NASA - National Aeronautics and Space Administration

The Helios Prototype flying wing is shown near the Hawaiian islands of Niihau and Lehua during its first test flight on solar power from the U.S. Navy's Pacific Missile Range Facility. The Helios Prototype aircraft was a proof-of-concept solar electric-powered flying wing designed to operate at high altitudes for long duration flight.

Part of a family of aircraft under the umbrella of ERAST (Environmental Research Aircraft and Sensor Technology), Helios was the culmination of the group’s solar-powered aircraft that, in August, 2001, reached an official world record altitude for a non-rocket powered aircraft, of 96,863 feet during a maximum-altitude flight.

On 26 Jun 2003 the aircraft was making the second in a series of shakedown flights leading toward a planned 40-hour long-endurance mission, when the aircraft broke apart in flight. The lightweight, highly flexible flying wing took off at 10:06 a.m. local time. About 16 minutes into the flight the aircraft encountered atmospheric turbulence causing abnormally high wing dihedral (upward bowing of both wingtips). Unobserved mild pitch oscillations began, but quickly diminished.

Thirty minutes into the flight the aircraft again encountered normal turbulence and then experienced an unexpected, persistent high wing dihedral configuration. As a result, the aircraft became unstable as pitch oscillations grew. Airspeed deviated from the normal flight speed, and the deviations grew with every cycle of the oscillation, soon exceeding the aircraft’s design speed. The resulting high dynamic pressures caused the wing’s outer wing panels to fail and the solar cells and skin on the upper surface to rip off.

The report is available at: http://www.nasa.gov/pdf/64317main_helios.pdf