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A little history. The IV-P door latch system
was developed empirically
and iteratively over the span of several months nearly 20 years
ago. I was
involved in the process at the concept development level,
participated in
design reviews and I designed and built a couple of prototype
latches. Latch systems
were installed in the cycle pressure test article (a fuselage
section stuffed
with blocks of hard foam to reduce air volume) and then exercised.
I am not
aware of any FEA studies done. However, due to the elastic nature
of the
fuselage, variations in builder technique and difficulties in
accurately
modeling the window to fuselage bond interface I would be highly
skeptical of
any FEA results. IMHO, testing is the only sure way to qualify the
system.
I have examined several failed door/windows and
repaired one
for a friend. During construction, that aircraft and mine were
both proof
tested to 7.5 PSIG (150% operational) before first flight. During
proof
testing, my friend’s airplane suffered a failure of the aft
pressure bulkhead
at 6.2 PSIG. Poor bonding of the skin to the core of the factory
part resulted in
delamination. Moisture present during oven cure was the apparent
cause. The
bulkhead was replaced and the plane was successfully re-tested.
My theory on the failure of door windows is
that the latches
and their maintenance are at the root of the problem. If a latch
does not close
fully over-center, under load it can release or fully latch. The
shock of this
event creates a pressure wave that propagates through the window
causing a
rupture of the extreme fiber, likely at an imperfection such as a
scratch,
mounting hole or tool mark. The window then fails like a balloon,
albeit a very
thick, highly pressurized balloon.
Prevention of window failure therefore should
be focused on
latch maintenance and inspection at regular (100 Hr/ annual)
intervals. The
latches should be cleaned, inspect and lubricated. Mounting and
adjustment
fasteners should be checked for torque. Roll pins should be
inspected for
position and sheer failure. I have seen roll pins “back out” of
their holes. Latch
preload adjustment should also be checked for uniformity.
Finally, preflight inspection should also
include a positive
confirmation of latch engagement. I use my index finger to feel
that the fore
and aft top latches are fully engaged after closing the door. On
one occasion
the aft latch did not fully engage because an article of passenger
clothing was
caught in the door jam.
I believe the IV-P door latch system is
satisfactory for a
properly constructed and maintained aircraft as is evidenced by
the numerous
aircraft in service. However the failures show that the door latch
system is a
critical maintenance item that should be part of the regular
maintenance
schedule.
Regards
Brent Regan
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