I have completed replacement of the door window in my IV-P and N437RP is at Brad Simmons’ shop for painting. I promised to write a complete report on this, and have made careful notes and pictures for this. It appears the report will need to be delayed a bit, so I thought I should provide a summary of what I believe to be the cause. Hopefully this will help some of you in avoiding my mistakes.

The door window in N437RP, and IV-P, developed a crack parallel to the back edge of the window at approximately 500 hours and two years in service. The crack was completely covered by the inside and outside door frame carbon fiber, and only visible as  a 2 inch “crackle” in the otherwise dark edge of the Plexiglas when viewed at a sharp angle from inside or outside of the door. I continued to fly the airplane for about 10 hours, but did not pressurize it during this time.

I removed the transparency from the door by grinding away the outside carbon cloth layers to reveal the outside edges of the Plexiglas. By doing this carefully, I was able to determine that the crack was approximately 4 inches long, parallel to, and about ¼” under the back edge of the carbon window frame. Neither end of the crack extended to the edge of the Plexiglas, and did not intersect either of the door latch mounting holes that had been oversized, filled with Hysol and re-drilled to size.

This discovery about the nature of the crack led me to conclude that it was caused by flexing/twisting of the door frame in that immediate area. While this could be caused by the latch located in the immediate area, this seemed unlikely as the crack was almost an inch  away and the mounting through-holes showed no stress or cracking at all.

The other source of bending/twisting stress in that area is the attachment point of the gas lift strut that holds the door open, and I believe this to have been the source. On two recent occasions, the door has been allowed to swing open hard in the wind against the air strut, and one or both of these almost certainly caused the crack—although I did not see it at the time. I have routinely taxied the airplane over rough ramps/etc with the canopy open, allowing it to bounce against the restraining strut… a practice I won’t do again.

I doubt this is the cause of all door window failures; in fact, mine might have been unique, but I plan to take much better care of my door in the future!  Detailed report to the LML when I can.

Bob