Autopilot doesn’t decend

[warren.huisman]

I have a century trident 2 axis autopilot with altitude hold. Nice autopilot but lately it refuses to descend. It will climb and hold altitude on command but not descend. Ground check, up and down seems ok. Any ideas or is my auto pilot going Tago Uniform?

 

 

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[LANCECASPER]

1 hour ago, warren.huisman said:

 

I have a century trident 2 axis autopilot with altitude hold. Nice autopilot but lately it refuses to descend. It will climb and hold altitude on command but not descend. Ground check, up and down seems ok. Any ideas or is my auto pilot going Tago Uniform?

 

 

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I’ve never seen that Autopilot in a Mooney before (https://sarasotaavionics.com/avionics/triden-series)

I’ve seen a Century IIB, 21, 31, 41 and 2000 in a Mooney but never a Triden 2.

What happens when you turn off ALT and hit the DN button? Are you saying that pre-select to a lower altitude doesn’t work? Or that it won’t come out of Altitude hold without turning off the autopilot?

@Jake@BevanAviation will know the answer I’m sure once he gets some information.

 - - - -

Turns out your airplane was the Millennium Mooney in the 2000 AOPA sweepstakes and it’s probably the only Mooney ever with that autopilot:

https://www.aopa.org/news-and-media/all-news/2000/october/flight-training-magazine/easier-than-you-think

[warren.huisman]

This autopilot always has pitch control active, either altitude hold or attitude hold. I did try to put it in attitude hold and then select and hold down without success as well. (It does not have altitude pre-select).

It’s just very strange that it will initiate a climb but not a descent…. It’s the same servo as far as I know…


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[LANCECASPER]

27 minutes ago, warren.huisman said:

This autopilot always has pitch control active, either altitude hold or attitude hold. I did try to put it in attitude hold and then select and hold down without success as well. (It does not have altitude pre-select).

It’s just very strange that it will initiate a climb but not a descent…. It’s the same servo as far as I know…


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You probably have the manual, but here’s a link just in case: https://www.dropbox.com/scl/fi/2pxn533t0sap9w6oqkunh/Triden_autopilot.pdf?rlkey=ym3zoucd0ou9csf77rjrsxdf9&dl=0

If it’s not doing what it usually does you need a Century A/P guru - @Jake@BevanAviation

[Jake@BevanAviation]

Unfortunately, I have never worked on the Trident system and I don't have any service manuals or test sets for that system.

[LANCECASPER]

12 minutes ago, Jake@BevanAviation said:

Unfortunately, I have never worked on the Trident system and I don't have any service manuals or test sets for that system.

OK, where is Jacob Kinsey and what have you done with him .. lol?

First time for everything, I've never seen Jake without an answer.

[warren.huisman]

Update on the autopilot. Altitude hold started to get really lazy as well.  I could easily be plus / minus 3-400 off target altitude.  So the next step I took was to remove the attitude indicator and have it overhauled by a local instrument shop.  He found several things wrong which plausibly explained my issues.  However, when the gyro was reinstalled into the airplane, as soon as the autopilot is engaged, it immediately pitches down or up.  It is not consistent whether it pitches up or down. 

I took it to an avionics shop and we checked the harness between the gyro and the autopilot and it all checked ok per the wiring diagram.  We also measured the excitation voltage with a meter and it was approx 1.2v rms, (we didn't put a scope on it).  According to the instrument shop it should be 5 volts ac and 5000 hz. Any ideas for next steps?

[Z W]

Unfortunately, the next step may be to bite the bullet and install a GFC500, or the newly-approved Dynon autopilot.

Been where you are with an old King KAP150 that nobody could keep working.  Every fix was another thousand+ dollars, weeks of downtime, and trip to the shop.  I believe the upgrade has paid for itself by now, though it's hard to tell.  Hope you find a better and cheaper answer.

[LANCECASPER]

3 hours ago, warren.huisman said:

Update on the autopilot. Altitude hold started to get really lazy as well.  I could easily be plus / minus 3-400 off target altitude.  So the next step I took was to remove the attitude indicator and have it overhauled by a local instrument shop.  He found several things wrong which plausibly explained my issues.  However, when the gyro was reinstalled into the airplane, as soon as the autopilot is engaged, it immediately pitches down or up.  It is not consistent whether it pitches up or down. 

