Pitch oscillation with Century 2000 Autopilot

[mikefox]

We have a 1989 M20J with a Century 2000 autopilot and attitude indicator.  We have been having problems with a pitch oscillation (sharp changes in pitch attitude both up and down) for years now, and have traced the issue to the attitude indictor pitch electrical signal - last time we found broken slip rings inside the attitude indicator.

This issue has recurred 3 times now, with attitude indicators from 2 different shops.

Has anyone else ever encountered this problem?  One shop suggested that we might have some vibration issue causing the problems, but there is little noticeable vibration from the panel as far as I can tell.

I would really appreciate any thoughts on this issue.

Regards,

Mike

[Piloto]

I had similar intermitent problems with autopilots. All of them were related to contacts. Either on the computer tray or cable connectors. Suggest the following:

1. Remove the autopilot computer from its tray and spray contact cleaner on all the contacts. Reinstall

2. On each cable connector between the computer and the gyros remove the connector cover and gently pull on each wire to make sure they still attached to the pins.

3. Spary contact cleaner on the connector pins.

From my experience I would say that about 80% of avionics intermitent problems are related to contacts problems. Contacts have a tendency to oxidize with time causing intermitent connections.

José 

 

[Parker_Woodruff]

Can't help you, but I don't envy you having to chase these things.

[Mac201]

I have had the same problem with my Century 2000 along with other recent expensive repairs at Century, including rebuilding the pitch and roll servos.  When troubleshooting the sharp pitch changes, Century tech support recommended that we send the attitude indicator and computer in to them for service.  We did and they rebuilt the pigtail on the attitude indicator reporting that it was sending intermittent signals and they bench checked the computer.  All components were reinstalled last week.  I test flew on Monday and still have the same pitch problems.  The AP works well for about five to ten minutes then begins aggressive pitch down movement in both attitude and altitude modes.  Other times it has been pitch up movements.  I am very frustrated and tired of paying!  I guess it's back to Century.

[Jeff_S]

Mac201 was crying about this on my shoulder the other night, right up until I opened the fridge in the hangar and gave him a beer! (Well, I offered him a beer, but he was going flying or somesuch so he declined.)

[mikefox]

We also sent the computer to Century - they upgraded it for a lot of money but didn't find any specific problem.  The pitch oscillation continued.  So we changed the attitude indicator.  It went away for a while but came back.  We then re-wired everything, with new harnesses.  Still a problem.

Finally changed the AI again, and the problem disappeared - and the techs found broken slip rings inside the AI.

Now the gremlin is back with us.  So it is off to the instrument shop again!

[Mac201]

My avionics A&P had a long conversation with Autopilot Central today.  I have decided to send the attitude indicator and AP computer to them to troubleshoot.  After spending plenty of money at Century on repairs, I just do not have confidence in them.

I will post how it goes.

[jetdriven]

Quote: mikefox

We also sent the computer to Century - they upgraded it for a lot of money but didn't find any specific problem.  The pitch oscillation continued.  So we changed the attitude indicator.  It went away for a while but came back.  We then re-wired everything, with new harnesses.  Still a problem.

Finally changed the AI again, and the problem disappeared - and the techs found broken slip rings inside the AI.

Now the gremlin is back with us.  So it is off to the instrument shop again!

[FLYFST]

Mikefox & Mac201,

Please continue to post your progress on resolution.  I recently had what you described happened to me during the post-purchase ferry flight for my M20J at 10000 ft, in and out of the clouds; really disturbing to see "George" command +/-700 fpm for no apparent reason in non-turbulent conditions.  I have flown the airplane several times since then, carefully monitoring "George" during cruise flight (6500, 5500) and doing coupled approaches, and thankfully the problem has not reappeared -- but I bet the gremlin still lurks somewhere, waiting to come out an bite me when in hard IMC. 

[mikefox]

Well, today we had the AI removed and bench tested - no trouble found.  They cleaned the brushes and re-installed.  Test flight showed no problems at all!  I'm not sure whether to be happy or not - would rather have found a smoking gun, but now I know it is associated somehow with the AI....

