Jeff Ryndak:
Today I saw a small crack about 1/2 inch long extending up from the bottom center screw of my windshield. I just had this windshield put in one year ago, when I put in all new glass (when the new interior went in). Is there any repair that can be done, like on auto glass, to fix this? I hate to have to stop-drill a new windshield.
Bob Steward, A&P-IA:
Stop drilling is the START of the repair. Perhaps an examination of the
CAUSE of the crack is in order, so that you might investigate preventing
new cracks at other locations.
All screw holes in plexi need to be significantly larger than the actual
screw size. Too often I see plexi with holes only slightly larger than the
screw shank, and cracks radiating out from the hole. Because Plexi has a
MUCH higher coefficient of expansion than aluminum or steel, it grows and
shrinks much faster with temperature than the aircraft structure. This
means that the hole may move relative to the screw and push the side of the hole up against the shank of the screw If it’s an “all thread” screw,
that is so much the worse, with those sharp edges of the threads biting into the soft plexi.
A good rule of thumb is a minimum of 50% over size for the screw holes, and where edge distance and the trim around the plastic allow, even 100% over size is not out of the question. We want to CLAMP the plexi, but never bear against the edge of the hole. Therefore a 3/16″ screw should have a 9/32″ clearance hole at a minimum, and as much as 3/8″ is good if you have room.
As for repairing the crack, stop drilling, or better yet, stop MELTING the
end of the crack with a fine pointed tool heated enough to melt the hole at
the end of the crack, would be the starting point. Some Methyl Chloride to
soften the edges of the crack and allow them to fuse together can be a good follow up step.
Any acrylic glazing shop will have MC in a small quantity. Avoid CA or any
other “cement”. We are NOT gluing the crack, we are softening the base
material and letting the edges flow back together. Once the MC has fully
evaporated (over night) the cloudy area can be polished to restore the optics.
For an explanation of what the approved methods of repairing the plastic
windows, consult the AC 43.13-1B “Acceptable Methods, Techniques and
Practices” Chapter 3, section 3 for repairs to Transparent Plastics.
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/99C827DB9BAAC81B86256B4500596C4E?OpenDocument
Sorry for the nasty wrapping URL, but that is the FAA for you!
Here is another link to the same page that has been “fixed” with the “URLizer”.
http://urlizer.com/00/1877/
Obviously some of the repairs are worse than the initial crack. Drilling
holes and lacing with safety wire is one example that one would only use
to ferry a plane to get it really fixed. Gluing a slab of plexi on the
inside or outside is an ugly but very effective long term fix.
Editor’s note:
Great advice from Bob, as always. As has been written before on MML and BAC, improper installation is the number one cause of window and windshield cracks. The windows in our BAC aircraft are designed to be bonded in place. The screws only provide positioning while the Chemseal cures out, by clamping the acrylic with rubber washers. In the case of the windshield, the screws also provide backup security in case the bonding sealer fails. If you look at the side windows, you’ll see that they only have a couple of screws, to position the acrylic while the Chemseal cures up. In too many cases, A&Ps who are used to Brands P and C install Aero Center aircraft windshields the same way; using felt or sealing tape, neither of which is a curing binding sealer. That leaves the screws to hold the windshield in place; and without adequate screw hole clearance, eventually the acrylic shifts, contacts a screw shank, and vibration starts a crack.
Make sure that you get this crack fixed right away. It might grow slowly, or it might go halfway up your windshield the next time you start the engine; especially in cold weather.