NTSB Identification: WPR11LA459
14 CFR Part 91: General Aviation
Accident occurred Monday, September 19, 2011 in Greenleaf, ID
Probable Cause Approval Date: 08/15/2012
Aircraft: MCARTHUR STEEN SKYBOLT, registration: N77VW
Injuries: 1 Uninjured.
NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.
The pilot was performing aerobatic maneuvers in the plans-built experimental biplane. He successfully completed an aerobatic routine and then elected to perform it a second time. During the initial dive, as he began to level the airplane, he observed the outboard trailing edge of the upper left wing fail, with fabric material in trail from the aft spar. He regained partial control, but decided to bail out a short time later. The airplane subsequently rolled inverted and entered a near-vertical descent that continued to ground impact.
The 15-year-old airplane, which was purchased by the pilot 16 months before the accident, had fabric-covered wooden wings. The pilot described the build quality of the airplane as "average" and stated that he had noted chips and cracks in the paint, which were acceptable when referenced to the inspection criteria in the fabric manufacturer’s installation manual.
The airplane sustained extensive impact damage, and sections of the upper left wing separated from the airframe in flight and were not located. Fabric material from the recovered wreckage was examined, and the paint was cracked, fragmented, and separated from the majority of the fabric surface. It appeared that the fabric had been incorrectly treated prior to painting and then painted with a brittle automotive paint, which was prone to cracking. The fabric manufacturer’s installation manual cautioned that automotive paint can lead to premature failure of fabric cover because cracking can expose the fabric material to damaging ultraviolet light, which will substantially reduce the fabric's strength. The failed area was on the upper wing, which would have been exposed to sunlight on a regular basis. Although the improperly treated and painted fabric may have led to the in-flight failure of the upper left wing, a determination of the wing’s failure mode could not be made because the separated section of the wing was not recovered.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: Structural failure of the upper left wing during aerobatic flight. Full narrative available
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