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During the annual inspection on our glider, we discovered that the maximum downwards extension of the right aileron (i.e. full left stick) was outside the manufacturer's tolerance. The measured value was 32mm and the manufacturer allows 40mm ± 5mm.

What are the possible causes of a change in the geometry of the system such that the ailerons no longer move in the range they did last year?

For reference, the glider is a 1983 Schleicher ASW19b, with standard l'hotelier control connections and aluminium push rods over bell cranks in the fuselage and wings.

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    $\begingroup$ We found the problem. The control stops are under the seat and are made of plywood. It looks like at some point in the past someone had added a small layer of plywood to reduce the control deflections but that over time the glue had deteriorated or the wood had swollen, so that there was now too much stop. Removing the extra ply and sanding the glue back to the wood fixed the problem. $\endgroup$
    – bwduncan
    Commented Jul 18, 2017 at 10:57

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We found the problem. The control stops are under the seat and are made of plywood. It looks like at some point in the past someone had added a small layer of plywood to reduce the control deflections but that over time the glue had deteriorated or the wood had swollen, so that there was now too much stop. Removing the extra ply and sanding the glue back to the wood fixed the problem.

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    $\begingroup$ plywood does tend to swell when it gets moist. $\endgroup$
    – jwenting
    Commented Jul 20, 2017 at 5:36
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Possibly an obstruction somewhere, causing it to not move through its full range because it gets blocked by something. Or a cable that's become too long (stretched...) causing full deflection to no longer move the surface through its full range of motion.

My guess if the aircraft is regularly cleaned and inspected in between flights is the latter, as it's something that's creep in slowly and gets worse over time.

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    $\begingroup$ Thanks for your answer! There are no control cables in this case (only the rudder). Obstruction is a possible and worrying explanation, I will send a camera along the control circuit. $\endgroup$
    – bwduncan
    Commented Mar 17, 2017 at 14:25
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Coming to this question late, but a recent experience I had might suggest a reason.

While assembling our club's Open Cirrus last weekend with another pilot, we noticed the gap tape on the elevator was missing. My compatriot applied new tape while I attended to other assembly items. After we completed the rigging, we did a positive control check, and, to our surprise we were not getting full deflection on the elevator. After some examination I discovered the issue...the tape was applied in manner that prevented the elevator from moving completely.

When you apply tape to an elevator (or aileron) you have to apply the top tape with the control surface down and the bottom tape with the control surface up, thus allowing full movement.

Is this your issue?

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    $\begingroup$ We have experienced this in the past. In some cases the tape can shrink over time, so you get full deflection when it's first applied but weeks or months later the tape prevents full movement. You have to make sure there is a bit of slack in the tape when the surface is at full deflection, and it's worth checking this at every DI. $\endgroup$
    – bwduncan
    Commented Jul 18, 2017 at 10:54
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It's very hard to say exactly what is wrong in this case, but much if it is worrying. As it is only one of the ailerons which is not deflecting that does narrow the possibilities. It could be that there is an obstruction which is preventing full deflection, this could be something as simple as foreign object debris working its way into somewhere, or something has come loose. A bell crank could be loose and shifting rather than transferring all of the movement. One of the linkages in the pushrod system could be loose. The wing could have flexed somehow, changing the length of rod needed to deflect the aileron fully.

Try having someone hold the stick still while you wiggle the aileron, if you get play in it then it's likely a loose bit somewhere. In any case I would get some professional help on this if you haven't already.

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  • $\begingroup$ Thanks for your insight! The other measurements aren't perfect (don't have them to hand) but are within the tolerance. The tolerance for the downgoing aileron is less (±5mm) than the upgoing aileron (±10mm), which is to be expected with differential ailerons such as these. So it's possible that the other aileron is affected in the upgoing sense, but is still within the limit. $\endgroup$
    – bwduncan
    Commented Mar 17, 2017 at 15:44
  • $\begingroup$ Will also check explicitly for play, but I would have hoped that we would have noticed 8mm of play! We tried loading the wings by hand in order to induce the sort of flexing length change you describe, but the deflections of the ailerons did not change. $\endgroup$
    – bwduncan
    Commented Mar 17, 2017 at 15:46

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