The board determined the mishap resulted from the inability to predict, using available analysis methods, the aircraft's increased sensitivity to atmospheric disturbances, such as turbulence, following vehicle configuration changes required for the long-duration flight demonstration.

The Helios Prototype aircraft involved in the mishap was a proof-of-concept solar electric powered flying wing designed to operate at high altitudes for long duration flight.

The failure occurred during a flight from the U.S. Navy's Pacific Missile Range Facility (PMRF) on the Hawaiian island of Kauai on June 26, 2003.

The propeller-driven aircraft was flying under guidance of ground-based controllers from AeroVironment, Monrovia, Calif., the plane's builder and operator, with assistance from NASA's Dryden Flight Research Center, Edwards, Calif., personnel.

The aircraft was destroyed when it sustained structural failure and fell into the Pacific Ocean. No other property damage or any injuries occurred as a result of the mishap.

The lightweight, flexible flying wing took off at 10:06 a.m. local time. At 10:22 and 10:24 a.m., the aircraft encountered atmospheric turbulence, typical of conditions expected by the test crew, causing abnormally high wing dihedral (upward bowing of both wingtips).

Unobserved mild pitch oscillations began, but quickly diminished, according to post-test data analysis.

At about 10:36 a.m., the aircraft again experienced normal turbulence and transitioned into an unexpected, persistent high wing dihedral configuration. As a result, the aircraft became unstable, exhibiting growing pitch oscillations.

Airspeed deviated from the normal flight speed with the deviations rapidly increasing with every cycle of the oscillation.

The aircraft's design speed was subsequently exceeded. The resulting high dynamic pressures caused the wing leading edge secondary structure on the outer wing panels to fail; the solar cells and skin on the upper surface were ripped off.

The remotely piloted aircraft came down within the confines of the Pacific Ocean test range, northwest of PMRF.

"The mishap underscores our need to assess carefully our assumptions as we push the boundaries of our knowledge," said Dr. Victor Lebacqz, NASA's Associate Administrator for the Office of Aeronautics.

"It should not, however, diminish the significant progress AeroVironment and NASA have made over the past 10 years in advancing the capabilities of this unique class of aircraft on many successful flights, including Helios' record setting flight to just under 97,000 feet in August 2001.

It is important that we learn from this experience, and apply the board's findings and recommendations to help ensure the payoffs of such vehicles are fully realized," he added.

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