WONDERLAKE, IL, USA
N12MV
VAVRA, MARTIN A. REBEL 300
A witness to the accident, who was a professional pilot and an aerobatic judge certified by the International Aerobatic Club (IAC), stated the flight originated as a practice aerobatic session. He reported that the right wing folded while the airplane was positively loaded to 6 to 7 G's. Investigation revealed the main wing spar upper cap strip failed in fatigue. Tests disclosed that the strength of the composite carbon fiber material was significantly lower than the design specification due to high porosity, excessive fiber waviness, and low fiber volume.
HISTORY OF FLIGHT On April 23, 1994, about 1035 central daylight time, an experimental Rebel 300 airplane, N12MV, was destroyed during aerobatic maneuvers at the Galt Wondelake Airport in Wonder Lake, Illinois. The solo private pilot sustained fatal injuries. The personal flight originated about 1030 and was conducted under 14 CFR Part 91. A flight plan was not filed for the local flight and visual meteorological conditions prevailed. A witness to the accident, who was a professional pilot and an aerobatic judge certified by the International Aerobatic Club (IAC), stated the flight originated as an aerobatic practice session to calibrate the airplane sighting device which is used to visually obtain precision vertical flight for aerobatic competition. The pilot had performed three or four entries to the center of the aerobatic box at an estimated altitude of 1700 feet AGL. He performed a smooth, consistent radius, 90 degree vertical pull each time to evaluate the sighting device. The witness, who was judging the airplane's performance at the time of the accident, wrote in his statement that, "On approximately the fourth or fifth vertical pull...as the nose was pitching up through approximately 30 to 40 degrees nose up, the right wing was observed to fail and at the same time the still attached left wing caused an immediate right hand roll of the aircraft. The visual perception of the breakage can be best assimilated to clapping your hands over your head." He stated the engine sounded like it was at full power. The right wing separated and made a "paddle wheel descent." He reported the right wing section impacted about 300 feet north east of the of the center of the aerobatic box. The remainder of the airplane impacted approximately 4000 feet north. The witness estimated the acceleration force on the airplane at the time of the accident was 6 to 7 G's. PERSONNEL INFORMATION The pilot, according to the President of the International Aerobatic Club, had advanced through the IAC four categories of proficiency. He had flown aerobatic competitions for about ten years, averaging four to five competitions per year, and was certified in the unlimited category. The most recent entry in the pilot's log was dated May 15, 1993. His total logged flying time was 1170.3 hours. AIRCRAFT INFORMATION N12MV, a Vavra Rebel 300, an amateur built airplane owned by the accident pilot was registered on April 21, 1993, and issued a special airworthiness certificate on May 12, 1993. The airplane was built from a kit manufactured by the former Akro Designs, Inc. of Aurora, Colorado. AERODROME INFORMATION At the time of the accident, the pilot was flying in an aerobatic practice area established on the eastern edge of the Galt Airport. WRECKAGE AND IMPACT INFORMATION The NTSB on scene investigation began April 28, at 1030 central daylight time. The wreckage had been moved into a T-hanger at the Galt Airport. Examination of the accident site revealed a crater in a level, grassy field about 20 feet by eight feet, oriented in a northerly direction. The depth at the deepest part, near the northern edge, was about five feet. Clumps of earth were scattered north of the crater for about 50 feet in a fan shaped pattern. Examination of the wreckage revealed the right wing was mostly intact with a 12 inch strip of the aft, inboard, upper wing skin missing. A semi-circular line of clear, fragmented plexiglass was impregnated in the upper skin from the vicinity of the spar outward and aft for several feet. The main wing spar was fractured several inches inboard of the right fuselage side. Examination of the surrounding spar revealed multiple inclusions in the adhesive which bonds the main wing spar to the wing skins. The wing attach bolt and surrounding spar structure were intact. The wing carry through spar was repositioned next to the right wing spar revealing a V-shaped fracture area about four inches inboard of the right fuselage side. Several pieces of spar material from the fracture were located, however, several sections were not found. The right forward wing attach fitting was intact with a three inch section of fuselage tubing still attached. The right aft wing attach fitting was delaminated from the aft wing spar with a six inch section of fuselage tubing attached to it. The upper end of each tube was bent outboard. The fuselage structure in the wing carry through area was reconstructed. The welded steel tubing which comprised the rectangular wing mounting structure was fractured into many small sections. The left dog bone link and bolts were intact. The right dog bone link was fractured near the center where it was bent upward. The bolts were intact. Both dog bone links had black abrasions on the lower surface. The left wing was fragmented into multiple pieces. The forward fuselage and cockpit area were fragmented into many short sections of bent and fractured steel tubing. The aluminum, fuselage mounted fuel tank was ruptured. The empennage aft of the elevator bell crank was intact. Examination of flight control continuity, including the elevator bellcrank linkages, revealed no evidence of preimpact malfunction. All three blades of the composite propeller were fractured near the hub. Visual inspection of the engine revealed no