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Thread: Autopilot malfunctioning

  1. #11
    Advanced Flight Systems Shawn McGinnis's Avatar
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    Sep 2012
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    Have you verified your AP MIN and MAX speeds are correct? The flight director will nose down if your minimum speed is set too high. Your MIN and MAX speeds should be a little above Vs1 and little below Vne.
    Shawn McGinnis

    Advanced Flight Systems
    support@advanced-flight-systems.com

  2. #12
    Doh! Just checked and yes, the min airspeed was 135. I can't believe I missed that.

  3. #13
    My autopilot is now functioning as expected under almost all circumstances. However, I still have a problem where the plane will sometimes dive below glidepath partway through a fully coupled approach. Min Airspeed is set to 70 on my AFS5600, which I have triple-verified after my previous error above, and I've confirmed through flight logs that my actual airspeed was well above that minimum (I saw 76 knots minimum in my log for the most recent approach where this occurred).

    After experimenting with this at altitude in slow flight, flaps-down configuration (normal behavior observed) and then on multiple approaches (where the plane sometimes but not always dives below glideslope), I have a theory. I believe this is due to the aircraft being out of trim, and the autopilot not being able to overpower the out-of-trim condition. On my last flight, I was on glideslope to KPWT on ILS 20 approach with autopilot on and properly tracking glideslope on final. Then about 1/2 way down final, it started to dive below glidepath with autopilot still engaged. I disengaged autopilot and could feel that the stick was pretty heavy in the forward trim state. I'm sure that's because I had trimmed for cruise on my way to KPWT and did not re-trim when I slowed down and lowered flaps on final. After taking control from autopilot, I trimmed the plane correctly, leveled off and re-engaged the autopilot, which then correctly followed glideslope again. This strongly suggests to me that this is an out-of-trim problem, although I don't recall feeling any servo slippage from the autopilot before it dived below glideslope, which seems odd. Have you seen any other issues where the autopilot can't overcome an out-of-trim condition?

    Related to this... I have searched multiple AFS and Skyview manuals trying to find out whether the AFS5600 displays any sort of "out of trim" indicator when autopilot is on. It doesn't appear so. There is one mention of a "pitch slip alert" in the Pilot's guide, but no mention of how I enable that or see it anywhere. Maybe it is an audible?? Don't know. Let me know if I'm missing that somewhere.

  4. #14
    OK I'm going to reply to my own post... I have confirmed that the problem I reported above was due to an out of trim condition. I did this by conducting experiments at altitude - engaging autopilot and then manually forcing the plane into an out of trim condition. Autopilot loses the battle and some point and the plane dives (or climbs) depending on which direction you are out of trim. The surprising thing about this is that the autopilot seems to suddenly 'release' without warning and the plane begins to dive or climb aggressively thereafter. My hands are always on the controls even with autopilot engaged and I would expect to feel the servo slipping for a while prior to this happening but I haven't noticed it doing so. I did notice that the Autopilot portion of the EFIS does show a flashing yellow up arrow or down arrow when the autopilot detects this out of trim condition, so it is possible for the pilot to manually correct the trim problem before the autopilot gives up. When flying an IFR approach, there are quite a few other things on your mind, however, so it's quite easy to miss that yellow arrow.

    As I mentioned in my post above, I was unable to find anything in the Pilot's Guide, the AFS installation manual or Skyview installation manual pertaining to this 'out of trim' indicator. It must be somewhere because after I saw it on the screen I vaguely recalled seeing it in one of the manuals... but I've groped through all of these manuals again and still can't find it.

    I think it would make sense to add an audible warning for the situation where autopilot is engaged but encounters a significant out of trim condition. This condition is likely to occur when you are flying a fully coupled approach because the trim condition changes significantly when reducing airspeed and bringing in flaps on final but you might not notice that while the autopilot is still in control. An audible warning would help avoid any surprises. Maybe this is already a configurable option and I missed it? Anyway I think it would also help to document the out of trim arrow in the manual in some more prominent way.

    Love your products. Just trying to offer some suggestions.

  5. #15
    The Vizion autopilot head has scrolling bars to indicate which way it is out of trim, would be nice to see a display of some kind on the efis.
    Dave Ford
    RV10

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