Date: 27 Aug 99 03:07:44 From: Dean Wilkinson <email@example.com> References: 1
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boeing707 wrote: > I work for USAirways in FLL. Recently, I have seen Southwest 737 > (models -300/-500 and the -700) landing lights blinking on and off > during their approach. Have only seen it on Southwest. > The entire set of landing lights on the wings alternately blink on and > off. Then when the airplane nears the inner marker, the blinking stops > and the lights are on steadily. > I have also seen this on a few Falcon 30's and Lears, but WN is the > only airline I've seen this on. > Can anybody give me the story on the blinking landing lights? > And, doesnt it lower the life of the lamp? Blinking landing lights are used for enhanced visibility for collision avoidance, especially in the daytime. Blinking landing lights are much easier to see against ground clutter or a hazy sky at much greater distances than a steady landing light. In fact, the Navy found that placing bright steady lights on a warship could make it disappear visually against the horizon since they essentially matched the brightness of the background sky and eliminated the silhouette of the ship. The same can happen to an airplane, so blinking is a good way to enhance visibility. As long as the current through the landing light filaments is ramped up and down rather than being abruptly started and stopped, and the current doesn't go all the way to zero, the Tungsten filaments are not stressed in a way that can reduce their life, and in fact the lower overall power dissipation can actually extend their life. Tungsten-Halogen lamps die mostly because of evaporation of the Tungsten which makes the filaments thin and brittle. The Halogen helps return the Tungsten atoms back to the filament where they are redeposited. Thermal shock is one of the biggest causes of failure since it can cause the filament to shatter, so as long as the blinking keeps that filament from cooling or heating too abruptly, it shouldn't hurt.