1. Field of the Invention
The present invention relates to protection of aircraft windows having an acrylic outer surface from chemical attack, crazing, discoloration and impact erosion. More particularly, the present invention provides a thin chemical and erosion resistant outer protective layer of glass, highly cross-linked acrylic, or other environment-resistant transparent material laminated to the exterior acrylic surface of the aircraft window by a thin urethane interlayer.
2. Prior Art
Aircraft windows including cockpit side windows and cabin windows are typically fabricated using acrylic plastics in homogenous or laminated form or acrylic and glass laminated structures to meet weight and impact resistance requirements. In many of these window configurations, the external layer or surface comprises stretched acrylic plastic. Stretched acrylic provides a tough, reasonably durable finish, however, crazing, pitting, erosion and discoloration of the acrylic surface can be caused by particulates, hail and chemical attack. The toughness gained in stretching the acrylic reduces hardness thereby increasing the susceptibility to degradation.
In recent years a worsening problem with aircraft windows has been created by significant volcanic eruptions which not only introduce significant quantities of high particulate ash into the upper atmosphere but also create a corrosive chemical environment severely degrading acrylic aircraft windows. Severe degradation of cabin windows for commercial airline aircraft is exemplary of this problem. Most regular airline travelers can attest to the significant degradation of visual quality in aircraft windows. The windows are crazed and/or pitted, significantly impairing vision. Studies have indicated that chemical attack from volcanically introduced substances at higher altitudes is the major contributing cause to such crazing.
In prior industry practice, airline cabin windows were removed and the exterior surfaces were ground and polished to restore the visual quality of the windows. Typical aircraft window configurations such as those shown in FIGS. 1A and 1B of the drawings had initial representative thicknesses of approximately 0.350.+-.0.015 inches with a minimum structural thickness requirement of 0.280 inches. This provided approximately 0.070 inches for grinding and polishing of the windows during refurbishment. This tolerancing allowed approximately three refurbishments of the window before reaching the minimum structural thickness. This machine grinding and polishing operation depends on flattening these relatively thin panes during the process using vacuum tooling or other means. The much thicker laminated cockpit side windows do not lend themselves to being flattened, so they are limited to less effective hand polishing and premature replacement if the surface damage is too deep to be removed in this manner.
Current atmospheric conditions are resulting in rapid degradation of windows and more frequent repair of the windows as described, which, along with more frequent replacement of aircraft windows due to increased frequency of repair, has created an extensive cost burden for commercial carriers. Consequently, modifications to original windows and a means to accomplish repair of acrylic aircraft windows to reduce damage and eliminate repetitive repair is sought.