Gentlemen,
After much exchange of ideas and techniques with several of you,
I finally cut into my cabin door today to replace a cracked window. I
discovered this crack—only visible by looking at an angle into the sandwich
between the inner and outer skins in the upper back corner—during pre-flight.
The crack did not extend into the transparent part of the window. This was one
of the early “fast build” doors from Lancair; purchased in
2004-2005. I will post a detailed report when I’m done, but thought you’d
be interested in what I found today…I was surprised.
As this fault was observed directly opposite a door latch, I
assumed it resulted from the holes drilled through the Plexi to accommodate the
latch. As the door/window was still intact (not blown out), I was careful to
grind away carbon from the outside to free the window without further damage so
we could confirm this. As I removed the carbon, I exposed a 4” crack starting
about an inch below the latch and extending toward the top of the door and
around the corner radius. The crack was about ¼” “under” the
carbon skin, and had not progressed to the edge of the Plexi at either end. It
did not originate at the bolt holes, but was generally parallel to the edge of
the Plexi and about ½” farther from the edge than the bolt holes. The crack
appeared to be caused by forceful flexing of the Plexi parallel to its edge.
I am not a structural engineer, but observed two possibilities.
First hypothesis: the latch exerts a twisting moment on the
door frame when closed and the door is pressurized. Possible, but I checked the
latch alignment and function carefully during initial installation, and all of
them are aligned the same, and work very easily…no binding and no stress upon
opening or closing. They also engage fully, and only very slightly “pull”
on the door frame when going over-center. No other latches showed any damage or
stress in the area, and one of the forward latches is also bolted with holes
drilled through the Plexi. The process of pressurizing is a gradual increase in
force on the door/window, allowing them to conform—within their stress
limits. This concept seems to work for all other areas around the door/window.
Second hypothesis. The FB door re-oriented the lift strut
location so that its door attach point is just above the”offending”
latch location, and its mounted to the door stiffener. On at least two known
occasions, other people have opened the canopy door in the wind and allowed it
to extend the lift strut forcibly against the lift strut internal stop. This would
cause a sudden twisting of the door frame in the location of the crack. Although
both violent door openings occurred some time ago, I believe this to be the cause
of the failure--subject to further review/ideas/comments from you.
A couple of related observations. The door was well constructed
in the areas I opened. I found no voids or weak areas. Holes in the Plexi for through-bolts
had been drilled to approximately ½”, filled with Hysol and redrilled.
All four of them were intact, and these “spacers” quite forcefully
held the inner and outer skins together. The window had been bedded against the
outer skin with Hysol, and bedded with micro around the edges. The window was
well attached to the door skins, although it could be separated with a thin
putty knife. The bond appears to be mechanical—the Hysol adheres by
conforming very closely to the rough surface without making a chemical or
adhesive attachment. More to follow as I have data…
Bill Harrelson and I took a lot of pictures, and I’ll take
more as we complete this job. I’ll write up the details for posting on
the LML and LOBO web sites for those interested. I’d like to extend a special
thanks to Laurie Fitzgerald (Australia), Brent Regan and Brad Simmons for their
exceptional advice, pictures, information—and encouragement. As with most
things done on this airplane, I’m combining their excellent
recommendations to hopefully make a long-lasting and reliable fix.
See you at Oshkosh!
Bob