In search of perfect plex
You usually first notice that your windshield has suffered the ravages of time upon turning final into the sun in the worst of summer's haze. Runway...what runway? About then, most pilots would dearly love to be able to jump out and give the plexiglass that loving polish job that sometimes just seems like too much trouble. Wouldn't it be nice to reach for a button and have the washers and wipers — just like in your car — take care of all that?
Obviously car and airplane windows are vastly different creatures. In fact, unless you're talking about turbine equipment, using the word glass in reference to airplane windows is incorrect. To discuss the average Cessna's or Piper's windows, we need not launch into a dissertation of high-tech multi-ply glass sandwiches or nearly bulletproof polycarbonates. For reasons of weight (and to a lesser degree, cost), light airplanes employ windows, or transparencies, made out of acrylic, the most common of which is plexiglass.
Prime advantages of plexiglass include light weight and simplicity in forming. Look at the complex curves of a Cessna 172's windshield, and you can see how that would be difficult to form in glass. Typically, windshields come in various thicknesses, depending upon the airplane and its intended speeds. These sizes range from eighth-inch-thick for lighter, slower models to quarter-inch or three-sixteenths-inch thicknesses for most GA airplanes. Consumers looking for improved windows have spawned a good-sized industry providing thicker transparencies, in thicknesses of up to a half-inch. Several sources can not only provide thicker than stock windshields and side glass, but STC'd kits that eliminate the windshield center post used on earlier models. Popular and desirable, these kits not only give you new transparencies, but better aerodynamics and improved visibility as well. Industry experts warn to be certain that you have all the correct STC paperwork with the new windows. Reports have been received of installations lacking the official STC that are materially inferior to the approved items.
When you opt for a thicker windshield, you are getting two things: better protection from birds and debris and greater sound-deadening capability. Thinner plexiglass flexes enough to resonate slightly with prop pulses, increasing the cabin noise level noticeably; think of it as a large, see-through loudspeaker in your face. Save for the weight penalty, there's really no drawback to having thicker windows all the way around, as long as the installation is done properly.
Extra acrylic thickness doesn't add onerously to the price, either. On a Cessna 172's windshield, the difference between eighth-inch and three-sixteenths-inch plexiglass is just 10 percent on a $250 windshield, and you can add about another 10 percent to get a quarter- inch-thick transparency. You can, however, run into additional labor costs for installation of thicker windows if any airframe alterations are required. Figure 10 to 15 man-hours to replace a stock windshield. Dennis Wolter's Aircraft Modifications and Upholstery, Incorporated, of Cincinnati, performs several sloped-windshield upgrades in Beech singles and twins. Wolter figures on 25 man-hours over two days to complete the modification. Budget between $2,000 and $2,500 for the full upgrade.
Wolter is an advocate of thicker transparencies, up to a point. "There is a diminishing return for plexiglass thickness," says Wolter. "I think three-eighths-inch is about as thick as you want to go, since anything thicker can lead to installation problems and might create some lensing," or optical side-effects in sharply angled windshields.
In choosing a shop for plexiglass replacement, you should look first at that facility's experience with your airplane model. As Wolter points out, "We do a ton of Bonanzas and Pipers, but if a Mooney owner comes in here, I would recommend he seek out a shop familiar with Mooneys." Wolter says that numerous tricks to installations become obvious only after some experience. Of course, most quality shops, his included, can do a good job on an airplane model they've never seen before, but you'll save time and money going to a shop that has worked on your kind of airplane in the past.
Deciding when the old transparencies need to be traded for new ones can be a difficult task. Generally, you should look at the overall condition of the acrylic, the thickness, and the security of the mounting. Deeply crazed glass is not just an optical eyesore, it's also probably a structural problem. Crazing is the result of intergranular cracks at the bottom of scratches. As the acrylic panel flexes, these miniature cracks propagate outward and can break down the structural integrity of the window.
Cracks in windows form from advanced crazing or from stress risers set up at the edges of the transparency. Should the window — which normally flexes in the airframe a bit — come in contact with a nearby bolt head, for example, a crack could start behind trim moldings and not be seen until it appears at the edge of the window mask. At that point, there's little you can do except stop-drill the crack and find the source to keep it from happening again at some other point in the window. (Proper installation plays a huge role in the propensity for cracking; it's possible for an installation error to set up the scenario for cracking that might not actually occur for years, at which point any warranty will be but a memory.) A few in the industry believe stop-drilling of cracks to be dangerous, but most agree that judicious stress-relieving in this manner is reasonable. But if you've got cracks coming at you from every corner, it is certainly time to think about new windows.
