Windshields (2 & 4 place) and Canopy (2 place)
From Bondline
Contents |
Introduction
Windshield replacement in all models and Canopy replacement (2 place)
Mechanical Difficulty: 
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Unlike side window replacement, windshields may NOT be signed off for return to service by the owner/pilot. You will have to have someone with an Airframe rating to supervise and make the log entry approving it for return to service.
Only the most intrepid (and some would say foolhardy) owners would attempt to install their own windshield or AA-1x canopy without the assistance of someone with experience doing Grumman bonded windshield replacement. No one step is particularly difficult, but the process does not allow for much error, and the cost of failure is high ($400-$500 in parts and shipping) for another windshield if you crack it drilling holes, or bond it in crooked such that the canopy won't seal tightly, etc.
Most of the steps for the windshield and canopy are very similar, especially the removal of the bow, the cleaning and then re-bonding to the new plexi. Only areas with significant differences will be shown under the "Canopy" heading. All the rest of the explanation can be applied to either part. It is not necessary to remove the windshield to work on the canopy or to remove the canopy to work on the windshield, however both tasks are explained below.
Unbolting the windshield
There are 2 AN3 bolts on the lower end of each side of the windshield bow. To access them, remove the plastic trim and the screws holding the forward edge of the upholstery on the cabin sides. The plastic trim is held in by a several #4 sheet metal screws. On the AA-5x series the pilot's side has a rocker switch and a map light on a flexible stalk to be removed. Many AA-1x and most AA-5x have the compass located on the windshield bow. The wiring follows the forward edge of the windshield bow down the right side. The ground wire is attached with a screw to the honeycomb, and the power wire is crimped in the same connector as the panel light lead and must be cut off. A new connector can be installed later to allow the bow to be removed without cutting wires.
After accessing the AN3 bolts at the bottom of the bow, there are 7 screws around the lower edge of the windshield. 2 screws on each side of the fuselage, one at the bow and one a few inches forward, and 3 screws at the forward edge of the windshield. It may be easier to remove the 3 forward screws if the glareshield cover on top of the instruments is removed.
The bolts on the right side are the same, but less wiring to deal with.
Unbolting the Canopy
The canopy comes off the sliding rails with the removal of the bottom row of screws. A second person to help support and lift the canopy is strongly suggested.
After all the bottom row of screws is out, the canopy can be lifted straight up and off of the plane. This ought to be done by at least 2 people, and 3 would offer an additional level of safety. With a helper on the opposite wing root, lift the canopy straight up, taking care not to scratch the quarter windows with the sharp corners at the aft edge of the canopy. Once the canopy is above the turtledeck height, carefully hand it over to one person on the wing root. The other person then gets off the wing and comes to the trailing edge of the wing so that the person holding the canopy can hand it down.
Removing the Windshield
At this point all the wiring is loose, all the screws are out, and all that is holding the windshield in place is the adhesion of the old sealant around the forward edge (which on some planes has incorrectly been replaced with RTV silicone by those that did not read the maintenance manual), and the spring tension of the aluminum bow. As needed, insert a thin bladed putty knife into the plexi-aluminum joint around the front of the windshield to pop the windshield loose. Push inward on the bow and attempt to slide the legs of the bow upward and aft to lift the windshield clear of the fuselage. Some more scraping with the putty knife may be needed as additional areas of adhesion are discovered. Lift the windshield and bow out of the plane and hand the complete assembly to a helper on the ground.
Removing the Bow
Early models (69-73) have multiple holes drilled through the plexi and the aluminum bow, and screws to hold the assembly together. It is a simple matter to remove the screws and clean and residue of the gasket material from the bow.
Late models (74 on) have a bonded bow and plexi, with some later production planes having 2 screws, one on either side of the latch assembly to help resist the shearing force caused by the latch. On the bonded assemblies it is assumed that the windshield is not being reused, so cutting the bonding with a variety of methods is possible. A piece of safety wire wrapped around some wooden handles can be used to cut the RTV bonding. A putty knife can be used to slice the bonding. A single edged razor blade can be worked around the aft face of the bow to slice through the sealant about 1/2 way across the bow. In many cases the bonding may be failing and large sections of the bow may already be coming loose.
Tip: In the case where the RTV bonding is failing and the windshield or canopy is still in good shape, care to separate the remaining bond is needed to avoid damaging the plexi. The razor blade or putty knife methods have worked well for me in the past. Usually, once the RTV is starting to let go, it all comes apart pretty easy. Masking the inside if the windshield (or canopy), with some aluminum tape or a couple layers of duct tape from the bond line towards the part that you look through, would be a good idea. You may find that a sharp single edged razor blade held perpendicular to the inside of the plexi and used as a scraper can remove most all the old bonding material. I've followed this with a "DA" or random orbit sander and some 400 grit paper to remove scratch marks and the last of the RTV. The fogged apperance of the plexi will diminish sharply if the DOW 1205 primer is applied and the plastic allowed to soften slightly, as recommended for the original RTV bonding method.
Removing the Canopy Skirt
At the aft edge of the plexi, there is an alumiunum skirt that completes the distance from the aft edge of the canopy to the turtle deck. A row of screws secures it to the plexi and the aft canopy bow. Removing the screws and taking the lock cylinder off the pilot's side will free the plexi and the aft bow from the skirt.
Cleaning the Bow
Once the bow is released, the next step is to thoroughly clean the remaining bonding agent off the aluminum. Scraping with a putty knife works well. Sometimes small areas may need to be scraped with a single edge razor blade to remove particularly well attached bonding material. Sanding until smooth, followed by a solvent wipe to remove the oil from finger prints is a good final cleaning step.
