by Kerry McIntyre

(This version is slightly different from the MAPA Log article)

In this month’s ShopTalk, we will walk the reader step-by-step through this lengthy process of replacing the windshield in your Mooney M20J and newer aircraft.


The 201 and later Mooneys have a one-piece bubble-type windshield that offers very good visibility. As airplanes age, the Plexiglas windshields and side windows will craze and become hazy or clouded. This hazing is a natural process that occurs as plastic gets old and is exposed to direct sunlight. Where and how the aircraft is stored will determine how quickly the hazing will occur. If the plane is stored in a hangar, the windows will stay clear a lot longer. Up here in Wyoming we don't get the intense sun that the southwest or southeast states get. Typically a windshield in the northern states will be more damaged by dirt scratches from improper cleaning long before it hazes up. The opposite is likely for a southwest or southeast windshield.


The first steps in windshield replacement are to mask off the area around the base plate enclosure and to remove the glare shield. If you can't see the flush rivets that hold this enclosure on, then you will need to strip the paint and Bondo that is covering these rivets. Often the paint must be stripped to locate all the rivets which often will be Cherrymax rivets. Using a snap punch, punch all the flush rivets on the base plate and drill them out. Use a drill stop to avoid drilling through into the cabin and damaging any wiring that may be under the glare shield. Use caution not to elongate the holes in the base enclosure and in the airframe.


Now that the rivets are drilled out, the base enclosure can be removed using a thin putty knife and a lot of patience. Because the windshield is installed with fuel tank sealant you may need to cut the sealant from under the base enclosure to remove the enclosure strip and not warp or bend it out of shape. You will want to reuse this enclosure strip, so don't destroy it, take your time.


Once the base enclosure strip is off the aircraft, clean all adhesive and tank sealant from the airframe and enclosure strip using a razor blade and rotary Scotch-Brite discs. Clean all remaining paint chips from the base plate using paint stripper and a Scotch-Brite pad. Don't forget to scrape any old sealant from the side and roof channels of the airframe and clean out all debris from these channels after you have removed the windshield.


The next step is to peel off all protective plastic or paper on the new windshield. Hold the new windshield up and look at an 8 foot florescent light through the windshield while moving the windshield around. The purpose of this is to see if the new windshield has any flaws in it. The flaw will bend the florescent tube image. Once you are satisfied the new windshield is flaw free, then re-mask both the inside and outside using the paper you just removed. Leave 3 inches of windshield exposed all the way around all edges, inside and outside. Now you can handle the windshield without scratching it. Try fitting the windshield into the airframe. Once it is lined up and in place, make a mark on the cabin roof center using a piece of masking tape and put an alignment mark on the center top of the windshield, these marks will help to properly align the windshield in the same spot each time you fit it to the airplane. It is a good idea to do the same type of marks at the bottom center of the windshield and the airframe.


Most windshields will not fit on the very first try; they are typically made big because no two airplanes are the same size. Trim the windshield using a rotary fiber saw blade, 1/16 inch thick, until you have an equal amount of windshield in the airframe channel. Once you have the entire windshield fitted properly, feather all the edges of the windshield before you do your final fit.


Your final dry installation of the windshield will include cleco-ing the base enclosure strip in place and hand‑forming it so no gaps exist between the airframe and the base enclosure or the windshield and the base enclosure. Mask off the windshield to the airframe using blue ¼ inch fine edge tape and masking tape to cover exposed areas. Mask off the airframe using masking tape and don't forget to mask off the base enclosure strip to the airframe also. Cover the glare shield area and the control yokes with a large towel or sheet.


By now you should have the headliner plastic pushed out of the way and masked as necessary. Before installing the windshield now is a good time to repaint the black fuselage tube that supports the compass. The new windshield can be installed with high or low adhesion fuel tank sealant. We prefer to use high adhesion sealant on windshields and low adhesion sealant on side windows. The low adhesion sealant typically will break down sooner than high adhesion sealant and the window will leak. So why not use high adhesion on side windows? The cabin skins are very thin and not reinforced, so when trying to remove a side window, especially a back one that was installed with high adhesion sealant, it is possible to damage the side skin. Now you have a lot more work to do than just changing a window.


At this point, the windshield has been trimmed and fits in its entirety. The airframe and windshield are masked off and center lines are marked top and bottom. Mix the sealant using the slowest curing activator and pack the gap around the cabin roof and side skins half‑full. Place a bead of sealant the diameter of your index finger on the base where the windshield will lay on the airframe. Set the windshield in place aligning the center marks. Press the window into the airframe until sealant flows out of all skins and your masking lines match the airframe. Fill all voids with sealant and place extra sealant along the base of the windshield and the base enclosure. Reapply a small amount of sealant to the airframe (under the base enclosure) before the base enclosure is cleco-ed on. That way, sealant will ooze out of the rivet holes where the Cherrymax rivets are installed.


Starting in the center and working towards the ends, cleco your base plate in. Having sealant ooze out of the front of the base plate all around the airframe and out of the top of the base plate and all around the windshield including the rivet holes will keep water from getting into the cabin. When riveting in the windshield, also start at the center of the base plate and work both directions around the radius.


After the base plate is riveted in, clean off all the excess sealant from the airframe and windshield. Now you will be grateful that you did a good job of masking. Caution! Do not slam the cabin door while the sealant is curing. Allow the sealant to cure 72 hours indoors at 65 degrees of higher before flying the airplane. However, before the sealant is cured, remove all the masking on the window and clean the window. This allows time to remove any sealant that may have gotten on the window or airframe. Repaint the airplane as necessary.


After reading this article, you may decide to use a Mooney professional to do this job. We will typically quote 20 man-hours plus paint, materials and a windshield. Most of the cost of this job is labor.


Just recently KNR completed and was issued an STC for replacement of 1968 and earlier Mooney instrument panels with flat machined modern powder coated panels. Currently KNR is obtaining the PMA so this kit will be available for installation by any FAA certified airframe mechanic. That's why MAPA readers have not seen a shop talk article in quite awhile. This STC/ PMA process is extremely difficult and in a future ShopTalk we will describe why STC'd and PMA parts are so expensive.


As always, if you have a question about this or any ShopTalk article, you may contact me at our aircraft repair facility by the old‑fashioned telephone: 307-789-6866 or by e-mail. Until the next ShopTalk, enjoy flying your Mooney.