[Mac201]

Well, we have not found the problem with my Century 2000 AP.  We sent the AI and the computer to Autopilots Central in Tulsa; both bench checked OK.  Then we cleaned all the connections again in the back of the computer tray, no joy.  Now have rebuilt the connectors and all wiring from the computer.  I tested it today, no joy.  Not sure where to go now.  Does anyone have a recommendation for a great AP troubleshooting shop in the Southeast?  Thanks.

[jetdriven]

Could be a servo.  perform a preflight test and note the amount of force required to overpower the servo clutch.   Do Corrosion-X lately? Century in Mineral Wells TX is the only place I'd have my autopilot repaired.

 

Also, check your static lines especially in the tail. Ours had an issue with them, one was disconnected. The pressure inside the tailcone is less, and with changing airspeeds and pitch oscillations, the autopilot chases it.  But the pressure inside the tailcone changes with airspeed.

[Mac201]

Thanks Jetdriven.  I have had both servos rebuilt by Century.  They also rebuilt the pigtail on the AI and bench checked the computer.  The system and servos pass the preflight test.  We can check the static lines, both the AP has abrupt pitch changes when in attitude and altitude modes without speed changes.  I do not think it is reading static pressure in attitude mode but not sure.

[mikefox]

We have had our AI bench-checked ok, but still had pitch-oscillation problems.  Then changed out AI and everything worked perfectly.  I've heard from some shops that the pitch output is the weak point on that AI.  But I haven't heard any good solutions yet.

 

Still waiting for Garmin to come out with the magic box for the G500 that will replace the AI for autopilot pitch and roll signals!

[Jeff_S]

Mr. Mac201, you bring up an interesting thought about static lines. The A/P will definitely need those intact to hold altitude, but I would have thought that the folks who have worked on your plane so many times would have checked that already. I know Epps did find some old and cracked static lines to my A/P and replaced those to address some minor pitch porpoising. That is something to look at. Although I still think you may have some grounding issues somewhere based on what you've described as the behavior.

But let's go up one evening this week and I can fly the bird while you troubleshoot things and maybe we can isolate the problem!

[jax88]

Interesting.  After having my Century main board replaced and adding the GPSS unit, my avionics shop is having the same problem with my autopilot, pitch oscillations.  They have tried pretty much what you've tried and no love on mine either.  I'll mention the static lines just in case.

[Mac201]

Thanks for the suggestions.  In further conversations with the Century tech, the static line would not effect attitude mode.  I am having problems in both attitude and atltidude modes with aggressive pitch changes, sometimes nose up, but usually nose down.  The AI and the computer have now been bench tested by Century and Autopliots Central. 

My avionics guy wants me to now take the plane to Autopilots Central, while the Century tech on the phone says he does not have any more suggestions for field repairs and that I should bring the plane to them.  Does anyone have experience with onsight repairs at either Century or Autopilots Central.  Thanks

[mikefox]

Well, problem solved!  (For now...)

We had an instrument shop tighten the brushes that carry the pitch signal.  The shop didn't think it would solve the problem, but we just completed a flawless test flight.  So I am doubly convinced that the sharp, short duration pitch movements (jerky, and in both altitude and attitude hold modes) were due to the pitch signal coming off the AI!!

Beware - the AI bench-checked perfectly when it was not working in the aircraft, so I think the vibration may have something to do with the quality of the pitch signal and the need for the brushes to maintain good contact.

[ScubaMan]

For what it's worth, I had the exact problem with a Century Autopilot that plagued me for about 2 years. We could not duplicate the problem on the ground and it was intermittent in the air. We finally traced the problem to a bad cable between the attitude indicator and the autopilot. Once that was replaced, the autopilot worked perfect. I might add that Dan at Executive Autopilots is great with the older century autopilots.

[Piloto]

On ‎8‎/‎14‎/‎2011 at 11:21 AM, Mac201 said:

Thanks for the suggestions.  In further conversations with the Century tech, the static line would not effect attitude mode.  I am having problems in both attitude and atltidude modes with aggressive pitch changes, sometimes nose up, but usually nose down.  The AI and the computer have now been bench tested by Century and Autopliots Central. 

 

My avionics guy wants me to now take the plane to Autopilots Central, while the Century tech on the phone says he does not have any more suggestions for field repairs and that I should bring the plane to them.  Does anyone have experience with onsight repairs at either Century or Autopilots Central.  Thanks

 

Mac, did you ever found a solution to your A/P problem? Thanks

José