SAE ARP 6621:2019 pdf free

SAE ARP 6621:2019 pdf free.Predicting Photometric Degradation of Exterior Aircraft Lights
Aircraft cover lenses degrade with typical use, which affects their performance. The effect of this degradation on optical performance is reduced light transmission and light scattered in unwanted directions. The amount of light loss will vary according to the material used and its level of exposure to various environmental factors. The effect of degradation on mechanical performance is a reduction in the physical integrity of the cover lens. Surface degradation is allowable so long as testing (or other acceptable methods) demonstrates that the lens still performs in a manner that maintains regulatory compliance.
Abraded surfaces一If the cover lens is exposed to abrasive conditions, such as wind, rain, ice, or particulates in the airstream, the exposed surfaces of the lens become damaged. This material erosion is seen as cloudy, opaque, or diffuse areas on the lens. The effect of this degradation is reduced light transmission and light scattering in unwanted directions. The amount of surface abrasion and subsequent light loss for an aircraft light fixture will vary with the material used and its exposure to airstream. The location of the lens on the aircraft will influence the degradation of the lens material; for example, erosion is worse on the leading edge of wings. With prolonged exposure, the transmission rates will stabilize after a significant reduction in transmission. Figure 1 gives an example of the relative degradation of a glass material and a plastic material after equivalent particulate exposure. Here, sand erosion testing was performed at UDRI. The sand erosion test conditions were equivalent for both materials, but the rate of surface degradation and transmission loss was different for the two materials. Abraded surfaces are acceptable so long as the light output is not reduced below required levels. This figure is included as an example showing the general effects of erosion on lenses of different materials. The relationship between the sand erosion test and the actual rates of erosion due to flight conditions is unknown.SAE ARP 6621 pdf free download.