NTSB Identification: WPR09FA141
14 CFR Part 91: General Aviation
Accident occurred Tuesday, March 03, 2009 in Antelope Island, UT
Probable Cause Approval Date: 11/29/2011
Aircraft: STEPHENSON ZODIAC 601 XL, registration: N3683X
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.

Radar data and a performance study depicted the airplane flying on a steady northerly course adjacent to mountains at 113 knots calibrated airspeed. The pilot changed course slightly to fly close to or over the mountains. The airplane subsequently experienced an in-flight breakup. All of the airplane's structural components and flight control surfaces were located at the main impact site. There was no evidence of excessive airspeed or maneuvers that would lead to a structural overload and failure of the wings. Turbulence was present in the area, although it was not believed to be severe and likely did not contribute to the failure.

An examination of the airplane wreckage revealed that the left wing spar had buckled upward near the fuselage and the left wing had wrapped around the fuselage near the cockpit. There was compression buckling of the lower spar cap of the left wing's rear spar and compression buckling of the upper and lower spar caps of the right wing's rear spar. The compression damage to both rear spars and the upward buckling of the left wing's main spar are evidence of upward and downward bending of both wings. The upward and downward bending of the wings is consistent with aerodynamic flutter. The structural loading at the inboard section of the left wing was further increased as the trailing edges of the outboard sections moved up and down. Ultimately the left wing failed as it bent upward near the root. Aerodynamic flutter can occur when there is insufficient stiffness in the structure or when the flight controls are not mass-balanced. Counterbalanced flight controls can protect less stiff surfaces at higher airspeeds. The ailerons did not have counterbalances, which would have offered direct protection from aerodynamic flutter.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

The in-flight failure 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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