Adjustment to solve Century II hunting in roll?

Jim_R

Jim_R

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I've got a Piper Autocontrol III (same as Century II, with the three adjustment screws on the control head--not a Century IIB with 4 screws) with a radio coupler in my Cherokee 180. Ever since I got it, it's had a (very) small/gentle roll oscillation...small enough that I don't think pax ever noticed it. It's tied into my IFD540 via GPSS and has been a solid performer for years.

A year or so ago, my AI went TU and I had it overhauled. After the AI was reinstalled, I noticed the autopilot was much more twitchy, perhaps because the freshly-overhauled AI is more responsive? If yokes are allowed free motion, the oscillations will ramp up to become semi-violent; it is definitely an unpleasant experience.

I've lived with this behavior now for many months, but it's either gotten a little worse or else I'm just getting tired of it. I have to continually manually damp out the oscillations, and it can be quite a fight. In smooth air, it's not too bad...it'll fly for smoothly for ~15 sec before it will gradually walk off, so it's easy to keep it damped. But in any level of turbulence, it's a struggle to keep George under control, to the point that it's no longer worth using the autopilot and I just turn it off.

My mechanic has had struggles getting the Altimatic III in his Arrow dialed in, so I'm not sure he's interested in trying to solve my issue, and honestly I'm not sure I want him to try. I'm not sure where to look next.

I'm in SE TX. Should I be considering a trip to Autopilots Central in Tulsa, or is this something I should expect to be easier to solve closer to home?
 
If you are dead set on wasting a lot money on antique autopilot at least go to someone competent.
 
Any chance the rigging cable tensions aren’t correct? Too loose? Could that cause it?
 
Jim_R said:
I've got a Piper Autocontrol III (same as Century II, with the three adjustment screws on the control head--not a Century IIB with 4 screws) with a radio coupler in my Cherokee 180. Ever since I got it, it's had a (very) small/gentle roll oscillation...small enough that I don't think pax ever noticed it. It's tied into my IFD540 via GPSS and has been a solid performer for years.

A year or so ago, my AI went TU and I had it overhauled. After the AI was reinstalled, I noticed the autopilot was much more twitchy, perhaps because the freshly-overhauled AI is more responsive? If yokes are allowed free motion, the oscillations will ramp up to become semi-violent; it is definitely an unpleasant experience.

I've lived with this behavior now for many months, but it's either gotten a little worse or else I'm just getting tired of it. I have to continually manually damp out the oscillations, and it can be quite a fight. In smooth air, it's not too bad...it'll fly for smoothly for ~15 sec before it will gradually walk off, so it's easy to keep it damped. But in any level of turbulence, it's a struggle to keep George under control, to the point that it's no longer worth using the autopilot and I just turn it off.

My mechanic has had struggles getting the Altimatic III in his Arrow dialed in, so I'm not sure he's interested in trying to solve my issue, and honestly I'm not sure I want him to try. I'm not sure where to look next.

I'm in SE TX. Should I be considering a trip to Autopilots Central in Tulsa, or is this something I should expect to be easier to solve closer to home?
Click to expand...

Autopilots Central in Tulsa is hard to beat. If it were me, I'd take it there.
 
WOW did I get poofed? The more you know about that system the cheaper it will be to fix it...

IMG_20191216_175521.jpg
 
Clip4 said:
If you are dead set on wasting a lot money on antique autopilot at least go to someone competent.
Click to expand...
Yeah, I know the current fad would be for me to rip out my probably-just-needs-a-Century-savvy-person-to-spend-an-hour-ish-adjusting-it ancient autopilot and spend ~$10k putting in a modern autopilot, but I'm not there yet. So now I'm trying to figure out where "someone competent" might be. But thanks for the help.
 
Jim_R said:
Yeah, I know the current fad would be for me to rip out my probably-just-needs-a-Century-savvy-person-to-spend-an-hour-ish-adjusting-it ancient autopilot and spend ~$10k putting in a modern autopilot, but I'm not there yet. So now I'm trying to figure out where "someone competent" might be. But thanks for the help.
Click to expand...

I've used Autopilots Central in Tulsa on a couple of old autopilot systems I once owned. If the system you have was working to your satisfaction previously, then have them go over it and get it in order.
 
Well, after talking with my mechanic, he's convinced me to let him give it a shot. I'm not certain that's the absolute best approach, but part of the decision is keeping good relations with my mechanic, so I'm going to take that approach in the near term. If that doesn't get good results, I'll be casting farther afield. Bob, thanks for throwing your hat in the ring--I may be in touch with you in the next few weeks! :)
 
Clip4 said:
If you are dead set on wasting a lot money on antique autopilot at least go to someone competent.
Click to expand...

:eyeroll:

The Century II is a decent little autopilot and the alternative is probably a $15k or $20k adventure, so I think it's probably worth a little tweaking.

@Jim_R a couple of thoughts from using my Century IIB: I have a very similar, subtle oscillation in roll. I haven't gotten around to playing with the "gain" yet and that might take care of it entirely, but I have noticed a couple things that might help: 1) FYI when I got a newly overhauled AI, mine worked a lot better (not worse), and 2) I'm starting to suspect that my turn-and-bank "ball" is not quite perfectly calibrated - and yours might not be, either - as the left/right roll hunting of the autopilot is greatly reduced if I fly with the ball just ever-so-slightly left of center. I have read in the Century manuals that if you fly uncoordinated it will cause the AP to hunt like this. It could be the case that your ball is centered when the plane isn't actually perfectly coordinated. You might play with your rudder trim or rudder pressure to see if the AP performs better slightly left or right of "coordinated".
 
