Ed,
"Markings on the fractures on the top and bottom spar caps were consistent with an initial break at the center of the top spar that progressed outboard."
"The fracture surface on the top spar cap contained radial lines emanating from the screw holes, shear lips at the edges, and necking at the edges consistent with tensile overstress in the top spar cap. There was no evidence of fatigue. The radial lines from the aft screw hole emanated from the top surface of the spar cap. The radial lines on the forward portion of the fracture surface appeared to originate near the tip of the broken screw."
The NTSB report now merely describes the evidence. Failure was "ductile overstress", and that overstress was negative G...not as would result from the pullup described by witnesses, but such as would result from attempting an outside loop at excess speed. Ultimate failure began with the top spar cap in the middle (where the holes were). "Radial lines emanating from the screw holes" appear to be the result, and not the cause, of catastrophic structural failure following aerodynamic loading clearly in excess of required design standards.
"Examination of the fracture surfaces revealed an apparent modification to the top spar cap that reduced the cross-sectional area by 10-percent." "Three screw holes were observed in the spar cap with the tip of a broken self-tapping screw still in one of the holes. According to information provided by the airframe manufacturer, two screw holes, spaced 1-inch apart, oriented longitudinally across the spar (fore and aft), were specified in the original design of the airplane (to fasten the seat pan to the spar)."
The 10% comment is utter nonsense. If there were three holes, two of which were "factory", only the one added is a "modification". For that one hole to reduce the cross sectional area of the top spar cap by ten percent, what diameter would it have to be? With a design "safety factor" in calculating structural wing strength of 50% or more, a reduction of 10% or even 20% would be of NO significance so long as required design stress capability remained. Had the NTSB deemed the presence of that one non-factory hole of significance they would have calculated or speculated as to how much it reduced overall wing strength from that required by regulations. They did not.
When this wing was overstressed to the point of failure in the air, it failed at the weakest point...where there were holes. The NTSB obviously did not regard that as in any manner unusual. There are "methods acceptable to the Administrator" for holes in spars and spar caps that do not materially reduce strength. There is absolutely no reasonable basis for the FAA to further pursue their original "unauthorized holes" (plural) witch-hunt by means of NPRM and AD from this final NTSB report of one unfortunate accident.
Our spar is designed to carry a greater load in positive G (i.e. pulling out of a dive where forces push one down into the seat) than in negative G (where forces try to eject seat occupants from the plane, as actually happened here. All other things being equal, only holes drilled in the bottom spar cap would reduce positive G structural capability...obviously not the case here. As this plane disintegrated occupants AND the seat structure departed the fuselage and came down in a different location than that of the fuselage center section.
"The broken self-tapping screw was found in the forward-most of these two holes. The third hole, not specified in the design, was located in-between and less than 1 inch from the other holes, and was threaded for a larger size than the other two holes."
OK, so at some time prior to the accident the original self-tapping screw failed. Instead of drilling out and replacing that screw, a new (and bigger) hole was drilled to secure the seat. That doesn't sound like the work of a professional mechanic. Some "handyman" type owner attempted a "fix" and whatever screw was handy was the design basis. It is not known when or by whom this act of inappropriate butchery, lamentable though it may be, was done; but it was NOT found to be the cause, or even a contributing cause, of this accident.
Just my "take", William R. Bayne .____|-(o)-|____. (Copyright 2010) -- On Mar 6, 2010, at 07:51, Ed Burkhead wrote:
The final NTSB probable cause report has been issued for the golf course crash at Sebring, Florida, in December, 2008.
