The “Boeing Maintenance Manuals” all state that the filament must be horizontal when fitted.
When in the horizontal position vibration & shock is pronominally in the vertical plane ie landing & turb.
The lamp filament being horizontal will bow up & down across the entire length of the filament and
the both attach points share the load. A vertical filament will concentrate the load at the upper or lower
attach point (one or the other not both sharing the load) where the entire force is felt. A warm filament
is not as brittle as a cold one, therefore less likely to break from shock & vibration. Some lamps have focusing
ridges internal to the front glass to aim, concentrate or scatter beams as designed for their positional use.
They are critical in installation orientation.
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