Article Content Courtesy of Thomas Chenock, Materials and Process Engineering
Navigation and strobe light covers (Figure 1) are made from an acrylic thermoplastic resin, poly-methyl methacrylate (PMMA), also known as Plexi-glass®.
Certain chemicals can cause the acrylic cover to develop surface craze cracks that can lead to larger cracks, especially in the outer machined perimeter of the cover where residual stresses could be present (Figure 2). The crazing is initiated by chemical absorption at the surface, which swells the plastic, leading to the small surface fractures.
Figure 1 – Navigation and Strobe Light Cover
Figure 2 – GIV Navigation and Strobe Light Cover with Cracks Indicated
FAA Advisory Circular (AC) 43-206 Part 2 (Par 602 j.) suggests non-metallic covers and housings should be cleaned with MIL-C-85570, Type II, (non-solvent; alkaline-based) cleaner diluted with 9 parts fresh water to 1 part cleaning compound, applied with a brush or clean cloth, then rinsed with fresh water and wiped dry with a clean cloth.
FAA AC-43.13-1B paragraph 3-25 advises cleaning transparent plastics by using mild soap and water, and advises not to use window cleaning sprays. This would include common household window sprays.
Do not use cleaning compounds that contain the following chemicals that can attack and damage the acrylic cover: