Re: WN 737 landing lights

Date:         27 Aug 99 03:07:44 
From:         Dean Wilkinson <razor@micron.net>
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.