Trim gets disabled on High RPM

[Czen77]

2020 RV-12iS Electric Pitch Trim / Garmin
Plane has 211 hrs.

Trim & Autopilot Trim Troubleshooting

Looking for some help troubleshooting an electric pitch trim issue on my E-AB RV-12iS.

Equipment:
• G3X Touch (GDU 460)
• GTN 650
• GMC 307 Autopilot Controller
• GSA 28 Roll & Pitch Servos
• Electric Pitch Trim

Observed behavior:

1. During takeoff and climb at full power, the electric pitch trim does not respond.
2. The trim remains inoperative in cruise at higher RPM settings.
3. When RPM is reduced to approximately 3500 RPM, the trim immediately begins working normally.
4. When the trim is inoperative, pressing either trim switch results in no trim movement and the trim position indicator on the G3X does not move.
5. Once RPM is reduced and the trim starts working again, the trim position indicator moves correctly and reflects trim position changes.
6. The trim motor passes the G3X configuration/test menu checks and operates normally in both directions.
7. The trim works normally during ground testing.
8. When the autopilot is engaged, the G3X repeatedly announces “TRIM TRIM.”
9. If I press either trim switch while the autopilot is engaged, the autopilot immediately disconnects.

Additional information:

• Aircraft is Experimental Amateur-Built.
• No trim or autopilot fault messages have been observed.
• Garmin ESP was previously active (instructor reported the airplane pushing back during steep turns) and has since been disabled for troubleshooting, but the trim issue existed before that change.

Questions:

• Is it normal for manual trim input to immediately disconnect the autopilot on a G3X/GSA 28 installation? Even if correct direction of correction is pressed ?
• Is there any trim inhibit logic that could be tied to RPM, airspeed, flap position, autopilot state, or a discrete input?
• Has anyone seen a condition where trim works perfectly on the ground and at lower RPM, but becomes completely inoperative at higher RPM?
• Are there specific G3X configuration settings or wiring areas I should inspect first?

Any ideas or similar experiences would be appreciated.

 

[Carl Froehlich]

Amit,

I know nothing about the RV-12 and even less about your G3X and Garmin autopilot. But….

Based on your post I’d try to recreate this problem on the ground. Make a simple manometer and connect it to you pitot. If the problem shows up at some indicated airspeed the I would look at how you configured the G3X.

Carl

 

[Brantel]

Has it ever worked properly?

Yes, the AP will disconnect if the trim button is press is pressed while the AP is engaged and the auto trim option is enabled.

Even though you are EAB, does the plane use Vans power module and wiring harness?

Yes, there are two things to check that can impact the trim motor speed:

One is the trim speed control potentiometer on the Vans provided power module. This needs to be cranked up for Garmin installs with the AP option.

Next is the ability for the Garmin pitch servo to control the pitch trim motor and its speed. If the pitch trim motor control option is enabled, there are min and max airspeed thresholds and trim servo speed settings that can speed up and slow down the trim motor speed based on airspeed.

If either of these are improperly configured, weird stuff can happen based on airspeed.

 

[j4cbo]

Next is the ability for the Garmin pitch servo to control the pitch trim motor and its speed. If the pitch trim motor control option is enabled, there are min and max airspeed thresholds and trim servo speed settings that can speed up and slow down the trim motor speed based on airspeed.

This is my first guess too. There’s no feature designed to be trim inhibit, but if the trim-speed-at-high-airspeed setting is misconfigured then it could cause the trim to not work at all above some airspeed.

Does the trim function properly in a full-power Vx climb?

 

[Czen77]

This is my first guess too. There’s no feature designed to be trim inhibit, but if the trim-speed-at-high-airspeed setting is misconfigured then it could cause the trim to not work at all above some airspeed.

Does the trim function properly in a full-power Vx climb?

At take off, the air speed is not much, but RPM is at max. Still it won’t work. Also won’t work on cruise settings.

The trim works great on the ground and in the hangar. I need to test it during run up. Will do so asap.

 

[Czen77]

Has it ever worked properly?

Just spoke with the builder/previous owner. He said it worked for him. But my buddy and I ferried the plane in 1st week of May this year from New Mexico to Houston, Texas, the trim did not work with auto pilot or during cruise /climb.

Yes, the AP will disconnect if the trim button is press is pressed while the AP is engaged and the auto trim option is enabled.

Should I keep the auto trim disabled ?

Even though you are EAB, does the plane use Vans power module and wiring harness?

Yes. Stein made the harnesses for Vans.

Yes, there are two things to check that can impact the trim motor speed:

One is the trim speed control potentiometer on the Vans provided power module. This needs to be cranked up for Garmin installs with the AP option.

Will check in the next few hours as I head to the hangar.

Next is the ability for the Garmin pitch servo to control the pitch trim motor and its speed. If the pitch trim motor control option is enabled, there are min and max airspeed thresholds and trim servo speed settings that can speed up and slow down the trim motor speed based on airspeed.

Any idea on what the ideal speeds should be set ? I can share pictures of current settings in a few hours.

I did go over them with Marc at Garmin support and he did not find anything unusual.

If either of these are improperly configured, weird stuff can happen based on airspeed.