This Q&A is an extract/compilation of older posts on MML.
How do I replace my windshield (windshield replacement)?
Editor:
Virtually all window cracking is the result of installation errors or physical damage.
• The windshield should be bedded in PRC890-B1/2 (or B2), or an equivalent curing sealant. LP sells the correct compound. The screws are to hold it in position during curing, and for structural support during a bird strike or crash. They should never be holding the windshield under normal circumstances. The sealant “glues” it in place. The sealant is a flexible adhesive bedding compound when cured, and lets the acrylic flex as needed to prevent cracking.
• The holes are always much larger than the screws; at least a quarter-inch in diameter. The screws are usually #08 AN525’s (cad-plated high-strength washer-head structural screws). You can also use a structural grade stainless steel truss-head screw (if you can find some), if your AI permits it. I think that the AN525’s hold up fine if they are kept painted with touch-up paint. They are “flatter”, stronger, and won’t create corrosion on the aluminum surface under them. It is always easier to replace a screw with deteriorated cad plating, than to fix the aluminum under a stainless steel screw. They are designed so that any corrosion will attack the cad plating before the aluminum. The screws should never touch the acrylic. The bedding compound should ooze up into the quarter-inch screw holes and surround the #08 metal screw shanks. Bear in mind that I am not near my materials; the screw size may be #10 (-3) rather than #08, but the principles remain the same.
• The holes must be drilled with a proper acrylic bit that “rubs” its way through the plastic. A standard bit will cause tiny chips in the perimeter of the hole, where cracks will begin. You should not feel any “chattering” while the hole is being drilled; the bit should just smoothly wear its way through and out. I try to back up the plastic with a wood block behind each hole, as I drill it. This helps keep the bit from “breaking out” the back of the hole. LP sells the proper bits.
• The screws should be installed with a rubber washer against the acrylic, with a metal back-up washer and nut. They are to be tightened only until the rubber is flush against the acrylic; the rubber should not appear to be compressed. The rubber washers can be made from old inner tube material if necessary.
• Clamps, towels, and wood framing should be used to hold the windshield in place while curing occurs (over several days). The acrylic must not be allowed to settle down from its weight, and rest on the screws. This is a major and frequent installation error.
• The windshield has to be dry-fitted. The edge of the frame is marked on the plastic protective sheet. A layer of wider high quality masking tape is applied to the plastic along the frame line (and on the frame as well), after moving the protective film back out of the way. A second layer of narrower tape is applied to the plastic, on top of the first layer.
• The plastic is then removed, and the area between the tape and the edge is scuffed with a rotary Scotch-Brite pad or similar tool. The metal frame surface is also scuffed clean, then wiped with Alodyne. After a few minutes for the Alodyne to create the protective anti-corrosion coating, the aluminum is wiped with a wet towel (to rinse off the Alodyne). It must then be allowed to completely dry. A heat gun can speed this up if necessary.
• The adhesive is mixed and applied. The easiest method is to buy it in the 6-ounce tubes, and use a sealant gun to apply it (similar to a caulking gun, but a standard gun won’t fit the cartridges). One tube should do the windshield, but you should have a second tube on hand “just in case”. These have a shelf life, and can be unexpectedly bad in the tube. Sometimes the two-part feed system fails, or the mechanical mixer in the tube fails. Usually two narrow beads are applied around the frame, rather than one wide one, so that the sealant spreads out across the width of the bedded area, without having too much ooze out around the edges. Whatever application method works for the mechanic is fine; this is just personal technique and preference.
• The windshield is put in place, with all the holes aligned, and the screws installed. A nice professional appearance touch is to make sure that the Phillips screw notches are aligned with the perimeter and each other. The acrylic sheet should be clamped in place using any means desired to hold it in place without marring it. If rivets also have to be installed, they can be done or can be temporarily Cleco’ed (and riveted later). There should be a visible bed of sealant all around the perimeter, with some slight amount extruding outside over the tape. If there are any slight gaps on the outside, they can be filled with extra sealant, as you would caulk a seam, as long as there is a good bedding width visible from the inside.
• The adhesive is checked periodically. When it has cured enough to be “rolled” with a finger without “stringing” like bubble gum, but is still soft, the first tape layer is peeled off the windshield. This should take 90% of the extruded mess with it. The window is then allowed to cure out. B1/2 should cure out in 36-48 hours, unless the weather is pretty cool. B2 can take from three to six days, depending on weather. The cure must be allowed to complete before the plane is significantly disturbed; you don’t want the sealant to get stress tears in the bedding area.
• After the final cure, a phenolic scraper with a beveled edge can be run around the perimeter, and all the tape removed. Do NOT let anyone scrape the acrylic with any metal tool, for any reason. Any nick or scratch can lead to early failure, just like a scratch in an aluminum spar.
• The blocking and clamps can then be removed, and all the screws should be checked to ensure that the rubber washers are not compressed. Try not to turn the screws themselves, as they will have sealant extruded around them, which keeps the acrylic off them, and also seals them since they are not tightened. If you have to loosen something, hold the screws and turn the inside nuts.
• Everything can be cleaned, and any remaining riveting can be done.
1:Concerning the rubber washers. Are these plain ordinary FLAT washers? Do you use just one between the acrylic and the metal washer or do you also use one between the acrylic and the frame?
