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."
The report is available on the Web at:
http://www.nasa.gov/pdf/64317main_helios.pdf
Still photos related to this release are available on the web at:
http://www.dfrc.nasa.gov/Newsroom/ResearchUpdate/Helios/Previews/index.html
For more information about NASA aeronautics research on the Internet, visit:
