Bob's door window failure has the appearance of a bending failure of the acrylic at the mid aft latch. A possible cause could be the improper adjustment of the latch. A contributing factor could be a small scratch in the surface of the window.

The IV-P latches react forces in two primary directions. One is in the direction normal to the AC's skin. This force is the result of the internal pressure multiplied by the area of the door. The moment arm of the latch produces a torsion bending of the door frame. Because of the moment of inertia of the door frame and window system, the strain is concentrated at the frame the window transition. This is the location and likely mechanism of Bob's window crack.

The other force is parallel to the latch arm. When it is pressurized, forces work to transform the shape of the IV-P fuselage into a sphere. Imagine a balloon.  The force reacting the internal pressure is a tension in the skin of the balloon. This tension force works to remove any non spherical convolutions of the surface. This force works against any bending force in the surface.

Imagine a slack piece of rope hanging from a tree. It is easy to bend the rope. Now imagine that same rope suspending  a car off the ground. Now it would not be easy to bend the rope.

Likewise, tension load on the door latch counters the bending load due to the normal force. The total force on the latch is greater but the local stress concentration in the window due to bending is reduced.

The lower three latches must be adjusted to pull the door into position. All three latches should have equal preload.

The five vertical latches must be adjusted so that they have no slack and no preload in both normal and longitudinal axes. To compensate for the flexible drive shaft compliance the latches should be phased so that the top vertical latches seat slightly before the bottom vertical latches.

Proper latch adjustment can take the better part of a day. Positioning in the normal axis is accomplished by shimming or milling the latch striker. Positioning in the longitudinal axis can be accomplished with the striker adjusting screw.

Remember that all the latches work as a system so a failure near the aft latch could be a result of the forward latches not being properly adjusted.

Proper periodic latch maintenance (cleaning, lubrication and adjustment) is essential for door reliability.

Be sure to always have the tools available inside the AC to remove the latch covers and open the door in the event of a latch roll pin failure.

Regards
Brent Regan