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Hello Jurgen,
Didn't see any replies to your question on repairing the bird strike damage
to your 320 wing leading edge, though there may have been some sent to you
privately.
There are some very good repair guidelines and recommendations for restoring
damaged composite material in the latest edition of AC43.13 -- Acceptable
Methods, Techniques and Practices -- Aircraft Inspection and Repair.
The following procedure assumes the glass fibers have been structurally
damaged, and that the glass itself should be replaced (e.g. not simply a
cosmetic blemish). We use the following method when repairing composite
leading edge sections that are not acting as the wall of a fuel cell:
The repair is easier to make if the wing panel is removed and oriented
leading-edge-up, but it can be done with the wing panel on the aircraft if
you are careful, and a contortionist.
First, "tap out" the area of damaged fiberglass to determine the size of the
repair. Using a very small (2-oz) brass hammer, or even a quarter-dollar
piece, continually tap the area of apparent damage, starting well away from
the damaged spot, moving toward the damage until the sound of the tapping
changes. This signals a density change in the material due to the damage.
The tapping should produce a sharp high-pitched sound on healthy structure,
and a dull thud on damaged areas. Mark the transition zone with a
felt-tipped pen. Tap the area from every direction, moving toward the
damage, and make a mark where the sound begins to change. Soon you will have
a "map" of the damaged area. (Also, be aware that any pre-existing structure
behind the leading edge in that area such as a rib, etc., will cause the
sound to change as well.)
Then, make a piece of pre-molded 2-ply Bi-Directional (BID) cloth layup in
the exact shape of the leading edge that has been damaged. This layup will
be used as a backing plate for the section under repair. Make this layup by
taping a large piece of thin plastic (.35 mil to 1 mil thick) barrier sheet
over the leading edge of your wing, immediately adjacent to (but not right
on top of) the damaged area. Cut two pieces of BID cloth large enough to
easily cover the entire damaged area, with plenty of overlap around the
edges. Alternate the orientation of the weave of these two fiberglass pieces
by 45-degrees.
Brush a layer of resin onto the plastic sheet and then add the first piece
of BID, wetting it out and chasing the bubbles by stippling with the brush
ends. Add the second layer, with the cloth weave at a 45-degree angle to the
weave of the first piece, and wet this out as well. Though not necessary,
you could also lay on a piece of peel-ply at this point and squeegee out any
excess resin and air. Be quick to catch and wipe up any resin drips heading
for your nice finish at the edge of the plastic sheet... (Use a big piece of
plastic.)
Allow the layup to cure, then remove it and all the plastic and tape from
the wing. You should now have a thin piece of fiberglass, pre-molded exactly
to your leading edge shape.
Now the scary part. Using a Dremel tool with a cutoff wheel, cut the damaged
section of leading edge out of the wing. Sand an extremely wide feathered
bevel (shallow angle) into the edge of the hole by hand with 100- or
120-grit so that the sanded margin resembles a wide beach all the way around
the hole. For example if the area you cut out was 1-inch by 2-inches, the
sanded margin should should extend at least several inches beyond the hole
all around, for a total area of at least 4-inches by 5-inches. The existing
fiberglass wing-skin thickness of the sanded margin area should feather down
to almost nothing at the edge of the cutout. Technically, we are looking for
a 30-1 bevel, so if the skin is 1/8" thick, the sanded margin should extend
back 30 x 1/8", or 3-1/2" all around.
Now lay your pre-molded BID layup piece directly over the sanded hole and
draw a line all the way around the actual hole with a 1-inch margin all
around. Cut this piece out with scissors or tin snips, and then drill a 1/8"
hole in the center of the piece at the apex of the leading edge curve. Run a
string through this hole into the interior of the curved backing piece and
tie it to a piece of wooden dowel or a wooden coffee-stirrer slightly
shorter than the long axis of the backing piece. Hot glue or epoxy this
stick in place. Carefully make sure you can insert the backing piece by
folding it gently and slipping it into the interior of the wing leading
edge, then pulling it tight with the string up against the interior of the
wing leading edge and centering it in the cutout hole. If you are repairing
a larger hole, several of these string-pulls may be needed, spaced out
around the backing piece.
Next, simply brush resin onto the perimeter of the backing piece and into
the interior surface of the prepared hole, and pull the backing piece with
the string-pull tightly up against the interior curve of the leading edge,
centered in the hole, and allow to cure. Use your creative ingenuity to rig
up something that will keep a nice tension on the string-pull while things
cure. Wipe up any excess or squeezed-out resin on the exterior of the area
under repair before it sets up. Make sure you have bonded the dowel or
coffee-stirring stick into the interior of the backing piece too, since you
don't want it rattling around inside your wing...
Once this backing piece has cured, cut the string off flush with the
exterior and lightly sand the overall repair area again to break the glaze
of any resin that has cured on the outside surface.
Cut several pieces of BID cloth, with the first piece just fitting the area
of the original hole cutout. The next piece should be 1/2" larger all
around, and the next 1/2" larger and so on, alternating the orientation of
the cloth weave. Since each layer of BID cloth when cured is about .040"
thick, it will take about 4 layers of BID to repair a 1/8" skin. Wet out
each layer of BID right on the repair with resin, starting with the smallest
piece first and ending up with the largest piece, which should overlay most
of the sanded area. Cover this with a final layer of peel-ply, and cover the
peel ply with more of the thin plastic sheet. Squeege out any excess resin
and air. Take care not to dent or deform the basic layup when using the
squeege. Wipe up any excess resin and allow the repair to cure.
When fully cured, remove the plastic and peel-ply, and you should have a
nearly-finished repaired surface. Fill with Poly-Fiber's Super-Fil or a
similar dry micro-balloon/resin mix, and once cured, sand to a
ready-for-primer surface. We've got .jpg photos of most of these steps, so
if you want them, let us know & I'll post them to the list or send them
privately to you.
Hope this helps!
Larry Graves & Dave Saylor
AirCrafters Builder Assistance Center
5810 Obata Way Unit 7 Gilroy, CA 95020
Tel 408-847-5957 Fax 408-847-6957
www.AirCraftersLLC.com
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Please send your photos and drawings to marvkaye@olsusa.com.
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