evidence of preexisting malfunction. Examination of the instrument panel revealed two needles on the G meter were captured at +6.5 G's. The negative G needle was captured at +2.5 G's. The airspeed indicator needle was captured at 239 miles per hour. SURVIVAL ASPECTS Photographs taken at the accident scene by an FAA inspector revealed the pilot was wearing a backpack type emergency parachute and was wearing a five point aerobatic restraint system. A witness, who was in radio contact with the accident airplane, reported that he made constant radio transmissions to encourage egress and stated he believed the pilot "was incapacitated when the wing folded". MEDICAL AND PATHOLOGICAL INFORMATION The pilot had a current third class medical certificate with no limitations. The FAA airman medical record contained no remarkable medical history. The report of autopsy remarked no preexisting disease. The autopsy was performed by the McHenry County Coroner in Woodstock, Illinois on April 25, at 0845. Toxicological testing was performed by the Armed Forces Institute of Pathology (AFIP) in Washington, DC and by AIT Laboratories (AIT) in Indianapolis, Indiana. AFIP tests were negative for all tests conducted, including the examination of liver and kidney specimens for the presence of ethanol by headspace gas chromatography at a cutoff of 20 mg/dL. AIT tests were negative for all tests conducted except for examination of liver tissue which resulted in positive for caffeine, and 0.0826% (W/V) ethanol. At the request of the NTSB investigator, AIT reexamined the liver tissue and gastric contents for the presence of ethanol which resulted in .095% and .028% (W/V) respectively. Further investigation revealed the specimens were transported at ambient temperatures from Woodstock, Illinois to Indianapolis, Indiana. Michael A. Evans, Ph.D., the director of AIT Laboratories wrote in his laboratory report: "Review of both the case history and the data from analysis of specimens in this case indicate that the presence of ethanol in the above specimens is due to microbial fomentation (sic) processes". TESTS AND RESEARCH Weight and balance data was recovered from the airplane. An estimated 42 pounds of fuel were on board. The pilot's weight was listed on his most recent medical certificate. The aircraft gross weight was calculated. Aircraft 1,146 Fuel 42 Pilot 175 ____________________ Total 1,363 pounds During a telephone interview, Mr. Martin Hollman of Hollman Aircraft Designs, Cupertino, California stated he had performed the composite engineering design work on the wing. He said it was originally designed to withstand a 16 G ultimate load based on a 900 pound gross weight and that the limiting factor, for positive loading, was the compression strength of the upper spar cap. Additionally, he stated that he used a theoretical compressive strength of 70 kilopounds per square inch (KSI) for the unidirectional carbon fiber cap strip material. Mr. Bob Jones of Akro Designs, Inc. reported that design changes had been incorporated to strengthen the wing following the original Hollman design. He reported the design load factor for the airplane was 10 G's at a maximum aerobatic weight of 1,368 pounds. He reported that a test wing had been statically loaded to test the strength of the wing design but no test data was provided to the NTSB. He provided a graph which indicated the calculated maximum compressive stress in the top spar cap was 36.4 KSI. The wing spar center section was examined at the United States Air Force Wright Laboratory, Dayton, Ohio. The laboratory reported that the upper spar cap strip material "revealed striations indicative of fatigue." The report indicated that "fatigue failures occur at stresses even lower than the ultimate strength, and therefore must also be accounted for in the design of the component." Specimens were machined from the wing spar section and tested at the Wright Laboratory. Six specimens of unidirectional carbon fiber material were removed from the upper cap strip and tested in compression using a standard test method. The average stress at failure was measured to be 38.7 KSI for the six specimens. The minimum strength was 23.1 KSI and the maximum was 50.2 KSI. The report indicated that some of the measurements might be slightly lower than the actual strength due to shredding and delamination in the specimens. Image analysis techniques were used to determine the fiber, matrix, and void contents of the spar material. The analysis revealed there was an average of 12 percent porosity in the spar cap material. Additionally, the fiber volume fraction was 45 percent. The report indicated that typical military composites have less than two percent porosity and 57 to 63 percent fiber volume fraction. The report stated the carbon/epoxy fibers in the upper and lower caps exhibited excessive fiber waviness. This waviness was evident in all directions, but the waviness along the length of the spar was particularly detrimental to the compressive strength of the upper cap under normal flexural loading conditions. ADDITIONAL INFORMATION Parties to the investigation were the Federal Aviation Administration, Flight Standards District Office, West Chicago, Illinois, and the International Aerobatic Club, Oshkosh, Wisconsin. Following the on-scene portion of the investigation the wreckage was released to Robert Russell, the Galt Airport Manager. Several wing and fuselage sections were retained for subsequent investigation and were returned to Dan Vavra, the brother of the pilot.
FATIGUE AND CATASTROPHIC FAILURE OF THE MAIN WING SPAR RESULTING FROM THE DESIGNER'S EXCESSIVE, UNVERIFIED ESTIMATE OF THE STRENGTH OF THE SPAR CAP MATERIAL AND THE MANUFACTURER'S FAILURE TO ATTAIN THE SPECIFIED STRENGTH DURING THE PRODUCTION PROCESS.
Source: NTSB Aviation Accident Database
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