Another reason to retire a transparency is due to fogging — that milky haze you see in airplanes left out of doors for decades. Ultraviolet light penetrates the acrylic and breaks down the material. Clouding occurs all through the glass; you cannot cure a milky windshield with abrasives. It must be replaced.
Other window problems include air and water leaks. High-wing Cessna windshields and Piper side windows seem infamous for leaks. In the Cessna case, leaks usually occur because the felt that sandwiches the upper portion of the windshield has come unseated, according to the Cessna Pilots Association technical staff. And the fix is not to whip out the silicone seal and smear it around the outside of the window, although this will generally provide a short-term fix. The high-wing Cessnas' windshields are designed to float in the airframe, and proper orientation of the felt is critical to sealing. For the Pipers, using the proper sealant and securing the channels in which the windows ride helps a lot; beware of commercial RTV (room-temperature vulcanizing) sealant, which contains salt and can lead to rapid corrosion of the airframe it comes in contact with.
Compared to chasing down leaks or cracks, the simple matter of keeping the transparencies clean is, well, not as simple a matter as you might think. One thing the experts recommend is to remove any debris with a generous helping of water, plain old water. Don't even consider going after the windows with a dry cloth, no matter how clean the transparencies appear or how soft the cloth. Most agree that you should just use a bare hand, devoid of jewelry, so you can feel any imperfections in the plastic.
Once the window has been washed off, you can resort to a combination of warm water and isopropyl alcohol, wiped dry with a clean flannel or terry cloth towel; beware, too, of towels with synthetic materials, as they may leach out chemicals dangerous to the plexiglass. They can also impart static electricity to the plastic, which only attracts more dust and dirt.
Some sources say don't even use the alcohol, but you might need something stronger than water to remove dried-on bug parts. If in doubt, start with a weak alcohol/water mixture and work up in strength to about 50/50. Also, you should avoid commercial window cleaners unless you know absolutely, positively that they won't harm the plexiglass. Obtain a throwaway piece of acrylic as a tester if you want to be really careful. (The official line from the acrylic makers is that they have not tested every cleaner, and many remain unknowns as to their effects on the windows.) Never, ever use anything with a petroleum base or containing a solvent.
If you have perfect windows, you can stop right here. But few of us do, and so some polishing of the inevitable scratches occasionally is in order. There are two distinct schools of thought here. One is to use a wax-like product that fills in the minor scratches. Pledge furniture wax is a perennial favorite and one almost universally praised as a short-term scratch-filler and light cleaner. The wax also helps keep bugs from adhering so easily and promotes the shedding of rainwater. Pledge doesn't last long on the window, however, so you will have to apply it often to get best results. You can also use lemon Pledge on the inside, and make your airplane smell nice and fresh.
Another wax-and-fill method is to use a product like Meguiar's plastic polish. Meguiar's is actually a very mild abrasive with a wax, so the application (with the bare hand, preferably) helps smooth out the fine scratches. The wax fills in the deeper scratches — temporarily — and offers the same insect- and rain-shedding qualities of Pledge.
John Archer, manager of sales and marketing at Micro-Surface Finishing Products, thinks there's a better way to deal with those scratches. His company produces the Micro-mesh kits (under $25 for the basic kit), which include several grades of very fine sandpaper. If you consider acrylic sheet as a clear laminate, you can see that removing a scratch should require nothing more than working through the laminate to the deepest point of the scratch, blending it with the surrounding material, and polishing the scratches out.
Micro-mesh kits include several very fine grades of special sandpaper; the cutting surface of the paper is affixed to a thin resilient backing that is supposed to make for a more level cutting plane. You start with the most coarse grit appropriate for the kinds of scratches you're trying to remove (the instructions provide clear guidelines) and work through ever-finer grits until you have polished the surface smooth and clear. It's a fair bit of work — count on spending several hours if you plan to do the entire windshield, for instance — and requires substantial patience. But the results can be worth the effort because you can save a windshield from premature retirement and yourself a fair bit of pocket change.
And with your windows rid of those scratches and pits, you might even be able to see the runway when facing the setting sun. At least you'll stop wanting to reach for the wipers.
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