Altimatic IIIc autopilot trim seems very stiff.

yelahlance · November 2022

I have a 76 piper lance and the altimatic IIIc autopilot works great but the trim wheel when operated manually seems very stiff. The elec. trim works good in both directions. Wondering if it was and easy thing to check. What maint. things in the trim system seem to usually get missed?

Scott Sherer · November 2022

Hi, if you look in the magazine archives you'll find an article on cleaning and lubing the pitch trim mechanism. We are rerunning the article soon. Take a look back in the archives to 2016 - 2018 and you'll find it. If you don't, let me know and I'll find it for you.

empannin · November 2022

Hi Scott,

It was Oct 2016, page 10. https://piperowner.org/pipers-magazine-october-2016-2/

also a good opportunity to check the play in the trim - no slop is critical for flutter margin. You do not need a grease gun as there is no grease fitting. The right grease here is critical as you pointed out. It needs to provide smooth travel at low temps. For the Seneca III they specify a grease that does not exist....

The rudder trim gets less use but has the same concerns and a good time to clean up and check play. Because of where the rudder trim barrel is I would not recommend spraying brake cleaner as it will fall inside the airframe.

The roll trim in a Seneca is often mounted under the middle row seats so it is warmer than the tail but since it is secured to the fuselage it is going to get cold soaked too. If your AP wanders in roll when it is cold soaked consider similar.

I suspect a ton of fruitless AP troubleshooting has a root cause in issues like this. The older analog and digital systems can provide smooth performance - but they often have built in assumptions on control response. If you are oscillating in pitch or in roll check how stiff the trim controls,

Since it is that time of year inspect the tail area for trapped water and/or ice. It is a W/B issue and if it freezes the trim in place an immediate hazard.

Scott Sherer · November 2022

Thanks Eric!

yelahlance · February 2023

Finally getting to the bottom of this. Anyone know a place to overhaul a trim servo? The clutch part of the servo seems to be the part that needs a bit of help. Thanks all for the help.

Seneca38173 · February 2023

I've had luck buying servo cores on Ebay, and having them repaired by Mid-Continent. Win is you minimize the down time on the A/C:

https://www.mcico.com/

You can also try Autopilots Central, though they likely send it to M/C for repair:

https://www.autopilotscentral.com/

Altimatic IIIc autopilot trim seems very stiff.

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