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Dynon Skyview 'Autopilot Pitch Disconnect'

F

F18Sailor

Well Known Member
Patron
I do have an inquiry with Dynon technical support, but figured a smart person here on VAF might be bale to help.

I receive an 'Autopilot Pitch Disconnect' message every so often in my Vans RV-9A. This behavior existed previously, and appears to be continuing, despite installing a newly overhauled and upgraded (SV-32 to SV-42) pitch servo. I also sent the SV-AP-PANEL back to Dynon per SB-00090, so its unlikely to be that. Any ideas what may be causing this? The AP status on the Dynon display appears to be engaged the entire time. It also appears that the roll is disconnected at the same time in the log file, but this audio message is 'overwritten' by the newer pitch disconnect message?

I can post full alert data logs, but a snippet of the event is below:

AUDIOSTOPAP_DISC
AUDIOSTOPAP_ROLL_DISC
AUDIOSTOPAP_PITCH_DISC
AUDIOPLAYAP_CONN

All this takes place in under 1s, so no real impact to the control of the aircraft. It does appear to happen more in turbulence. Some thoughts:

1) Maybe a failing disconnect switch? Wouldn't explain the immediate re-connect.
2) I did check as much of the wiring as I could during the CI, no obvious issue with the servo wiring or disconnect wiring. I didn't check the wiring inside the control stick.
3) Maybe a gain setting is cranked up too high and the AP gets confused?
 
There are three things to check:

1. Ensure that your servo is rigged properly. The further out your linkage is rigged on the control arm, the more torque is required by the servo, to the point where it may overload and disconnect. A higher torque servo can help as well.

2. If you have autotrim, ensure that it has the proper range of motion and speed. This will offload the AP servo and prevent disconnects.

3. Follow the autopilot tuning guide provided by Dynon.
 
Iirc, the ap disco wire comes from the servo and if this wire is taken low or grounded the ap disconnects. Your disco switch is just connecting that wire to a grd. My guess is that the disco wire has worn away insulation from rubbing on something and allowing intermittent contact with the airframe (matches symptoms of increased occurrence with turbulence) or you have a problem in the the switch.
 
I had the same issue in my RV-6. Went crazy trying to figure it out over the course of years. It would happen very rarely but did happen. My cause was that I had installed both servos on one 5 amp breaker. My read of the instructions show a 5 amp breaker for the servos, that’s what I did. Once I added a second breaker, one for each servo, problem solved. What do you have feeding the servos?
 
vlittle said:
There are three things to check:

1. Ensure that your servo is rigged properly. The further out your linkage is rigged on the control arm, the more torque is required by the servo, to the point where it may overload and disconnect. A higher torque servo can help as well.

2. If you have autotrim, ensure that it has the proper range of motion and speed. This will offload the AP servo and prevent disconnects.

3. Follow the autopilot tuning guide provided by Dynon.
Click to expand...
1) I believe my servo is rigged properly. As noted, I went one size up (per the AFS manual) on the pitch servo recently. I don't have a good photo of the reinstall, but I believe I am on the outer arm of the servo.

2. No auto-trim (as far as I know)...I get a trim message every so often and manually re-trim.

3. Copy on the tuning guide
 
RFazio said:
I had the same issue in my RV-6. Went crazy trying to figure it out over the course of years. It would happen very rarely but did happen. My cause was that I had installed both servos on one 5 amp breaker. My read of the instructions show a 5 amp breaker for the servos, that’s what I did. Once I added a second breaker, one for each servo, problem solved. What do you have feeding the servos?
Click to expand...
I have a 7.5 amp breaker feeding both servos. Per the Dynon Autopilot Installation Guide (https://dynonavionics.com/includes/guides/RV6-7-9_Pitch_Kit_Instructions_Rev_F.pdf), the SV32 servo draws 1.33A and the SV42 2.03A, so combined I'm at less than half the current rating of the breaker. Seems unlikely that this is an issue, unless the wiring is incapable of feeding the current?
 
lr172 said:
Iirc, the ap disco wire comes from the servo and if this wire is taken low or grounded the ap disconnects. Your disco switch is just connecting that wire to a grd. My guess is that the disco wire has worn away insulation from rubbing on something and allowing intermittent contact with the airframe (matches symptoms of increased occurrence with turbulence) or you have a problem in the the switch.
Click to expand...
I agree that this is where to start. I checked the wiring where it exits the control stick and no signs of chafe there. Likewise with the wire runs aft of the baggage compartment bulkhead. I'm included to take a look at the switches themselves next.
 
Last edited:
Some update from ground testing at the plane today:

1) Autopilot configuration speeds are pretty open, but I lowered the minimum value to my 1.3*Vstall, or 64kts (down from 67kts), and raised the max value from 165kts to 170kts. I was well within this envelope when the AP disconnect occurred, so this is unlikely to change anything

2) I did as much stick wiggling, switch tapping etc. as I could. No false disconnects occurred and both switches worked properly.

