Thread: CTsw Under Fin damage

Just removed a wrinkled, delaminated, underfin from that CTsw that sat in AL for quite a while. Found evidence of water in the bottom and old wasp nests blocking the drain hole. Neglect. On the positive side, the rest of the airplane held up very well and has cleaned up nicely.

Still, I can't understand how the underfin assembly suffered massive delamination of the plies. There is no foam core and the worst areas are those above the water mark.

Has anyone else seen anything like this? I submitted a Service Difficulty to FD tech support.

Craig Payne
"Yakman"
LSRM-A Rotax S+M

Craig, I was just discussing this topic with a person who experienced this. He said that the under fin started showing "waviness". When he investigated, he found that the drain hole was blocked and water was trapped inside. The fin needed some cosmetic repair. We are warned that water can damage our airplanes and your experience and this other owner's experience verify this. I'm not sure if there is any foam in the fin but think there may be some in order that the water caused damage. I guess that the other "good to know" from this is that we should check that the drain hole is clear since we really don't want to be flying with a couple gallons of water in the tail!!

No foam in this fin, just a couple of plies. The "waviness" is due to separation of the plies and air in between. My suggestion to FD was to elongate the hole to 25mm lengthwise to compensate for different degrees of pitch when parked. Also suggested 1" wide tape around the seam. Their immediate reply was that owners should read the MM part about keeping the drain hole open.

Craig Payne
Yakman

Yakman, the reason there is no foam between the 2 layers of carbon fiber is becaused it was disolved by the water with a high alkaline content. The whole structure on the airplane including the under fin is made from a 2 layers of carbon fiber sandwiched over a layer of foam.

So there was foam in the under fin at one time? I have always heard that rain in that area was acidic, due to emissions from the refineries to the West. Certainly no one went out on the ramp and cleaned with Fantastic or something like that?

Yakman

Nothing wrong with making the hole a tad bigger and I think most of us out west have the old cloth tape or the Bolus tape around the underfin.

Originally Posted by 3Dreaming

... The whole structure on the airplane including the under fin is made from a 2 layers of carbon fiber sandwiched over a layer of foam.

i thought that the fin was fiberglass and not carbon fiber

The underfin I have worked on had foam on the sides and was fiberglass. Any chance that the water in the tail froze causing the plys to delaminate?