Structural Failure Leads to Fatal Ultralight Accident near Flensburg

Casualties unknown • Flensburg, DE

A structural failure of the wing spar in an Aerospool Dynamic WT-9 resulted in the death of one pilot and the injury of another during a return flight.

What happened

On April 11, 2002, an Aerospool Dynamic WT-9 ultralight aircraft was returning to Flensburg-Schäferhaus airfield following a maintenance flight. During the transition from a straight descent at Flight Level 17 to level flight at Flight Level 11, the aircraft's wing structure failed due to the breakage of the spar bridge within the fuselage.

As the left wing and side wall detached from the aircraft, the seatbelt of the occupant in the left seat failed. This pilot was ejected from the cabin and died in the accident. The second occupant, seated on the right, managed to activate the aircraft's integrated recovery system, which mitigated the impact of the subsequent crash. The aircraft struck the ground near Flensburg, resulting in the total destruction of the airframe.

The investigation

The BFU examined the wreckage and analyzed the structural integrity of the WT-9 design. Investigators found that the primary failure occurred at the spar bridge located in front of the left seat. The investigation noted that while the wing spar was designed as a C-profile with carbon fiber reinforced plastic (CFRP) caps and a glass fiber reinforced plastic (GFRP) sandwich web, the type certification documentation relied on Finite Element Method (FEM) calculations that modeled the spar bridge as a closed box profile rather than a C-profile. This discrepancy meant that the impact of control column forces on the stability of the spar bridge had not been explicitly addressed in the original certification.

Furthermore, the investigation reviewed the aircraft's loading. The aircraft's weight was likely at or near the maximum takeoff weight of 450 kg. The accelerometer recorded a peak load of +7.4 g, which was close to the documented breaking strength found during subsequent manufacturer testing.

Findings

  • The structural failure of the wing spar bridge was the primary cause of the accident.
  • The aircraft was likely operating at its maximum weight limit of 45 and 450 kg.
  • The failure of the left occupant's seatbelt led to the fatal ejection of the pilot.
  • There was a lack of specific structural evidence regarding the stability of the C-profile spar bridge under control column loads during the initial type certification.
  • The aircraft was subjected to high vertical loads, potentially caused by a large gust or significant elevator input, which approached the known structural limits of the component.

Probable cause

The exact cause of the structural failure could not be determined with absolute certainty due to a lack of flight data, but the failure of the wing spar bridge during a high-load phase of flight was the decisive factor.

Frequently asked questions

What happened in the 2002-04-11 Aerospool Dynamic WT-9 accident near Flensburg, DE?

A structural failure of the wing spar in an Aerospool Dynamic WT-9 resulted in the death of one pilot and the injury of another during a return flight.

What aircraft was involved and where did it happen?

The accident on 2002-04-11 involved a Aerospool Dynamic WT-9, at Flensburg, DE.

What was the probable cause of the accident?

The exact cause of the structural failure could not be determined with absolute certainty due to a lack of flight data, but the failure of the wing spar bridge during a high-load phase of flight was the decisive factor.

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