Autopilot pitching down in IMC

[randylsnarr]

I have a complete SV suite, with D1000 and D700.
I was making a flight from Utah to Nevada Saturday.
I had a brief period where I was completely IMC for a few minutes. During this time, the auto pilot suddenly pitched down and started descending on its own steeply.
I had to disconnect the AP and manually fly. The horizon kept moving up and down on the Skyview 1000. The problem went away when I left the clouds. This happened about a year ago also in light precip in the clouds.
I think it is static related..
Sorry if this has already been addressed..
Randy Snarr

Lancair 235/320
N694RS

 

[Dynon]

Did you happen to notice what airspeed was doing during that time? Also are you aware of our heated pitot service bulletin?

http://www.dynonavionics.com/docs/support_bulletin_101014.html

Finally, do you happen to have data logging turned on (the more valuable diagnostic log would not have anything older than an hour in it). But looking at any logged data if you have the recording frequency set high enough might yield some clues. Finally, if you do have a data log, we'd love to see it ourselves. We'll send you a note offline.

 

[carey.mills]

Randy,
Any time you have pitch deviations while coupled, note your airspeed and I bet you will have a/s fluctuations.  If you ever get the chance put a pitot static test set on you plane and dial up the speed while watching you attitude indicator.  Trust me, you will be surprised.  The units used a/s as part of the pitch attitude calculation, so any interruptions in a/s (dynamic pressure) will lead to and cause pitch changes, and if the interruptions are large you can see some very dramatic changes in pitch.  Add to the fact that you're coupled and you could potentially over G your aircraft.  Imagine being in the IMC and having a clog/ice up pitot tube.  For units that are feed GPS GS or already have it integrated the fix would be simple, use the GPS for the attitude integration.  That is what the units are designed to do in the event it looses IAS.  Most GPS units are 5 hertz and the signal is very reliable even if your pitot tube is clog.  Give us, the user the ability in the set-up menu to choose dynamic pressure or a valid GPS GS for the attitude integration.  I used to chase pitch auto pilot settings, I was always trying to improve the auto pilot performance and my ride quality, but I realized in the following flight conditions my IAS tended to fluctuated: moderate turbulence, gusty conditions, and light to moderate rain.  In all conditions, when coupled in pitch the performance was inadequate.  My fix, I bought a simple quick disconnect 1/4 refrigerator ice maker valve and installed it on my pitot line.  Now when the conditions are marginal (mod rain and turbulence), I simply rotate the valve 90 degrees, watch the IAS go to zero, the unit reverts to GPS assist and the auto pilot performs magnificent.  Warning: make sure you are un-coupled, because when the airspeed starts to drop, the pitch attitude will become erratic and if coupled the aircraft may pitch aggressively.  BLUF know your system and understand how the system uses dynamic pressure and/or GS and what can happen when you loose one and what the aircraft will do if its coupled in pitch.
Hope this helps,
Carey Mills
RV 7 N727CT
RV 4 N286RV