Jim_R said:
Yeah, I know the current fad would be for me to rip out my probably-just-needs-a-Century-savvy-person-to-spend-an-hour-ish-adjusting-it ancient autopilot and spend ~$10k putting in a modern autopilot, but I'm not there yet. So now I'm trying to figure out where "someone competent" might be. But thanks for the help.
Click to expand...

If it flawlessly as designed, it is still not a good autopilot by 20 years ago standards.
 
Clip4 said:
If you are dead set on wasting a lot money on antique autopilot at least go to someone competent.
Click to expand...

I spent a lot of money on an old autopilot like this including big names mentioned here - 20 years ago...and the thing never did work.
2000 hours of no autopilot in that plane - I think it really helped my hand-flying skills.
 
AA5Bman said:
:eyeroll:

The Century II is a decent little autopilot and the alternative is probably a $15k or $20k adventure, so I think it's probably worth a little tweaking.

@Jim_R a couple of thoughts from using my Century IIB: I have a very similar, subtle oscillation in roll. I haven't gotten around to playing with the "gain" yet and that might take care of it entirely, but I have noticed a couple things that might help: 1) FYI when I got a newly overhauled AI, mine worked a lot better (not worse), and 2) I'm starting to suspect that my turn-and-bank "ball" is not quite perfectly calibrated - and yours might not be, either - as the left/right roll hunting of the autopilot is greatly reduced if I fly with the ball just ever-so-slightly left of center. I have read in the Century manuals that if you fly uncoordinated it will cause the AP to hunt like this. It could be the case that your ball is centered when the plane isn't actually perfectly coordinated. You might play with your rudder trim or rudder pressure to see if the AP performs better slightly left or right of "coordinated".
Click to expand...


A flight adjust should always be performed after an attitude gyro "change" and is very likely to correct that issue. I can help there also!

A slow dutch roll can be caused by a few things, If offsetting the rudder settles the oscillation, it it is "loading" the system to one side.

This means the roll servo is powered up slightly and stalled, heating up the motor, and the drive transistors in the computer.

It is also creating a slight skid that will slow you down and burn more fuel.
 
Doc Holliday said:
Autopilots Central in Tulsa is hard to beat. If it were me, I'd take it there.
Click to expand...

This. The only owners I’ve ever known who were ultimately happy with their ancient autopilots went there. Many screwed around with local avionics shops for years.

If it’s fixable, the folks in Tulsa will fix it. First time usually. (I do know of one owner who had to visit twice. He wasn’t super happy about that, but he also didn’t pay for ten trips to the locals either, so...)

I don’t know what it is about most shops that they can’t troubleshoot with any consistency following a plan and a schematic, but ... it’s just really rare to find a place that does. There isn’t any rocket science in most of these old units.
 
I have the same Piper Autocontrol 3B.
The symptoms sound familiar to those I have had in the past, but no longer. Before fiddling with any adjustments, the first thing I would do is consider the connectors to be faulty. I noticed that sticking my hand behind the control head would jostle the connectors enough to cause a sudden but temporary engagement of the autopilot if it was set to follow a heading or radio signal. My IA knew how to fix the issue temporarily, but ultimately changed out the connectors for the permanent fix.

The other culprits were faulty AI or heading bug on the DG, but more often the DG, which caused problems when they needed to be replaced. Notably, when I installed my ASPEN, these issues have completely gone away. My unit has been rock solid ever since as it uses its electronic HSI for signals to Century unit. I have retained the vacuum AI gyro however for its signal supply to the autopilot.

I was tempted to adjusting the screws on the control head, which seemed like the thing to do, or read about changing the brushes on the servos, but these above suggestions IMO, should be tried first. I’ll bet the folks in Tulsa know all about the connector issue.
 
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MajorTurbulence said:
I have the same Piper Autocontrol 3B.
The symptoms sound familiar to those I have had in the past, but no longer. Before fiddling with any adjustments, the first thing I would do is consider the connectors to be faulty.
<snip>
I was tempted to adjusting the screws on the control head, which seemed like the thing to do, or read about changing the brushes on the servos, but these above suggestions IMO, should be tried first. I’ll bet the folks in Tulsa know all about the connector issue.
Click to expand...
You win the internets today. Turns out it was a loose connection on the back of the DG.
 
Glad it worked for you. Hope you didn’t have to shell out big bucks to discover that easy fix. All in all, I think that legacy autopilot of yours, and mine, is a mighty good performer despite the internet naysayers. Only thing it lacks is altitude hold, capture, or glide slope capture, but that, IMO, is no biggie relative to what it does offer.
 
A little late to the conversation but I had an issue with a similar Century IIb autopilot resulting in intermittent trouble holding a heading in heading mode. I tried several times to adjust the pots using the procedure in the manual but it didn’t help.

I’m fortunate to live in the Tulsa area where AP Central is located and after talking to their techs I dropped off the controller and radio coupler for them to test. They told me that the connectors were notorious for failing. Sure enough, one of the “ears” that secure the cable to the back of the DG was missing. AP Central tested the components (all tested good) and replaced the connectors for a reasonable cost and it worked perfectly for the rest of the time I owned that airplane.
 
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