3) I tried enabling "broken line detect" for the disengage switch. This resulted in a caution/warning that the line was broken, despite the switch working normally. This indicates that I do not have the optional 5k Ohm resistor across the switch

4) Interestingly, while engaging the autopilot at an aft stick position, with the target altitude set to 300ft and the current altitude ~260ft, as the autopilot commanded further up elevator, the current draw on the servo appeared to increase, despite no air over the elevator and the control stick being completely free and clear through the entire allowable range of motion. Is this normal during ground testing, or does this suggest binding in the pitch servo linkage?
 
F18Sailor said:
Interestingly, while engaging the autopilot at an aft stick position, with the target altitude set to 300ft and the current altitude ~260ft, as the autopilot commanded further up elevator, the current draw on the servo appeared to increase, despite no air over the elevator and the control stick being completely free and clear through the entire allowable range of motion. Is this normal during ground testing, or does this suggest binding in the pitch servo linkage?
Click to expand...
Was the stick fully aft, so that the elevators were up at their limit? if thats the case and the AP tries to command more up elevator, it might indicate that the limit calibration needs to be redone. I forget the exact process, but I think the on-ground calibration has you move the control surfaces to their limits so the AP controller knows those limits
 
N890GF said:
Was the stick fully aft, so that the elevators were up at their limit? if thats the case and the AP tries to command more up elevator, it might indicate that the limit calibration needs to be redone. I forget the exact process, but I think the on-ground calibration has you move the control surfaces to their limits so the AP controller knows those limits
Click to expand...
That would make sense to me, but this was done immediately following a servo calibration, which has you max the stick out in all 4 corners to determine the limits. I saw the increase in current draw on the pitch servo at a travel between 75 and 80, and the max up travel is 110 in my case. Certainly sounding like I should remove the aft baggage bulkhead and check that there is no linkage binding.
 
F18Sailor said:
That would make sense to me, but this was done immediately following a servo calibration, which has you max the stick out in all 4 corners to determine the limits. I saw the increase in current draw on the pitch servo at a travel between 75 and 80, and the max up travel is 110 in my case. Certainly sounding like I should remove the aft baggage bulkhead and check that there is no linkage binding.
Click to expand...

Yeah does sound like a physical check of the servo would be a good idea. Do you have the servo limit brackets installed on the servos?
 
Update, I have limit brackets installed on the servos. In the case of the pitch servo, the arm was engaging the aft limit bracket prior to the elevator itself engaging the stop. I corrected this by adjusting the linkage so the servo sits in the middle of the limit bracket range. The other change I made was to secure the D-Sub connector

I went up and flew the autopilot tuning procedure a few days ago. I had no servo disconnects on the way to the tuning area, despite some light to moderate chop down low-it was smooth above 4,000ft. Near completion of the roll tuning, I did have 2 "X's" show up on the pitch side of the autopilot control message window, but super brief. No errors during the pitch tuning, nor for >1hr of flight afterwards (incl. 3 coupled approaches, also in some chop). My thinking at the moment:

1) I tend to think this is pitch servo/wiring related. I didn't open up the d-sub housing and check the pins for bad crimps/loose solder.
2) It could be current related, maybe higher draw as the servo warmed up? Hard to diagnose; I am debating splitting the two servos across 2x 5A circuit breakers, or possibly increasing the existing circuit breaker from 7.5A to 10A. Thoughts?

I'll get another thread started with my tuning numbers. It would be nice to share tuning results across the RV-7/9 community!
 
Minor update here:

I'm about 99.9% sure its not wiring. We (experienced A&P/IA, EE to boot) crawled all over the plane for 3 days, couldn't find anything, went and flew, issue was immediately apparent as soon as we hit turbulence or entered a turn. Not a problem in straight and level flight, for the most part. Servo breaker (7.5A) is not getting warm.

I think vlittle pointed out tuning, which may be a factor, but I'm starting to think the root cause might be software/air data. What is known:

1) The pitch servo isn't receiving a disconnect signal. It is receiving a disengage signal. This is from the in-flight AP status menu. Hence, the wiring can almost certainly be eliminated. Servo data never drops offline.

2) The problem first appeared after updating to newer versions of the Skyview Classic software. I am on the latest revision.

3) At the moment, it appears worse than on my prior flights. What changed? I had an ADAHRS box fail on on me-sent back to Dynon and re-installed exactly as the previous one came out of the airplane, BUT I did not check that it was 0°/0°/0°. This is on my to-do list. For the record, Dynon said the box failed due to component age. The standby ADAHRS in the system was performing normally and both match when doing a compare.

4) I did not adjust the command bars to be level with the horizon with the aircraft in a level attitude; I had done this on the previous flight and the system behaved much better...

5) I am noticing "shear", i.e brief but quick changes in airspeed when in turbulence, and delays in the visual pitch attitude of the aircraft. I'm also noticing oil canning in the wing skin adjacent to my pitot-static probe. It is flexing more than is ideal and may be causing false readings?
 
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