Pitch control problems

[hobby]

I have been working with Tech support on a pitch control problem on my DigiFlight II VSGV. I have had this autopilot since Nov 08 and it worked perfectly until recently. I first sent in the programmer/controller, but the problem persisted. I then sent in the pitch servo. It also bench tested OK. Before reinstalling the pitch servo, I used the circuit tester that was sent to me and the circuits tested OK. After installing the pitch servo, I took a test flight and everything worked OK. I flew to Galveston this weekend and the autopilot flew perfectly on the outbound 2 hour leg. Departing into IFR weather yesterday, when I engaged the autopilot, it exhibited the same pitch problems as before by pitching up then down, not holding VS or altitude. I flew the trip home with the pitch function disengaged.
I am perplexed at what to do next. I plan to check the pin to pin continuity while physically shaking the wiring harness at both ends, looking for an intermittent short or open. Any suggestions would be appreciated.

[PhantomPholly]

Were you flying in rain?

In my prior airplane with a TruTrak, I had a less-than-perfect static system. I had two static ports on opposite sides of the cabin with a cross-connect, and for some reason it seemed to draw air into the system (perhaps one static port had lower pressure). The result was to draw water into the system, causing a similar experience to yours.

If you have a switch installed to use alternate static (cabin pressure), try changing to this in flight and see if the problem ceases (disengage the autopilot before doing this!!!!!!!! The instantaneous change in static pressure can cause a dramatic / traumatic pitch change. Ask me how I know...).

If the problem "goes away" then you have a static system problem rather than an autopilot problem.

[hobby]

I checked my static system today to ensure that it was not blocked. I also checked that my alternate static source was clear and working when opened. After these checks, I made a test flight. When I initially engaged the autopilot, it exhibited the same behavior as previously reported by significantly oscillating up and down. I then disengaged the autopilot and opened the alternate static port. Upon re-engaging the autopilot, it immediately went into the pitch oscillations again.
Some items to consider - this autopilot has worked perfectly until recently; I have tested the autopilot with the static line disconnected from the back of the controller with no change in the results; both the controller and the pitch servo have been returned to Tru Trak and checked out OK, and the wiring harness checks out OK with the circuit tester provided by Tru Trak.
My next diagnostic step is to try to check each wire from the controller to the pitch servo for continuity while shaking the wiring harness. I don't know if this step will prove anything, other than to eliminate one more set of variables.

[TrutrakTech]

Hobby,

What type of aircraft? One other thing to check, make sure you don't have an excessive amount of lost motion in the elevator control system or in the pitch servo linkage. If it's there, it can cause the autopilot to always be chasing itself.

[hobby]

Lucas,
The airplane is an RV6 with the two standard static ports on either side of the rear fuselage connected together with a single static line running to the instrument panel. The elevator control is via push-pull tubes with little to no slack. I received the pitch control servo back from Tru Trak just a couple of weeks ago and checked for control system slack at the time of re-installation. Note that the autopilot worked perfectly on the subsequent test flight and on a 2 hour flight thereafter. It was on the return trip that the autopilot again started the pitch oscillations.
When I say that the pitch oscillates, I mean that the autopilot initially pushes the nose to a 20-30 degree down pitch, at which point I am not willing to let the nose go any lower and I override the autopilot. It will then begin an upward oscillation which I eventually override.
Could the wiring from the controller to the pitch servo cause the reported problems? I am willing to replace this wiring harness if it could be the problem.

[PhantomPholly]

Ah, new information - work done on pitch servo.

Since the servos are identical, can you easily swap the servos? If the pitch problem goes away but the roll is goofed, you then have an answer...

:D

[TrutrakTech]

Hobby,

This is obviously a commanded descent or ascent. It sounds as there there is a gyro offset. Try a gryo set the next time before you fly. At the end of the runway or taxiway (while stationary), press and hold the knob in for 10-15 seconds with the AP disengaged. The display will read GYRO SET. Then go on your flight and see if that cleared it up. If the plane is moving during power up (even rocking from wind), the gyro can get an offset introduced into it. Let me know what you find. Thanks!

[hobby]

Lucas,
I finally got the opportunity to make a short test flight after the recommended "gyro set". On the first flight, I did the "gyro set" on the ground, without the engine running. When I engaged the autopilot on climbout, I experienced the same condition as previously reported of the autopilot pulling the nose down through the horizon even though I was climbing at 1,000 fpm when the autopilot was engaged. I disengaged the autopilot and trimmed for level flight, then re-engaged the autopilot. Same nose down pitch with me eventually overriding the autopilot, then disengaging it.
I then performed an in-flight "gyro set" while in level flight. Upon re-engaging the autopilot in level flight, I got the same nose down pitch. After landing, I performed a "gyro set" while sitting on the ground, this time with the engine running. On the ensuing climb out, I engaged the autopilot, but got the same nose down pitch. I guess that I struck out as the autopilot commanded nose down pitch was three for three.
Any additional suggestions that I might try?

[TrutrakTech]

Hobby,

I sent you a PM and my email was returned to me as blocked. Check your PM's here. Thanks!

[TrutrakTech]

What was the resolution to this pitch problem? I am experiencing the same exact issue.

Thanks,

Jason

Jason,

What type of aircraft do you have, which autopilot? Have you tried the suggestions in the other posts of this thread? Thanks!

[hobby]

Here's the status to date on my pitch control problems. After speaking with Tru Trak after my test flights and the recommended gyro sets, I returned the controller/programmer to Tru Trak a second time. I spoke with Tru Trak yesterday and it bench tested out OK again. They are sending me a different unit (same model - DF II VSGV) to test in my airplane. I am flying to Colorado and back this weekend so I will post an update when I get back.

If I still have a problem, I will probably pull out the existing wiring harness to the pitch servo and fabricate and install a new one, even though the Tru Trak circuit tester showed that the existing wiring was fine. There could be an intermittently short/open wire caused by vibration and/or temperature variations experienced in flight.

[hobby]

Since the last post, I purchased and installed a new pitch servo wiring harness from Stein Air. (They do beautiful work.) The original wiring harness removed was for the original Altitrak that was subsequently modified for the Digiflight II VSGV.
In installing the new harness, I made certain that both the roll and pitch servos and the controller were connected to the same power lead and ground lead. Previously the pitch servo was grounded to the air frame near the pitch servo.
I made a test flight today and the autopilot system worked flawlessly, even in hot, bumpy West Texas. I will be taking a trip to Utah in about 10 days so the autopilot will get a thorough workout during the trip.
Thanks to Lucas and rest of tech support at Tru Trak.

[TrutrakTech]

Since the last post, I purchased and installed a new pitch servo wiring harness from Stein Air. (They do beautiful work.) The original wiring harness removed was for the original Altitrak that was subsequently modified for the Digiflight II VSGV.
In installing the new harness, I made certain that both the roll and pitch servos and the controller were connected to the same power lead and ground lead. Previously the pitch servo was grounded to the air frame near the pitch servo.
I made a test flight today and the autopilot system worked flawlessly, even in hot, bumpy West Texas. I will be taking a trip to Utah in about 10 days so the autopilot will get a thorough workout during the trip.
Thanks to Lucas and rest of tech support at Tru Trak.

Great to hear!