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I had a Cessna 172 that got windshield crazing badly -- one spot on the left, one on the right, just above the defroster vents. The Cte of plexiglass is large, the Cte of the canopy rim is approximately zero -- a bad mismatch. I've plumbed my defroster to diffuse the hot air over a wide area before it hits the canopy, and plan to use it as little as possible.

On Jun 4, 2012, at 9:48 AM, Douglas Brunner wrote:

My presumption is that it came from the defrost. When I built my plane, I put in a duct system beneath the glare shield that directed hot air up to the canopy. In retrospect, I think I made it too effective.

I used to leave the defrost on long after the canopy had cleared. I had a long trip “through the soup” in Western Maryland, Pennsylvania and Ohio during which I believe the defrost was on the whole time. Immediately after that leg the canopy showed some significant cracking/crazing.

Now I switch immediately from defrost to heat (or off) as soon as the canopy is clear.