NTSB Factual Report

On September 19, 2011, about 1315 mountain daylight time, a McArthur Steen Skybolt, N77VW, collided with terrain following an in-flight structural failure near Greenleaf, Idaho. The pilot was operating the experimental amateur-built airplane under the provisions of Title 14 Code of Federal Regulations (CFR) Part 91. The airline transport pilot was not injured, the airplane sustained substantial damage and became completely fragmented during the impact sequence. The local flight departed Caldwell, Idaho, about 1255. Visual meteorological conditions prevailed, and no flight plan had been filed.

The pilot was performing aerobatic maneuvers in a practice area. He completed a routine uneventfully, and elected to perform it a second time. During the initial dive when he began to level the airplane, he observed the outboard trailing edge section of the upper left wing fail, with fabric material in trail from the aft spar, which appeared to be devoid of any wing ribs. He regained partial control of the airplane, but elected to bail out a short time later. The airplane subsequently rolled inverted, and began a near-vertical descent into a field.

The single engine biplane was comprised of wooden wings, covered in fabric. It was built from plans, completed in 1996, and purchased by the pilot 16 months prior to the accident. The pilot described the build quality of the airplane as, "average" and as such, had planned to ultimately restore the airplane. He noted sections of chipped paint and circular, "ringworm" cracks in the surfaces of the fabric covering, which he attributed to the airplane being painted with automobile paint.

The failed upper wing section was subsequently examined by an investigator from the NTSB, and an inspector from the Federal Aviation Administration (FAA). The wing structure had become heavily fragmented, with the spar fractured into multiple sections. The aft ribs inboard of the aileron were not located, and presumed to have separated in flight. All remaining ribs and spar fragments were free of rot.

Sections of the wing fabric covering material garnered from the upper wing were recovered for examination. The material bore the stamp, 'Poly-Fiber D-103 FAA PMA Stits Aircraft'. The samples were examined at the facilities of Consolidated Aircraft Coatings, the manufacturer of the Poly-Fiber material, in the presence of the NTSB investigator-in-charge. Representatives from Consolidated determined that the material was of the medium weight type, manufactured before 1992. A tensile breaking strength test was performed on two 1-inch-wide strips, utilizing a calibrated load cell. The materials yielded at a force of between 91.20 and 91.67 pounds. The minimum acceptable yield value for this material was 90 pounds.

According to the FAA approved Poly-Fiber installation manual "How to Cover an Aircraft Using the Poly-Fiber System," in order to utilize the fabric material on a certified airplane, it must be treated after installation with a sealant (Poly-Brush), aluminum impregnated ultraviolet (UV) protection coating (Poly-Spray), and a final color coat (Poly-Tone). Examination of the recovered material revealed that the paint had become cracked, fragmented, and separated from the majority of the fabric surface. The material exhibited light streaks of pink fabric sealer, which covered about 50% of the fabric surface. The installation manual states that a correctly sealed surface should be uniformly deep pink in color. Paint fragments were examined, and were brittle when worked by hand. The paint consisted of layers of the ultraviolet protection coating with a final coat of what appeared to be automotive paint. The installation manual states the following regarding the use of automotive paints:

"Premature failure of cover jobs is often caused by automotive paint cracking over Poly-Brush and Poly-Spray. When these brittle paints fail, they take subcoatings with them, exposing the fabric to UV damage."

The installation manual states that a raw, uncoated piece of Poly-Fiber material, left exposed outside for 1 year will lose about 85% of its strength.

The pilot stated that he did not observe substantial cracks in the area of the wing failure, and that the nature of other cracks was acceptable, when referenced against the installation manual's inspection procedures.

NTSB Probable Cause

Structural failure of the upper left wing during aerobatic flight.