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If a fire in the cockpit started, for whatever reason, what stops the plastic line from melting and adding fuel to the fire right underneath you?
Which is why a few extra dollars for a fire-resistant fuel line is a good idea along with fuel shut-off valves between the tank and the cockpit. Also, there are very few in-cockpit fires (that aren't fueled by spilled gasoline, etc in the post crash environment) that can't be controlled by isolating the electrical components. This is one of the major reasons why being a little cautious about your choice of materials (structural and insulation as well as things like fuel and electrical lines), the layout of systems, etc can make a huge difference in whether a crash is survivable or not.