STEC 55X pitch oscillation

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  • #84889

    Well, I jinxed myself. I previously posted that the 55x in my 340 had no pitch oscillations… or so I thought. But, the past three flights I’ve made have included pitch oscillations with both the auto trim on as well as excessive trim commands with the auto trim off….. ONLY when I’ve had a tailwind! Headwinds or no wind, it’s dead on stable but with tailwinds ranging from 11-22kts, the oscillations begin.

    Anybody have any ideas?

    #103830

    Hi Jim, I’m not sure if my problem with the 55x is what you would call a pitch problem.

    I am getting the problem looked into and will post findings here..

    Symptom:
    After flying for approx 1.5 hours at above FL200 with ALT hold the 55x commands a strong pitch up or down without warning. The FD bars also indicate the same pitch in the PFD. Altitude changes are up to +/- 2000ft per min. The deviations occur every 10 secs or so. OAT below freezing.

    There is a document on this site (tech docs section) that discusses a range of things to check with 55x pitch problems. These are being looked into on my aircraft.

    I am also looking for help from this forum.

    Tom.

    #103832
    jwarrington
    Participant

      Tom,
      I had this happen a couple of times and turned on pitot and stall and vent heat
      And it stopped
      Jack

      #103834

      Yes, that sounds like a static issue.

      Mine presents itself as a gradually increasing oscillation where the auto trim seems to chase the pressures on the bridle cable or servo but it lags so it never properly releases the pressure, resulting in it chasing it one way, then the other, each time getting worse.

      What I can’t figure out is why with a tailwind? External pressure on the elevator and unequal cable tensions producing slack on one cable?

      I’ve got the pdf found in the Tech pubs section of this website and have reseated the head unit in the panel and have tried turning the auto trim off. With the auto trim off, the display unit illuminates trim up or trim down requests, finally resulting in an audible warning.

      If I disconnect the autopilot at the exact altitude that I had initiated the altitude hold mode, there is no pressure either direction on the yoke, even though there may be a trim up or down command illuminated on the head. Almost like there is a gain setting, run timer, or something telling the autopilot to first initiate the trim command until pressure on the cable is relieved, then run for X seconds or until an opposite pressure of X developed.

      #103835
      jwarrington
      Participant

        Also,
        You might try calling Scott at S Tec.
        817-215-7600.
        He has been helpful to me.
        Jack

        #103836
        quote JBurnsIII1:

        But, the past three flights I’ve made have included pitch oscillations with both the auto trim on as well as excessive trim commands with the auto trim off….. ONLY when I’ve had a tailwind!

        Not sure about the tailwinds, haven’t had one in so long..

        Make sure cable tension are set right at the max of what stec recommends. I’ve had pitch issues when they were not.

        Also there is a run in procedure for the servos for the brushes. I’ve heard people have problems if this was not done. You can do the procedure again to even out the brushes.

        Andrew

        #103844
        quote :

        I had this happen a couple of times and turned on pitot and stall and vent heat
        And it stopped

        Thanks Jack, I had selected pitot heat on as was suspecting freezing moisture in the lines. Was in clear air all times. I have not tried turning elec trim off but do recall the trim wheel did not move during the excursions.

        #104327

        Flew on the 55x for quite a few hours this week after the techs improved the ‘strapping’ of the rack that the 55x sits in. I also flew with pitot heat on when near or at a freezing OAT.

        Happy to report I had no pitch issues.

        Others have since commented that the 55x needs a very solid rack with low vibration to operate at top performance..

        #104333
        quote TWhite2:

        Flew on the 55x for quite a few hours this week after the techs improved the ‘strapping’ of the rack that the 55x sits in. I also flew with pitot heat on when near or at a freezing OAT.

        Happy to report I had no pitch issues.

        Others have since commented that the 55x needs a very solid rack with low vibration to operate at top performance..

        The rack/connector is a known issue or point of weakness on the 55x. What do you mean by strapping?

        #104334
        quote MHangen1:

        The rack/connector is a known issue or point of weakness on the 55x. What do you mean by strapping?

        I understand that better rack strapping means tightening the rack to the panel or similar to prevent movement or vibration…

        #104346

        I talked to S-Tec again today, and they are waiting on approval to revamp the installation process of the pitch servos. They said they are weeks away still, but this could solve all of the pitch problems. For more information, I was directed to contact Chad Howard @ 817-215-7776, or chad.howard@cobham.com. He is out of his office a lot, but he is the point of contact for this new installation process.

        #104347

        Thanks! Hopefully, this will be good news. Did anyone elude to any clues on why they are suspecting the servo install to be suspect? I talked to Scott at STEC a couple days ago and he stated that many issues with 340/414 pitch oscillations can be traced to the static system, however, he did indicate that over sensitive servo switches can also be to blame.

        #104348

        No, no one gave any clues or reasons for the new installation process. He did use the term “beefy” a couple of times.

        #104425

        I had slight porpoising tonight. Was enough to annoy the passengers but not violent.

        The thing is I really watched the flight director bars on the PFD as the porpoising was occurring and the command bars dived and rose essentially leading the autopilot.

        Is this something anyone else has seen ? I would have thought a servo issue would not be reflected in movement of the FD bars ???

        #104426

        Are you sure that it wasn’t the other way around, that the slight porpoising wasn’t causing the FD to react? That is a more likely scenario. Very light porpoising can come from a slight loosening of the elevator cables tension.

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