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Bolts on 4-place tailwheel bracket

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  • Bolts on 4-place tailwheel bracket

    I thought it worthwhile recording my experience with the tailwheel spring retaining bracket:

    Background:
    Some months ago during a short landing contest, I noticed that the retaining U-bracket which holds the back of the tail spring to the fuselage had become loose, despite being correctly torqued when installed. I assumed that the springs had settled-in with use, and that the bracket had not been sitting correctly to begin with.

    In response to that, the bolts were re-tightened in the field. This was not done with a torque wrench because none was available, just torqued "by feel" until the bracket was tight again (read mistake was made!).

    Effect:
    Last weekend, one of these bolts failed during a landing on a rough strip.
    The tailwheel had unlocked and spun around after initial touchdown (tail touched as I entered a wheel landing). The wheel had been facing diagonally forward when it touched the ground again, as I lowered the tail. Marks on the runway showed what had happened. The tailwheel re-aligned itself very rapidly upon contacting the ground, and put a high side-loading on the bracket as it did so. This high load caused the bolt to shear off at the threads.

    Losing this bolt allowed the spring leaf attached to the tailwheel to swing out to one side as I struggled to steer the aircraft down a very small runway. The spring bent the retaining bracket out of it's way, until the spring interfered with the tailwheel steering horn and stopped moving (this also locked the rudder). I was still able to steer with differential brakes.

    Probable Cause:
    The failure surface was primarily brittle fracture, over 95% of which appears to be one final failure, by microvoid coalescence. This is almost certainly due to one large overstress event. There was a small fatigue crack which the final failure initiated from, with several large striations visible - but the crack only represented less than 5% of the failure area (damage from previous landings I assume). The fracture's shape shows it was single-direction bending stress which caused the crack, and a combination of torsional stress and bending which caused final failure.
    All this tells me that the bolt was under a lot of stress when it failed, and that the nut was certainly overtightened.

    Action taken:
    I replaced the bolts with more of the same AN4 bolts, and torqued them correctly.
    Reviewed with maintenance engineer - replaced washers with ground-back washers to allow the bolts to sit straight, the bend in the U-bracket was causing them to bend. Used a washer as a spacer to prevent the U-bracket from bending.

    I suspect this is a once-off due to over-torque. However, the initial cause of the bracket becoming loose could also be strain on of those bolts. Has anyone else had similar experiences with that bracket becoming loose?
    It has opened my eyes to the high operational stresses which tailwheel steering places upon the spring retaining bracket.
    Last edited by Battson; 06-15-2014, 05:28 PM.

  • #2
    In that area, I just recently found that my aft big bolt was loose, and much of the source of my shimmy. With the weight on the wheel, a visual inspection did not show the looseness. I concur that there is a lot of stress back there.

    Comment


    • #3
      I just spoke with Bob about your post Jonathan. He recommends to everyone that they check all those bolts after maybe 10 hours of flying. He said that things kind of settle in after some use, and need to be checked to make sure the bolts are still snug. When the bolts get loose - they get beat up terribly and make a failure of this type much more likely.

      I know it is hard to check them with the weight of the plane on the tailwheel. But not too hard to get the tail wheel off the ground slightly to check the tightness of everything.

      I was talking in Alaska to a man who operates a fleet of PA-18's. He really wants a Patrol but "can't make money with it". He was telling me that the Pipers are great for their purposes up there but that tailwheels & tailsprings etc are constant maintenance items. Different parts back there are always going bad because the tail gets beat up so badly. We don't abuse the tail parts quite as badly down here but still have to watch all those parts closely. Mark

      Comment


      • #4
        Mark, Great info.
        I fly a Piper Pawnee as the tow plane for our glider club in LaBelle FL. Non-Stop T & Ls all day on a grass field. Now I do consider myself to always have gentle landings, but ironically the only "whoops" we almost had was the tail wheel bracket close to falling off about 20 hours after an annual. the tow hook is also mounted on this bracket. We could not tell if it was the landings or glider yanking on it all day. Obviously a lot more happening back there than a typical tail wheel bird, but this area gets a very thorough look over during free-flight now.

        Oh, and the clue that something wasn't right, was when taxiing on grass or asphalt, the bird turned real easy one way, but difficult to maneuver the other direction. You should have heard all of us Monday Morning Engineers coming up with reasons for this taxiing issue until someone bent down and said, "Hey, that doesn't look right". I think that was one of our JR members. He was 14.
        John, Naples FL
        Bearhawk 4-Place Plans #1316
        Patrol Plans #006
        Experience is something you get, right after you need it.

        Comment


        • Battson
          Battson commented
          Editing a comment
          That turning issue is the rotation of the prop combined with the earth's spin, creating a magnus effect isn't it?

      • #5
        Makes perfect sense to me. Easier right turns here and easier left turns in your hemisphere. You'd fit right with the glider guys.
        Last edited by Jflyer; 06-05-2014, 10:13 PM.
        John, Naples FL
        Bearhawk 4-Place Plans #1316
        Patrol Plans #006
        Experience is something you get, right after you need it.

        Comment


        • #6
          I know this was posted three years age, but I have just put about 70hours on 587WP & probably should check these bolts. What is the proper torque for these bolts?
          Jim Lindner

          Comment


          • Mark Goldberg
            Mark Goldberg commented
            Editing a comment
            The proper bolt/nut torque for AN4 bolts (and all other sizes) is in AC43.13
            Mark

        • #7
          Anything special about a tailwheel that would make it any different from typical specs?

          P. 7-9, Table 7-1

          Christopher Owens
          Bearhawk 4-Place Scratch Built, Plans 991
          Bearhawk Patrol Scratch Built, Plans P313
          Germantown, Wisconsin, USA

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