On Thu, 7 Jun 2007 23:42:37 -0400, "Peter R."
wrote:

Today I spotted two four inch and one six inch vertical cracks in the side
window at the seat directly behind the pilot seat. The cracks are almost
perfectly straight and originate at the top frame more or less in the middle
of the window. The aircraft is a 1973 V35B and I would imagine that this
window is that old.

How does this happen? Old age? Cold to hot? Some other force that decided
that one month since my last aviation maintenance expenditure was too long?

my take on it is that the installation has probably always put extra
stresses in the plastic in that area. as the plastic is exposed to uv
radiation it gradually suffers depolymerisation, ie breaks in parts of
the chains of molecules that make up the plastic.

the plastic moves more with thermal changes than does the aluminium so
with thermal changes such as you experience in climb to altitude there
is some movement and stressing going on in the plastic. the plastic
shrinks faster than the aircraft.

over time the gradual reduction in strength and the stresses due to
temperature change meet each other and you get a crack.

how you treat the crack can add to it's severity.
in plastic the tensile forces pulling the plastic apart (it will never
crack in compression) are actually magnified by the shape of the crack
at the very tip of the crack. once a crack starts the stress buildup
at the tip of the crack will tend to make it propagate across the
sheet of plastic.
the method of halting the crack is to drill a small hole just beyond
the crack. the shape of the tip of the crack is then changed from a
sharp stress riser to a more broad distribution of stresses. if the
plastic in the area has sufficient strength to handle the reduced
stresses in the area then that is the end of the propagation of the
crack. an eighth inch drill is typically used.

in your guess above, deterioration due to age (uv radiation effects)
and Hot to cold leading to tensile forces in the plastic locally
exceeding its strength.

dont feel special. we all need to replace plastic windows
occasionally.

btw how your guy mounts the new window is important to having that
window last beyound the warranty period.

Stealth Pilot
Australia.