EAA Sport Aviation - September 2009

maintenance
& restoration

is perfectly clean, don’t use it. Soft cloths such as cotton diapers also work well, but avoid using polyester or rayon; it can scratch unprotected plastics. To eliminate any chance of scratching, use only disposable cloths. DuPont makes special disposable windshield wipes under the brand name Sontara.

Technique matters, too. A number of cleaner and polish manufacturers recommend applying their products in a straight back-and-forth pattern in line with the airflow over the window or windshield, so as to avoid swirl marks that will reflect the sun at any angle.

Keeping your windows clean and polished also has the

Aviation-specific products are available
for both cleaning and polishing plastic
windows and windshields. The simplest
ones consist of only acrylic-safe clean-
ers and anti-static compounds.

advantage of making it much more difficult for future contaminants to stick. A simple, once-a-month cleaning and polish should make it easy to remove almost all bugs and grime with simple water and your bare hand.

Repairing Versus Replacing

Most window damage can be categorized as cracks, crazing, hazing, or scratches. There are no long-term solutions to cracks. You can stop-drill the crack to deter progression, but the bottom line is that the window needs to be replaced. Depending on the crack type and location, there might also be an airworthiness issue.

Crazing refers to the myriad of “spider webs” that one would see through the glass when looking at the sun. It is caused by thousands of minute fractures in the plastic and typically occurs toward the surface of the window. Crazing can be caused by stress, UV exposure, or chemical damage from cleaning products containing ammonia, MEK, or other solvents. It can sometimes be treated by removing the damaged outer layer of the window. There are a number of excellent products on the market that do just that, but the labor is extremely intensive and has several risks. If not done properly, the process can result in optical distortion and/or losing critical window thickness during the process.

Hazing refers to a window that has lost its clarity. Hazing is caused by internal degradation of the plastic, making the window look milky or cloudy. It is most often caused by UV or chemical damage. Since hazing occurs in the core of the plastic, it cannot be repaired.

The most common window repairs are those to remove scratches. So long as they are localized, and the window itself isn’t suffering from hazing or crazing, there are some great systems out there to remove minor scratches.

Novus No. 2 and No. 3 as well as “210 Plus” Plastic Scratch Remover are mildly abrasive creams that can help with small scratches that you can just barely feel with your finger-

Eight Tips for Window Replacement

If you do have to resort to complete window replacement, here are a few tips you can use to make the job easier:

• Consider installing thicker glass than the original. Thicker windows and windshields can greatly reduce cabin noise, are much stronger, and should last longer in service. Many window manufacturers have supplemental type certificates to make this possible.

• Always work in a warm environment, above 70°F if possible. The warmer the windows, the less likely they will be to crack or chip while trimming and flexing them into place.

• Remove the protective film from the new window carefully and inspect the window for any manufacturing defects before starting work. After inspection, replace the protective film, leaving only the perimeter areas (that must be worked) unprotected.

• If the original glass can be removed intact and fits the aircraft well, trace its outline on the new glass with a china crayon as a guideline. Don’t make this your ultimate guide, as the curvatures may vary. But, use it as a reference point for your trimming job.

• For any holes that need to be drilled in the window, be sure to use a special acrylic drill bit. These bits are specially sharpened to scrape a clean hole in the window and will not chip or crack the plastic like standard drill bits. Keep in mind that acrylic expands and contracts, so the holes should generally be drilled oversize.

• Be sure to bevel and sand all cut window edges slightly and remove any tooling marks that may act as stress risers for future cracks.

• Use the proper sealing materials, as specified by both the window and aircraft manufacturers. Some of the adhesives, tapes, and sealing materials have evolved over the years, and there may be more than one option available.

• If the window installation requires flexing the window or feeding it into a channel frame, the process can often go smoother by spraying the window edges liberally with an approved window cleaner. This can act as a lubricant during installation, and most will evaporate quickly once the job is complete.