Yes, plain, flat, rubber washers. You can make your own from an old inner tube, unless your A&P has some. They go between the metal washer and the acrylic. The sealant acts like a giant adhesive rubber washer on the other side of the acrylic, after it sets up. The sealant should also ooze up around the screw shanks in the oversize holes, providing the equivalent of a rubber cushion all around the screws, once cured out.
2: My understanding is that the frame has 4 parts (top, bottom and 2 sides). Does one remove all these parts (lots and lots of rivets).
I’m pretty sure yours has one bottom strip and one wraparound upper strip. As Jeff said, don’t remove the bottom strip; just the screws that go through it. Since your old windshield is junk now, you can try to pull it out without removing any of the strap (just pull all the screws and braces); you’ll probably have to break it to get it out. Then you can see whether the new windshield can be maneuvered in with the upper strap still in place. I was able to do that with my Sierra back in 1988, but it depends on a lot of factors. For example, the new LP windshield had a slightly smaller perimeter than my original unit. You sure don’t want to overstress and break the new acrylic, trying to over-flex it into place. Getting the old one out will give you a good idea of what it can withstand. A major part of this job is the difficult and messy task of getting all the old sealant off. A Rol-Loc Scotch-Brite pad in an air die grinder works well. Cover everything you can as well as you can, including something like taping an old sheet over the instrument area with duct tape (or something similar). Crap will get everywhere that isn’t covered up.
If your current A&P has a lot of windshield experience, I am hesitant to insist that he do it my way. I do feel strongly about using the PRC890 rather than the tapes, etc., for bedding the windshield. Some other planes use a lot of screws or rivets to hold the windows in place. Beech used relatively few, because they bond all the windows into place with the adhesive sealant. For example, if you check your side windows, some or all probably only have two screws in them.
3: Is the windscreen first bonded to the frame, allowed to set and then the frame bonded to the aircraft or is all the bonding done at once?
You can probably get everything into place, and use the screws and Clecos to pull things together after applying the sealant. Unless your mechanic wants to do it his way/some different way, I would wait for the sealant to cure out before final riveting. That helps make sure that the acrylic doesn’t slip out of place (and touch screws) due to the riveting vibration. It also helps make sure the sealant doesn’t get squeezed out anywhere; the sealant has to have at least 1/16″ to 1/8″ thickness or it loses a lot of strength.
I’m afraid I disagree with any choice of tape and caulk rather than the adhesive sealant. It will almost certainly develop leaks during a windshield’s life, and probably within 2-3 years. Unless the owner just gets lucky. The clear RTV does not have nearly the bond strength of the 2-part Polysulphide sealants (and the windshield flexes a surprising amount). What’s worse, once it has been used, nothing else will stick there. It is all but impossible to get all traces of the silicone removed.
4: Could you describe in more detail how the windscreen is held in place during curing so it does not settle down from its own weight?
I can’t specifically say where to put what parts. You just have to use wood blocks, clamps, Clecos, and anything else that seems to help to secure it in place. Sometimes you can C-clamp or Quick-clamp a 2×4 across the pillars behind the windshield, with several more 2×4 pieces screwed down to the main piece. Then you put a thick layer of old cloth towels over the blocks, and then clamp the conglomeration up against the acrylic, pushing it into place (and helping hold it up). Sometimes you can fit small blocks under the lower edge; just take care not to bond in any of these things.
5: You refer to an “adhesive” and a “sealant”. Are they the same?
PRC890 is an “adhesive sealant”, as are most of the 2-part Polysulphide sealants. In a pinch, you can use 1440, 1422, etc. The materials books and packaging all refer to these as being largely interchangeable; the packaging often calls them “fuel tank sealant”. Some sealants, such as RTV, are only good for weather and water sealing, and only for relatively stationary parts. They do not have the adhesive strength to provide structural support, or to remain bonded to both surfaces despite some relative movement. It’s like the difference between white glue versus a 2-part epoxy glue. The Polysulphides are resistant to virtually everything that could get on them.
6: Finally, I am confused as to the order of doing things. .Could you briefly list the correct order?
1. Have the new windshield and sealant tubes on hand.
2. Remove the fasteners as needed, and get the old windshield out.
3. Remove all the old sealant from the metal surround (to bare aluminum). Treat with Alodyne, rinse, and dry.
4. Put the new windshield in place for dry fitting and hole marking. Drill the holes, apply the premium masking tape as described to metal and windshield, and scuff the perimeter of the windshield.
5. Have the blocking/bracing made up and ready, along with the clamps, screws, nuts, metal and rubber washers, Cleco’s, etc.
6. Mix the sealant, apply it, and install the windshield with the screws and fasteners. Block it in place until some sealant oozes out around the edges, and leave it until it cures. When the stuff gets a bit firm, remove (only) the first layer of tape on the windshield.
7. When the cure is complete (it will take days as described), run the edge of a sharpened piece of phenolic around the perimeter, and remove the tape.
8. Remove the blocking, and verify that the rubber washers are not being flattened out anywhere.
9. Remove all the protective film, rub down the edges of the sealant if any protrude, and clean the windshield. Any exposed sealant can usually be “rolled off” or “rubbed down” against the metal frame edge, just using your fingers.