Our Sierra has a cracked pilot side window that needs replacement. Does replacing a side window qualify as something that an owner can do without a mechanics sign-off? What is involved? What kind of sealant is used?
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An aircraft owner can legally replace side windows (but not the windshield). Don’t forget that a log entry is still required, with the owner’s signature. Window replacement is not particularly difficult. It does include a lot of grunt work; and if the proper materials and methods are not used, you will be wasting a lot of time, effort, and money (and will encounter future problems). Here is my personal approach to side window replacement; others may or may not agree with parts of this.
Do not use any form of silicone sealant, nor any form of strip caulk sealant. I won’t get into all the details why; just too much extra to write. Some of these products have their place in other airframes, but not ours.
You have to use a bonding-curing polysulphide sealant. The retention (and sealing) system is dependent on the cured sealant, not the screws. The window is actually glued in place. This provides both waterproofing and stress relief from flexing. These sealants are more commonly known as fuel tank and airframe filet sealants. Don’t let that put you off. Nearly any of the polysulphides will do the job. Examples are Chemseal B1/2 or B2, and PR1750, but there are many more that will work. If you call LP Aeroplastics, they will tell you what I am telling you, on the sealant type. Don’t be mislead by the fact that other aircraft may use different sealants (like felt tape or bedding tape). Our line of aircraft requires a bonding adhesive sealant, for proper window retention, water sealing, and crack prevention. Here are the key sealant and installation characteristics you are after, to do the job easily and correctly:
1. You want a two-part sealant that comes pre-packaged within an application cartridge. You follow the directions on the package to mix the sealant, inside the tube. A three-ounce tube would probably do the one window, but you might want to get the six-ounce tube to play it safe. You would be wise to avoid things like the pint cans and application via putty knife; it makes a mess.
2. You will need a special application gun to fit these tubes. You can often find a used one cheap on eBay, but unless you find a complete kit for sale, make sure that the gun size matches your tube size. There are different barrels for the different tubes. I strongly recommend the air gun, but the hand gun will work too. I have bought the air guns on eBay for twenty bucks. Along with being easy to use (they release air automatically when the trigger is released, to instantly stop flow), they also seem to get more of the sealant out of the tube.
3. The required sealant is a Class B two-part Polysulphide. It is usually available in Cure 1/2, or Cure 2. This is theoretically a specification for the usable pot life. If have found wide variations in actual pot life, apparently dependent on many factors (temperature, humidity, surfaces, frequency of pumping, etc.). In my opinion, the key factor will be how long you can let the plane sit without any movement, for final cure. If it will have to be moved in no more than three days, use the B1/2. If it can be allowed to sit for at least six days, use B2, just for the workability margin. With just the one window, I’m sure you’d be OK with the B1/2, if that’s what you most easily find.
4. This stuff isn’t cheap. It also typically has a max one-year shelf life, so you have to have a source of fresh tubes. Local prices may range from $30 to $60 for a six-ounce tube, but most commonly toward the lower end of that scale. Frankly, if you have a large FBO, aviation service facility, or repair station nearby, I would go by there and ask them if they have any B1/2 or B2 Polysulphide sealant. See what they have, check the shelf date, and the price. You might be just as well off with a convenient local purchase.
5. The window frame has to be cleaned down to bare aluminum. Do NOT use any kind of wire brush or wire wheel for this. You can use a stainless steel putty knife for gross removal, followed by a Scotch-brite hand pad, but a cup brush or pad made of coarse Scotchbrite, used in an air drill or die grinder, will do the fastest and best job (by far). This will make a mess, so cover up everything else, and try to do it with the door open and someone holding it for you. After bare metal is achieved, brush it with Alodyne, and wipe off the Alodyne with a wet cloth. Then let the surface dry completely.
6. Dry-fit the window, using the main fore and aft screw holes. Make sure that the window isn’t positioned too low for proper positioning of the storm window hinge and window operation, and sealing all the way around the perimeter. Note that the screw holes should just fit the screws at this stage (and will be enlarged later). Mark the frame perimeter with a safe marker (water erasable, china wax, crayon, etc.). Also mark the storm window hinge holes. Frankly, I also add an extra screw-hole and screw just aft of the storm window, using the same spacing as the leading screws. I do this to reinforce the small area on the lower aft corner of the storm window opening, as the window is prone to cracking there. The extra screw makes sure that both sides of the opening corner have the same support. After the dry fitting, remove the window, do any needed edge trimming and storm window opening trimming, and apply a layer of premium masking tape around the frame line. I like the 3M blue tape; I have never had it leave behind adhesive, when removed.. A narrow width will make the curves easier. You need the tape to wind up being at least an inch wide, and wider is better. Then use the Scotch-brite pad again to scuff the window perimeter where the sealant will touch it. After the scuffing, apply a second layer of blue tape over the first one (more on this later).
7. Open up the holes to the next screw diameter size. BTW, I’m assuming that you know you must use acrylic bits for drilling the windows. If you use an unmodified standard drill bit, even if you don’t crack the window at the moment, the bit will leave fissures in the sides of the holes that will lead to later cracks. If you find that your current screws are size 08, open the holes to size 10 (-3). If they are size 10, open the holes to size 1/4″ (-4). The plastic should never touch metal (neither screws nor frame), once installed.
