Altus, OK, USA
N25BM
Bruce Moore Skybolt
The pilot was performing aerobatic maneuvers when he noticed the fabric on the top right wing was separating and departing the airplane. The pilot reduced the engine power, airspeed, and altitude, and then returned to the airport. Examination of the airplane revealed the wing rib stitching had failed, and approximately 5 feet of the top right wing fabric and approximately 3 feet of the top right wing lower fabric was separated. According to a fabric manufacturer technical bulletin, "When light weight fabric is installed on an aircraft with a high horsepower engine and inadequate fabric forming structural spacing, coating fatigue cracks develop very rapidly...We recommend fabric weighing less than 2.4 [ounces] per [square yard] be used only on aircraft with engines up to 145 [horsepower]." The fabric covering weight on the airplane was 1.7 ounces per square yard, and the airplane was equipped with a 300-horsepower engine.
On May 23, 2004, approximately 1300 central daylight time, a Bruce Moore Skybolt homebuilt single-engine aerobatic bi-plane, N25BM, sustained substantial damage following a partial in-flight breakup while maneuvering near Altus, Oklahoma. The commercial pilot, who was the registered owner, and a passenger were not injured. Visual meteorological conditions prevailed, and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 personal flight. The local flight departed the Altus/Quartz Mountain Regional Airport (AXS), Altus, Oklahoma, at an unknown time. According to the Federal Aviation Administration inspector, who responded to the accident site, the pilot was performing aerobatic maneuvers when he noticed the fabric of the top right wing was separating and departing the airplane. The pilot reduced the engine power, airspeed, and altitude, and then returned to AXS. Examination of the airplane revealed the wing rib stitching had failed, and approximately 5 feet of the top right wing fabric and approximately 3 feet of the top right wing lower fabric was separated. One wing rib was damaged and two trailing edge ribs were separated. The fabric covering weight was 1.7 ounces per square yard. The airplane was equipped with a 300-horsepower Lycoming IO-540 engine. At the time of the accident, the airframe had accumulated approximately 116 total flight hours. The date and time since the last inspection was unknown. According to Poly Fiber Aircraft Coatings Technical Bulletin 89-1D, dated October 1989, Aircraft Fabric Drumming, "When light weight fabric is installed on an aircraft with a high horsepower engine and inadequate fabric forming structural spacing, coating fatigue cracks develop very rapidly...We recommend fabric weighing less than 2.4 [ounces] per [square yard] be used only on aircraft with engines up to 145 [horsepower]...Sometimes fabric is pressed or beat against close underlay structure which causes a crowfoot break in the coatings after 50 to 100 [hours]." Numerous attempts to obtain a completed Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2) from the pilot were unsuccessful.
The partial in-flight separation of the wing fabric and rib structure due to an inadequate material installed by the airplane builder.
Source: NTSB Aviation Accident Database
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