I took it to an avionics shop and we checked the harness between the gyro and the autopilot and it all checked ok per the wiring diagram.  We also measured the excitation voltage with a meter and it was approx 1.2v rms, (we didn't put a scope on it).  According to the instrument shop it should be 5 volts ac and 5000 hz. Any ideas for next steps?

Whenever you overhaul or change out the Attitude Indicator on an attitude based autopilot, the autopilot and the new indicator have to be aligned. 

The key would be to find someone who really knows this autopilot and not let anyone else touch it. I'm sure there are some Century A/P gurus out there that know it well. I would try to do that instead of going the Autopilot Central Route. @Bob Weber might be able to steer you in the right direction.

[EricJ]

5 hours ago, warren.huisman said:

Update on the autopilot. Altitude hold started to get really lazy as well.  I could easily be plus / minus 3-400 off target altitude.  So the next step I took was to remove the attitude indicator and have it overhauled by a local instrument shop.  He found several things wrong which plausibly explained my issues.  However, when the gyro was reinstalled into the airplane, as soon as the autopilot is engaged, it immediately pitches down or up.  It is not consistent whether it pitches up or down. 

I took it to an avionics shop and we checked the harness between the gyro and the autopilot and it all checked ok per the wiring diagram.  We also measured the excitation voltage with a meter and it was approx 1.2v rms, (we didn't put a scope on it).  According to the instrument shop it should be 5 volts ac and 5000 hz. Any ideas for next steps?

That was what killed my Century III as well.   The AI had to be overhauled, and after reinstallation the autopilot never worked right, except in my case the problem was roll.   The worst part was that it worked fine on the ground.   All the ground tests worked, and I even took the AI out of the panel and if you moved it by hand the controls would respond correctly, but in the air it was essentially dangerous.   I even bought a second AI but it behaved the same.   I've been hand-flying ever since, which has been more than five years now.   

[Bob Weber]

Warren emailed me, I will return with a call.

Eric I can help you get yours back up and running if you would like, contact me at 616 822 1999 9am-5pm Eastern.

 

 

[EricJ]

2 hours ago, Bob Weber said:

Warren emailed me, I will return with a call.

Eric I can help you get yours back up and running if you would like, contact me at 616 822 1999 9am-5pm Eastern.

 

 

Thanks, but I already deleted the vacuum system and replaced the AI with a G5, so I committed to not fixing it quite a while ago.   Several others took a crack at it and it got nowhere, and that was after spending an unexpected (and unauthorized) five figures on overhauling the whole thing a year before it quit.   It was just a bad idea to spend any more money on it.  

[Bob Weber]

Regretfully Eric, I hear that story all too often.

I wish I could have met you when you started down that road, I have assisted many with the same issues to success.

Troubleshooting is an art in its self, troubleshooting autopilots is a challenge without a lot of experience and training on them

[warren.huisman]

Update on the autopilot. Altitude hold started to get really lazy as well.  I could easily be plus / minus 3-400 off target altitude.  So the next step I took was to remove the attitude indicator and have it overhauled by a local instrument shop.  He found several things wrong which plausibly explained my issues.  However, when the gyro was reinstalled into the airplane, as soon as the autopilot is engaged, it immediately pitches down or up.  It is not consistent whether it pitches up or down. 
I took it to an avionics shop and we checked the harness between the gyro and the autopilot and it all checked ok per the wiring diagram.  We also measured the excitation voltage with a meter and it was approx 1.2v rms, (we didn't put a scope on it).  According to the instrument shop it should be 5 volts ac and 5000 hz. Any ideas for next steps?

Update on the autopilot. We finally figured out that the instrument shop messed up the overhaul. The output from the gyro was reversed which led to the immediate pitch up or pitch down. Frustrating couple of months but it is working again. Sometimes perseverance pays off!

Thank you to Bob Weber for his help as well.


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