Twin Commander flown by Erick Teeters & John Kelley. This article addresses inspection and care of the PPG heated glass windshields used on 690A through 695B models of the Twin Commanders.
In this context, the Critical Vision Area is the area of the transparency bounded by a two-inch wide periphery surrounded by the structural attachment frame.
Delamination, by definition, is the separation of the interlayer from any glass or plastic ply surface within a laminate. Delamination within specified limits is not considered to be a structural issue. Delamination is a vision issue.
In the case of a windshield it usually occurs around the edges of the windshield and is detected by looking at or through the windshield. Delamination presents itself as a shiny, flat, oblong-shaped bubble with smooth edges. If caused by moisture ingress it may appear cloudy or translucent; otherwise, it will be clear. Typically, as with any laminated construction, the delamination will continue to grow over time until the stresses that caused it are relieved.
Small bubbles may form within the vinyl interlayer due to an overheat condition or edge pinch caused by over-torqueing fasteners during windshield installation. Bubbles in the interlayer are not considered to be a structural problem. Bubbles in the interlayer are not a reason for windshield removal unless visibility is seriously impaired.
Haze or “fogging” has been noted on occasions when aircraft transparencies have developed haze across the daylight opening. This condition is typically exhibited in the non-heated areas of the transparency or after the aircraft has been inactive for a period of time. The cloudy or hazy condition observed in aircraft transparencies is the result of moisture within the interlayer material between the glass plies being subjected to certain atmospheric/weather conditions. Under these conditions, the moisture in the interlayer can go out of solution and appear as “haze” or “bloom.” The moisture is typically absorbed over time. Reports of this condition normally occur during winter months. It is frequently noted that the haze occurs after some period of inactivity during which the window heat has not been cycled (i.e. a maintenance check or otherwise being out of service for a period of days or weeks). The condition can often be treated by operating the window heating system in flight for several hours, which elevates the window laminate temperature to operating condition and drives out excess moisture. It is possible to utilize other heat treatments for the same effect, such as PPG Surface Seal curing kits (heating blankets) that operate in a controlled temperature range. Heating lamps may also be used, but only if the surface temperature of the window closest to the heat source is carefully monitored and does not exceed 150 degrees F.
Heating elements arcing can affect the operation of the windshield heat system. It does not initially affect the structural integrity of the windshield, however, continued arcing will eventually cause glass fracturing. Arcing of the lead wires or terminal connections within the windshield is cause for windshield replacement.
The windshield heat system keeps the heated area at approximately 107 degrees F plus or minus 11 degrees by cycling the power relays as needed. If a windshield does not heat, access the aircraft wiring and check the heating elements resistance terminal to terminal. The expected resistance values are: Low heat density element (the tall strip) = 1.24 Ohms plus or minus 15 percent and Hi Heat density element (narrow strip) – 0.99 Ohms plus or minus 15 percent. If the measured resistance value is not within the normal range, then replace the windshield.
Check the windshield temp sensor elements—there are now two, a primary and a spare. If the resistance value is out of limits relocate the aircraft temp sensor wires as instructed in the AMM to use the spare sensor. The temperature sensor resistance value is a relatively straight line as follows: Nominal 265 Ohms (±12–) at 0 degrees F and 346 (±4–) Ohms at 120 degrees F.
Galvanic attack and resulting corrosion of the heating element wires and bus bar can occur due to moisture ingress. Depending on the affected elements this can result in reduction or loss of heating ability, isolated overheat, interlayer bubbling, and possible fracture of the outboard glass ply.
Scratches may occur on the surface of the outer layer and are considered by PPG to be structurally acceptable as long as vision thru the windshield is not impaired. Acceptability of scratches on the inner glass layer is dependent on the “intensity” (compare the scratches to AASTM F 428 standards) or by the depth of the scratch. Scratches more than 0.002 inches in depth are cause for replacement of the windshield.
Surface Seal. The 13SS and -14SS windshields currently in use come with a proprietary Surface Seal coating on the outside layer of glass providing an additional layer of protection to the glass. PPG has a document that addresses care of the Surface Seal coating: Document number DSS1042. The document describes how to inspect the coating and reapply if necessary. “Developed to enhance vision during rain conditions, SURFACE SEAL COATED GLASS from PPG Industries incorporates a durable transparent water-repellent coating on the windshield surface for efficient rain removal,” the manufacturer says. The document provides instructions for testing the coating’s water-repellant ability and reapplying it if necessary.
Weather seal, or as it is commonly called, the “hump seal,” is the primary barrier to prevent moisture from getting inside the laminated assembly. Moisture getting into the structure can lead to delamination and electrical system damage. PPG recommends inspection of the seal every four months or every 1,000 flight hours (whichever occurs first) for erosion or degradation. Pay particular attention to the area at the top and aft edges. If the seal has eroded to less than 75 percent of the original width or is found to be cracked, then it should be repaired. Refer to Twin Commanders CK163, which addresses hump seal repair.
Cracking of the urethane interlayer that bonds the inner and outer glass plies together can occur. Cracking occurs when the interlayer is exposed to moisture. These cracks are very fine and could have a spider web appearance. The affected area could also have some cloudy appearance and delamination. This condition is not generally a cause for removal unless it impairs vision or affects the operation of the electrical heat system. If this condition is found, the windshield should be replaced if the delamination and/or interlayer cracking extends from two inches inside the vision area of the part (four inches in the corner).
Peel chips can occur on the laminated surface of either glass ply. Peel chips are not a cause for removal unless the pilot’s vision is seriously impaired. Peel chips are called adhesion chips or cold chips and are caused by the vinyl interlayer pulling glass out of the surface of the structural ply. Peel chips usually start to form in the lower forward or upper aft corners of the windshield. Peel chips differ in appearance from delamination in that they are usually not smooth at the edges and exhibit jagged progression lines as the chip grows. Because the strength of the glass structural ply is reduced by this type of defect, windshield replacement is required.
Glass fracture can occur in either glass ply. The outer and inner glass plies are typically not fully tempered. If either of these plies should fracture, the break pattern will be in relatively large fragments. If you observe the fracture pattern lines you can see that the pattern(s) form an arrow head pointing to the location where the overstress occurred. Should either glass ply fracture in flight, refer to the POH/AFM for action to be taken.
Windshield cleaning. Do not let any acidic materials come in contact with the windshield as acid may remove the protective coating on the surface of the glass. Do not wax windshields after cleaning.
PPG recommends the following cleaning agents: a 50/50 mix of isopropyl alcohol and water, or a mild liquid dish soap such as Dawn, Joy Palmolive, or Ivory. PPG does not recommend the use of powdered detergents, abrasive cleaners, or strong acidic or caustic cleaners.
Prior to beginning the cleaning procedure remove rings, watches or other objects that may scratch the windshield. Flush the surface of the windshield by spraying with clean water to remove any abrasive materials that may have been deposited on it. Wash gently using a 50/50 mix of water and isopropyl alcohol containing a mild liquid soap in a 2-to-5% concentration. Rinse thoroughly with clean water and dry with a damp chamois, sponge or cloth.
Eagle Creek Aviation Service’s Mike Grabbe has been servicing Twin Commanders since 1974. He has been a factory service representative, and is currently the Twin Commander Factory Authorized Training Provider. He can be contacted at [email protected].