Checking the Bow
Now is a good time to check that the bow is straight. Its not uncommon for the windshield or forward canopy bow to not lay flat, and this must be addressed before any trimming or bonding takes place.
Bonding the Bow to the Plexi
The OEM factory bonding and the instructions from LP Aero until recently were all based on a Silicone Rubber "RTV" bond. The instructions from LP Aero described the materials as Dow Corning 732 RTV and Dow Corning 1200 and 1205 primers. This method was used successfully for more than 30 years and resulted in excellent bonding, if the primers were used and care was taken to prevent contamination of the bond area. Because the smallest quantity the primers are sold in is 13 oz. and they require a Hazmat charge to ship them, it could cost $40 for the two primers and another $20 in Hazmat charges, and all that is needed is about 1 oz. of each primer. Because of these costs and inconveniences, another bonding process has begun to replace the RTV. Recently LP Aero changed its instructions, and a new poly-sulfide based bonding material was recommended. This material is "Pro-seal" fuel tank sealant and is compatible with MIL-S-8802. Its available in several brands, but its vitally important that you use the "Adhesion Promoter" AC-251 from Advanced Chemistry and Technology, or its equivalent in the brand of sealant you use.
On the Plane or On the Bench?
The bow can be clamped in place with the plexi and bow jigged on the plane, or it can be "bench bonded" and then installed on the plane. Both methods have their advantages and also some drawbacks. In the on plane installation you will need at least 12-15 2-3" C-clamps to apply the pressure that keeps the plexi aligned with the bow during the cure of the adhesive. With the bench method, you'll need a table large enough for the entire bow to lay flat and some sort of surface that can be drilled and screws installed to provide the needed clamping action.
The windshields are slightly oversize to accommodate production tolerances of the airframes. This means that the excess must be trimmed. The question is how much is excess and exactly WHERE it should be trimmed. Windshields that are bench bonded typically have the forward edge trimmed after the bonding is cured. Canopies that are bench bonded have the aft edge trimmed, and the bottom edges along the canopy rails also need to be trimmed. The problem begins with the lack of rigidity of the plexi. Trying to hold it in place at one location will cause it to spring loose on the opposite side. This is one of the reasons that first time installers have such a hard time doing a good job. They just can't get that big floppy piece of plexi to lay down so they can figure out where to trim and mark the locations.
To bench bond a canopy or windshield bow, one should start with the bow out of the plane and checked for flatness. Some metal work to make it lay down flat on the face of the angle that would be vertical when installed, may be needed. The exact distance between the mounting holes at the bottom of the bow is also needed, so that you can measure that distance out on your table. Marking the hole centers and then securing the bow to the table (#6x1" drywall screws are very handy for this) lets you measure the bow to find the center. A mark on the table is a good way to keep that location visible. You will be cleaning the bow and treating it with primer and covering it with bonding agent, so any mark made on the bow will be gone.
Measure the plexi and mark the center of the plexi with a china marker or with a permanent marker on the "spray lat" protective covering on the outside, because you will remove the inner covering when bonding, and your mark will be gone with it! Now you can position the plexi on the table over the bow and line up the center marks. The plexi should end at the same distance on both legs of the bow, as a quick check that you have accurate center marks. A mark on the table at each lower corner of the plexi will be helpful later in assuring that the plexi did not slide while clamping it.
The inner spray lat is removed, or at least the first few inches. The plexi is laid over the bow, and the edge of the bonding is masked with masking tape or duct tape. Make sure that you have burnished the edge down well, as the primer and bonding agent can wick under the tape edge and leave your bond line uneven.
Lightly sand the area to be bonded. 320-400 grit paper is a good choice. A wipe with Naptha on a clean cloth will remove the sanding dust and your finger prints along with the oils from your skin that would contaminate the bond. The bow is prepared with a solvent wipe (MEK is a strong degreaser that leaves no residue when it flashes off), and the primer is appied per the instructions that come with it. If using the 732 RTV and the 1200 and 1205 primer system, 1200 is used on the metal bow, and 1205 is used on the plexi. If using the Pro-Seal method, the AC-451 adhesion promoter is used on both the metal and the plexi.
The plexi is prepared with the primer, and the bonding material is applied to the bow and trowelled so an even thickness of about 1/8" thick. The plexi is placed over the bow and lined up with the center mark and the two end marks previously located. Because of the thickness of the sealant, the two end marks will not quite match the plexi, but the distance should be evenly split between them. It can help to have someone hold the plexi sides out just a bit (away from the cleaned and primed area) so that the plexi doesn't scrape off the bonding agent as it is being slid into position.
Secure the windshield by clamping the bonded area. The wood blocks and drywall screws shown in the picture work very well on a bench that can have screw holes placed in it. They are easy to make and very inexpensive.
Start in the middle and work out to both corners evenly. Watch that you have squeezed the excess sealant out of the bonded area, but don't press so hard as to force ALL the sealant out. You should stop clamping before you see the aluminum bow pressed against the inside of the plexi. Wipe the excess sealant off the inside of the bow/windshield joint. It is much easier to make a nice finished edge now and have the clean up work complete while the bonding agent is still soft.
Wait overnight, or longer if the temperature is not above 70 degrees F.
TIP: An electric blanket laid over the outside of the bond and draped up over the plexi can be a big help with the temperatures are low overnight.
Check the cure of the bond by testing an edge of the sealant in an area that you can't see after the plexi is installed. It should be firm and rubbery, and not at all sticky. If not cured enough keep it warm and wait another 6-12 hours. You really can not rush the curing, and releasing the clamps before the bonding is done will contribute to the bond coming apart.
