NTSB Identification: CEN10FA042
14 CFR Part 91: General Aviation
Accident occurred Friday, November 06, 2009 in Agnos, AR
Probable Cause Approval Date: 04/28/2011
Aircraft: ZENITH AIRCRAFT CO ZODIAC 601 XL, registration: N538CJ
Injuries: 1 Fatal.
NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.
Flight track data recovered from an onboard global positioning system, and a subsequent performance study depicted the accident airplane flying between the altitudes of 2,500 and 3,500 feet at airspeeds between 60 and 108 knots calibrated airspeed (KCAS). The calculated bank angle for the entire flight never exceeded 30 degrees and at the time of the accident, the airplane was climbing at 500 feet per minute through 2,800 feet and had accelerated to 100 KCAS. The estimated angle of attack was about 3 degrees during the last minute of the flight. There was no evidence of excessive airspeed or maneuvers that would lead to a structural overload and subsequent breakup. The wreckage was spread over 600 feet. An examination of the airplane wreckage revealed compression buckling of the upper and lower caps of both rear spars and upward and downward bending of both wings. The upward and downward movement, twisting, and flexing of the airplane wing surface was consistent with the occurrence of aerodynamic flutter. The structural loading at the wing roots were further increased as the trailing edges of the outboard sections moved up and down. Ultimately both wings failed in down bending at the root. The ailerons did not have counterbalances that offer direct protection from aerodynamic flutter. Aerodynamic flutter can occur when there is insufficient stiffness in the structure or the flight controls are not mass balanced. Less stiff structure can be protected to higher airspeeds with the use of counterbalances on the flight controls. In addition, damage was noted on the flap assemblies consistent with over travel in the upward direction. An examination of the engine revealed no anomalies.
The accident pilot was experienced in the accident airplane and had built the airplane from a kit. Medical and pathological examination of the pilot revealed the cause of death as impact related and no medical or toxicological issues that would have precluded him from operating the airplane in a safe manner prior to the accident.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: The in-flight separation of both wings due to aileron flutter. The aileron flutter was the result of inadequate wing stiffness and the lack of aileron counterbalances. Full narrative available
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