Incident AeroVironment Helios Prototype , 26 Jun 2003
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ASN Wikibase Occurrence # 277335
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Type:AeroVironment Helios Prototype
MSN: 1
Fatalities:Fatalities: 0 / Occupants: 0
Other fatalities:0
Aircraft damage: Written off (damaged beyond repair)
Location:16 km W off Kauai, HI -   United States of America
Phase: En route
Departure airport:
Destination airport:
Confidence Rating: Information is only available from news, social media or unofficial sources
On June 26, 2003, the Helios Prototype broke up and fell into the Pacific Ocean about ten miles (16 km) west of the Hawaiian Island Kauai during a remotely piloted systems checkout flight in preparation for an endurance test scheduled for the following month.
On the morning of the accident, weather forecasts indicated that conditions were inside the acceptable envelope, although during the preflight go/no-go review, the weather forecaster gave it a "very marginal GO." One of the primary concerns was a pair of wind shear zones off the island's coast. After a delayed take off, due to the failure of the winds to shift as predicted, Helios spent more time than expected flying through a zone of low-level turbulence on the lee side of Kauai, because it was climbing more slowly than normal, since it had to contend with cloud shadows and the resultant reduction in solar power.

As the aircraft climbed through 2,800 feet (850 m) 30 minutes into the flight, according to the subsequent mishap investigation report "the aircraft encountered turbulence and morphed into an unexpected, persistent, high dihedral configuration. As a result of the persistent high dihedral, the aircraft became unstable in a very divergent pitch mode in which the airspeed excursions from the nominal flight speed about doubled every cycle of the oscillation. The over-speed condition was exacerbated when the pilot turned off the airspeed hold loop instead of executing the correct emergency procedure and increasing the airspeed hold loop gain. The aircraft’s design airspeed was subsequently exceeded and 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 of the wing to rip off. The aircraft impacted the ocean within the confines of the Pacific Missile Range Facility test range and was destroyed. Most of the vehicle structure was recovered except the hydrogen–air fuel cell pod and two of the ten motors, which sank into the ocean."

The investigation report identified a two-part root cause of the accident:
"Lack of adequate analysis methods led to an inaccurate risk assessment of the effects of configuration changes leading to an inappropriate decision to fly an aircraft configuration highly sensitive to disturbances."
"Configuration changes to the aircraft, driven by programmatic and technological constraints, altered the aircraft from a spanloader to a highly point-loaded mass distribution on the same structure significantly reducing design robustness and margins of safety."



Revision history:

12-Apr-2022 06:19 PolandMoment Added
12-Apr-2022 06:21 harro Updated [Aircraft type, Location, Country, Narrative]

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