Crash was caused by atmospheric disturbanced following changes
to aircraft configuration
The board that
investigated the loss of the remotely operated Helios Prototype
aircraft during a test flight last summer released its final report
today.
The board determined that 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 had been flying under guidance of
ground-based controllers from AeroVironment, Inc., of Monrovia,
Calif., the plane's builder and operator, with assistance from NASA
Dryden Flight Research Center 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, highly
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 and the solar cells and skin on the upper
surface to rip 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, Associate
Administrator for NASA's 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 altitude 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."
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