick up my new 20, it was not ready to go. I was lined up to get #1500 but it would not pass flight testing so Cirrus pulled 1467 off of the shelf and reconfigured it as a 20 and threw in traffic and some other goodies. The reconfiguration of 1467 was done in the shop and not in the factory and was botched. Cirrus ultimately did the right thing and pulled the aircraft and apparently got it fixed but I went through 5 years of hell before getting out from under the problem. Check to see if 1458 was originally manufactured as a 20 or as an SRV. If the same tech worked on both planes, then the same mistake could have been made. I do not know what the final problem was that caused the problem on 1467, but Bill King at Cirrus could find out for you. Good luck.

Wow Steven that is quite the stunning story. I am glad Cirrus FINALLY did the right thing but what an ordeal that must have been! Thanks for the post.

Jim The rest of the story is that Cirrus(Bill KIng) worked out a deal to get me into an 05 22 and that airplane has been flawless.


Many thanks for sharing your experience and I am glad that you finally had a positive resolution.

As far as I know the aircraft was registered in the UK by the Cirrus dealer from new as an SR20-G2 (previously registered in US as N410CD).

I don’t think the SR20 G1 and early G2 had a good Autopilot set up, because in G3 they improved the design, which improved the reliability and performance (or at least in the one I use for training).

At the moment I am investing on a “Static Line Sump” (which is the SB2x-34-22R2) to prevent moisture getting into the system. I was told by the avionics engineer who tested the motor, the cause of the problem seem to be the pitch trim motor. Having said that, I won’t be fixing the problem, unless the maintenance company guarantees that replacing the motor will fix the autopilot otherwise they will have to refit the original.

I will contact Bill King tomorrow.



Mo I hope the proposed fix works for you. Bill knows the situation on 1467 and to my knowledge that problem was fixed. Try to avoid the replace and hope mentality. These gremlins usually have an uncomplicated answer. Best wishes.


For what it’s worth, my personal experience doesn’t mirror this.

I used to own SR20 G1, which had the S-Tec 55X. After resolving an issue that many early 20’s had regarding “stick wobble” (which proved to be an easy fix once it was figured out), my 55X setup worked perfectly, for 1700 hours of flying. (The originally installed 55 had a capacitor blow which simulated a fire, and after I sold the plane the TC went out.)

My G3 has the Avidyne DFC 90. I like that setup a lot.

The DFC 90 has some distinct advantages. Speed envelope control is considered by many to be a meaningful safety advantage. Many people find the DFC 90 to be much more accurate in tracking the localizer, especially in strong winds, although in my experience both worked great. The DFC 90 has a “straight and level” button and an “AP” button that has the plane continue doing whatever it’s doing, and I really like both of those.

At the same time, upon losing the HSI (actually AHRS), the S-Tec 55 will continue flying in GPSS mode, whereas the DFC 90 won’t. Different people have different opinions on the safety implications of this. To me it’s a big deal. I’m not one of the pro-type pilots, and I’m more worried about spatial disorientation if things go crappy than I am about letting the autopilot bring the plane to a stall. Yes, I routinely hand-fly the plane in IMC and under the hood, but disorientation seems to be a common issue under high-stress situations.

Finally, in my observation, both manufacturers have instances of bad and good customer service in their history. S-Tec has awful expense and time involved in repairs, and seems to have exploited the situation when the backlight failed. Many Cirrus pilots are justifiably upset over that. My observation and personal experience, is that Avidyne sometimes delivers exemplary service (I’ve experience that recently) and also poor service (I’ve experience that recently as well and to this day am upset about it). To say that one manufacturer has a consistent record in this area is contrary to my observation and I don’t think it’s supported by the facts, zealous advocates notwithstanding.

Net, I wouldn’t disqualify the chance to get your setup working properly. At the same time, if there’s meaningful expense to getting it right a DFC 90 swap is a very good option. Finally, Autopilot Central in Tulsa are incredible. Ralph Stahley is the autopilot whisperer.

Whatever you choose, I’d be sure to get the thing operational. Whether it’s the S-Tec or Avidyne unit, a functional autopilot is a huge safety advantage.