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IMHO IV-P window blowouts are due to improper fabrication technique. Either the latch through holes are improperly drilled and/or the window is not properly bonded in place.

Drilling acrylic is tricky as the bit tends to grab and cause compressive fractures in the surrounding material. These fractures range from over a diameter in length to microscopic and are the seeds of the structural failure cracks that occur later. The best advice is to practice drilling several holes in a scrap before trying the real thing. You will get the best results with a drill bit that is sharpened with a negative rake, not drilling a pilot hole and providing a solid back up block of aluminum or phenolic. The back up block is important as the worst damage to the part happens as the bit exits the material.

Proper bonding of the window to the frame is especially important as this joint is being held in tension, not just compression as with the other windows. Improper bonding causes the tensile stresses to be concentrated in the bolt hole area.

The blow-outs happen at altitude because the differential pressure is highest (remember the rate knob on the pressurization controller), the temperature is coldest making the acrylic brittle, and the air density is lowest thereby reducing the damping of vibrations on the skin of the airplane.

It is the vibrations from the engine and prop that work the tiny crack, made during assembly, through the brittle and loaded acrylic until there is a failure.

Compliments of the season.
Brent Regan

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