Trim control using AP module

[awmuhs@aol.com]

On May 4th I flew a practice ILS approach using a GTN650 and my skyview autopilot. I used GPS steering to get to the FAF. The autopilot and auto trim were holding 5000 feet and I had the Vnav armed to capture the glide slope. At the FAF the GTN switched to localizer and the airplane tracked inbound. At the glide slope intercept Vnav became active and I started down the GS. I knew auto trim was working as I could see the call for trim, the little green triangle was scrolling and I could see the change in the trim indicator on the skyview display.

At DH I hit the autopilot disconnect and increased engine power. As I did the nose started to pitch up. When pushing the trim button to lower the nose I got no response. I reduced engine power to make it easier to keep the nose down. As I flew away I tried switching the power to the trim system off and on with no improvement. I flew, at reduced power to minimize nose up tendencies, about 10 miles back to my home airport as I got close to the airport the nose down trim started to work again.

After landing I started trouble shooting the pitch trim system. I applied power directly to the trim motor using short pieces of wire. I checked the Tosten trim switches on both sticks. I bypassed the Tosten switches to operate the servo. I pulled out the AP module to check the connections. Everything worked/looked fine.

It seemed that the problem might be heat related since the initial problem occurred after a lengthy precision approach and then, after not using it for about 10 minutes, it started working again.

With that in mind I ran the trim up and down for about 5 minutes waiting to see if it would fail. it finally did. the up trim continued to work but the down trim would not.

I pulled the module out drove home and called tech support on Monday May 4th. After telling a Dynon tech my problem he asked if after the initial failure was the auto trim able to operate the pitch servo. I told him I did not know. So he asked me to fly again to see if the auto trim would control the trim properly.

On Tuesday May 5th, I took off and engaged the autopilot with auto trim enabled. I headed out to the practice area. I had set the autopilot to level off at 5000 feet. As it tried to level off I could see that the auto trim was trying to apply down trim. Right after that I started getting a slip indication on the pitch. When I clicked off the auto pilot the airplane pitched up hard and the down trim button had no impact.

While I was doing the ground trouble shooting on May 4th I added a backup manual trim switch (I should have included it when I built the plane) to the pitch servo so I was able to trim the airplane and return to the airport. During this portion of the flight the up trim button on my stick continued to work.

As directed I uploaded a Diagnostic file Tuesday May 5th in the early afternoon.

Based on the way it is behaving I do not think it is a software issue. I believe that what ever hardware is suppose to switch the polarity to the servo to drive it the other way is not working.

Any suggestions on what else I can look at?

 

[dynonsupport]

Alan,

The driver inside the system is a device called an H-Bridge. It's 4 transistors. Two of them turn on for one direction, the other two for the other. There isn't a "reversing" device. However, it would be possible for one of the four to be bad and for one direction to not work. This being heat related seems really, really unlikely. These devices are all in the same package, all share the same power and control circuits, and have the same heat and over-current protection.

Now, the other way for one direction to not work is a failure in the trim servo. It has diodes and switches inside, and if a switch sticks, you can't move the servo one direction (ask me how I know...). These switches are the limit switches if you get to one end of the travel. Heat makes a lot more sense in making a switch stick than a transistor failing but recovering. Most silicon doesn't fix itself.

The other possibility is your stick switches. This is an RV-10, right? Have you tried the other stick to see if it can move the trim while you are having issues? Remember, the switches do have timeouts so if you hold them for more than a few seconds, they disable for a few seconds.

The last possibility is that you have your trim speed set so slow that you are low on torque and can't fight the air loads in one direction.

If I had a suggestion to figure all of this out, it would be to put a light on the wires going to the trim servo. That way you could see if the light lit up when you requested trim, and if you got the light but no motion, it's the servo, but with no light it would be the AP module.

 

[awmuhs@aol.com]

I checked the switches on both sticks. They worked correctly.

When I did the original trouble shooting on the ground I had my DVM on the terminals that fed to the trim servo. I was able to see the 12VDC signal being sent to the servo. I ran the trim servo up and down. When it stopped working I could see that there was not any voltage being sent to the servo.

My trim speed is set as recommended by one of your techs and it has been working just fine for the last 10 months.

Respectfully, I understand that a transistor can't heal itself, but I have been working on electronic circuits long enough to know that heat does affect the reliability of electronic devices. Each time the pitch servo has stopped working it was after being used for 5 to 10 minutes. On my last flight when the auto trim failed to lower the nose I applied down trim using the manual switch that I had installed and was able to trim the airplane.

 

[dynonsupport]

Alan,
Thanks for the extra info on what you've done to troubleshoot. That does help a lot. We're not trying to deny that something could be up with our product, just making sure we get the right thing fixed.

Just to verify- when it won't move, it's only blocked in one direction, right? There are more obvious possibilities if it stops in both directions. That's the confusing part, but almost anything is possible.

Are you using a standard Ray Allen trim servo, or something else?

 

[awmuhs@aol.com]

I am using a Ray Allen servo.

It is only blocked in one direction.