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ACCIDENT INFLIGHT AIRFRAME FAILURE(From CP33, Page 5, July, 1982)
The thought of an airplane
coming apart in the air brings chills to most aviators and certainly to aircraft
designers. Despite many horror stories related to severe weather, drastic over
speed In dives, and even airframe flutter (unbalanced elevators), we had yet to hear of an
in-flight failure of a Rutan design - until June 21st when the caller described a winglet
ripping off a VariEze at 200 + mph during an airport buzz job. Within two hours Mike
Melvill and Dick Rutan were airborne in the Defiant for a non-stop flight to Dallas, Texas
to investigate. What they found, though, did not lead to grounding or flight
restrictions of other VariEzes. The cause was tantamount to leaving the wing attach
bolts off your Cessna and expecting the fairing strip to hold the wing on. Their
An aerobatic pilot witness standing nearby described what
happened when the winglet came off. The aircraft yawed, rolled, and pitched up 90
degrees. The calculated 13-g loads did not fail the wings but twisted the fuselage
enough to shed most of the Plexiglas from the canopy frame. The aircraft impacted
inverted on the prop and top cowling, then it slammed down, shearing the pilots rollover
structure, the top of the instrument panel and impacted the canard/fuselage fairing.
It then bounced back into the air, rolled left to upright, and struck the ground
upright, failing the main gear (pulled brackets and major glass structure from the
fuselage). The aircraft came to rest 90 feet from the initial impact point at a
heading of 110 degrees right of flight path. The nose gear was retracted. The
right winglet was located about 1900 feet short of the wreckage. Parts of the
Plexiglas canopy were found 1000 feet short. With the exception of the right winglet
and rudder assembly, and parts of the Plexiglas canopy, the wreckage was essentially
complete and in one spot . Although it had sustained major damage, the airplane was
located in a small area, not over 20 x 30.
The right winglet failed inward during the high-speed low pass.
Sample sections were cut out of the winglet - root/wingtip. Skin coupons were
burned out and the number of plies was counted. The type of glass and fiber
orientation was determined.
Figure 1 shows the VariEze design structure and the structure
found on the wreckage of N11CH. The major tension lay-up (#8) that was omitted was,
without question, the primary weakness, which allowed the winglet to fold inward and fail
at high speed. The winglets lift inward and, at high speed (with zero sideslip) have
an inward bending moment that is equal to that attained in a 15-degree sideslip at the
maneuvering speed. Note that with lay-up #8 omitted and with lay-up # 9 not
extending to the lower skin, the only structure opposing the bending was the foam core
acting through rib #6 to the bottom skin. It is conservatively estimated that the
structural strength of the winglet-to-wing joint of N11CH was less than 1/20 of what It
should have been. It is very surprising that it did not fail sooner.
The incredible thing that was not answered was how the builder
could have omitted the primary structure and why it had not been noticed. Even after
the final paint job, it was obvious that the #6 rib could be seen on the surface.
This aircraft throughout showed evidence of poor workmanship.
Poor workmanship In itself had not precipitated structural failure with these
construction materials. Prior to this accident the VariEze type had amassed approximately
150,000 hours flying without in-flight airframe failure, even though many of the aircraft
have relatively poor workmanship. The omission of important primary structure was clearly
the cause of the structural failure.