[Orcnet] Off Topic - simultaneous headlight bulb failure?

[Keith Lofstrom]

This is only slightly electronic in content, and probably off topic
for Orcnet.

Perhaps Chris Jacobs knows, or one of you knows something about 
light bulbs or automobile electrical systems.

My wife's lowbeams stopped working on her Honda.  She followed me
home the previous night, and both were working.  My working assumption
was that simultaneous failure would be along a common path (switch,
black box, relays, fuses, etc.) but that all checked out.  In fact,
both bulbs had failed open, almost simultaneously after 4 years of
use.  I asked a mechanic, and he said that the bulbs usually tend
to go out within hours of each other.  

Assuming the bulbs were very similar to within manufacturing tolerance,
a few hours out of thousands of hours of use still seems closer than
believable.  All I can guess is that they were both nearly worn out,
and the Honda received some sort of electrical or mechanical jolt 
that killed both bulbs.  Perhaps it is the onset of cold weather, such
that the thermal shock of turnon was maximized.

Has anyone else encountered this?  Is this a known mechanism?  I am
fascinated by the failure of electrical items, since I like to prevent
it in my designs.  And I like to know where to look first when repairing
stuff.

Keith

-- 
Keith Lofstrom          keithl at keithl.com         Voice (503)-520-1993
KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
Design Contracting in Bipolar and CMOS - Analog, Digital, and Scan ICs