8. Get clamping equipment and pads ready. You will need to make a clamping frame. This consists of a 2×4 that will span the door from front to rear, down the centerline of the window. Then screw two 2×4 cross-pieces to the stringer. These will be vertical within the window opening, so that they will push near the upper and lower four quadrants of the window. What you will be doing is putting thick folded towels over these two vertical boards, then pushing the clamping frame up against the window. You clamp the fore and aft end of the wood to the leading and trailing edge of the door, using rubber-padded clamps. Stanley Qwik-Clamps work quite well, but padded C-clamps will do. The intent is for the fore and aft screws to provide windowalignment, while the clamping system forces the entire window perimeter into the sealant. You don’t want to clamp it so tight that you squeeze out all the sealant, but you’d like to get the window surface to within about 1/16″ of the edge of the metal window opening. Don’t worry about this too much; a thicker layer of sealant is preferable to squeezing too much out; it loses much of its bonding strength when it is too thin.
9. Put a double layer of the blue tape around the outside perimeter of the window frame. Make a small cut-out in the tape where the heads of the screws will bear. Get all your screws and self-locking nuts ready. You should have truss-head aircraft-grade stainless steel screws, or AN525 washer-head cad-plated aircraft screws. You need rubber washers to go against the plastic on the inside, under the metal washers (remember, no metal touching the plastic). You can make these washers out of inner tube rubber, if needed. The nuts you use really need to be the special MS21042 all-metal stop nuts, commonly used by Beech all over our planes. These have a washer head and a reduced wrench size. You need to use these since you have to use self-locking nuts (because the screws do not get completely tightened down), and regular-size locking nuts will often interfere with the interior plastic trim.
10. Mix and apply the sealant. If you try to put one thick bead all around, it will probably drip and sag. Instead, try to apply two or three rows of a quarter-inch bead, making sure that the beads touch each other (no trapped air gaps). The larger metal contact area of multiple thinner beads will help prevent sagging. The sealant will probably be dark brown or black when applied, and will probably cure out to brown or tan.
11. When you first set the plastic into the sealant, you install the fore and aft screw assemblies, making sure to keep the plastic off the screw shanks, and snug the screws down until the plastic surface is about 1/16″ from the metal on the outside. Don’t just use the screws to pull down the plastic; use your hands and fingers to help evenly depress the perimeter of the plastic, so that the screws aren’t flexing the plastic at the holes. Then you install the clamping system, and snug down the clamps. Make sure that the clamping force is properly distributed, by adjusting the thickness of the towel padding as needed.
12. After seating the window in the sealant, there will be sealant oozing out on both inside and outside. Let it sit for maybe an hour, then peel off the first layer (only) of the blue tape on the outside. This will remove the vast majority of the extruded mess, and will leave a thinner bead for later trimming. You can scrape off any gross oozing on the inside, but usually the inside needs no attention. You should have at least a half-inch of sealant contact all around the window. Let the window cure. You have a couple of options for the remaining clean-up. My preference is to let it cure almost completely, then use a piece of phenolic with a beveled end as a knife. I slide the sharp end around the edge of the metal to break any layer of waste, then I peel off the final layer of tape. Some mechanics like to rub their fingers around the edge, to sort of “rub in and off” the waste line. If you do this, be careful not to cut your fingers on the aluminum edge. Bob S recommends 3M General Purpose Adhesive Remover for final clean-up. You can also call LP Aero for their recommendation. The stuff that I use can’t be had any more. Whatever you use, use it only as required, exactly where required, and follow it immediately with a good aircraft cleaner that is safe for acrylics. Chemical damage to acrylics may not show up right away.
13. I usually use Pledge Polish for general window cleaning and polishing, but any of the advertised plastic cleaners-polishers do a good job. Do NOT use most common auto or household glass cleaners, as many contain ammonia (death on acrylics). Be very careful what you ever put on the aircraft’s acrylic windows. Even if they don’t immediately melt the plastic, the wrong materials cause microscopic and molecular changes that may not appear as damage for months or years (such as crazing and hazing). Never use paper towels of any kind, and use only clean 100% cotton towels, for cleaning and polishing. The towels you use on your windows should be kept separate from other rags, should be in covered storage, and should not get used for anything else (including drying hands). I never clean the windshield when it is dry. I wait until I have flown in rain, or I flood it with a wet cleaner, and use my hands to remove any spots. Then I dry it and polish it with Pledge.
The storm window has to be fitted to the opening (whether new or used). It also has to be fitted to the hinge. This can get a bit tricky, to get everything right, so be careful and don’t rush. The storm window is the exception to “plastic touching metal”, as the hinge has to be riveted to the storm window. The original rivets are AD (hard) rivets, but I use soft rivets to guard against window damage. The inner flange secures the window in flight, so there are no structural issues. LP Aero sells the very thin, proper sealing gasket material for the storm window.
I hope all this helps. Take care and do it right. If you can do the pilot’s side window, you can do any of them.