What happened
On March 22, 2000, an ultralight aircraft, a WD D4-B Fascination, departed Heubach (EDTH) for a VFR flight to Bremgarten (EDTG). Radar tracking by Deutsche Flugsicherung (DFS) monitored the aircraft as it flew south of the Class C airspace, maintaining a flight level between FL 40 and FL 50. During the flight, the aircraft was carrying a total weight of approximately 483 kg, which exceeded the maximum allowable takeoff mass of 450 kg.
Near Simonswald, witnesses observed the aircraft performing a roll around its longitudinal axis before entering a steep dive. Observers noted that the aircraft appeared to be missing a portion of its left wing. The aircraft struck the terrain 50 meters below the Mooseckweg, resulting in the fatalities of both the pilot and the passenger. The impact destroyed the aircraft and caused minor damage to nearby forest vegetation.
The investigation
The BFU investigation focused on the structural integrity of the wing and the functionality of the aircraft's emergency recovery system. Investigators recovered significant portions of the left wing, with some fragments found up to 1.8 km from the main wreckage. Analysis of the wreckage revealed that the aircraft had suffered a major structural failure of the wing leading edge (nasenkasten).
Laboratory testing of adhesive samples from the wing structure indicated a deficiency in reactive hardener components, which compromised the long-term adhesion of the bond between the leading edge shell and the spar. Furthermore, the investigation examined the deployment of the ballistic parachute system, which was activated during the emergency but failed to fully deploy.
Findings
- Structural failure: The primary cause of the accident was the failure of the wing structure in flight. This was driven by several contributing factors, including manufacturing defects where inadequate adhesive bonds were present in the leading edge. These defects meant the actual strength of the wing did not meet the required safety margins.
- Overweight condition: The aircraft was operating above its maximum certified takeoff mass, which increased the structural loads during flight.
- Environmental factors: The aircraft was subjected to thermal turbulence, and the dark red paint of the aircraft likely contributed to increased surface temperatures, potentially affecting the mechanical properties of the composite materials.
- Parachute failure: The emergency recovery system failed to deploy successfully. The investigation found that the integration of the system within the fuselage caused physical obstructions. Specifically, the edges of the GFK airframe prevented the deployment sleeve from aligning with the direction of the rocket's thrust, causing the parachute to become snagged.
Safety action
Following the investigation, the BFU issued several safety recommendations:
- The aircraft manufacturer should redesign the integration of the recovery system to ensure successful deployment across the entire allowable flight envelope and maneuver range.
- The certifying authority (DAeC) should expand airworthiness requirements to include flight testing of the recovery system under various maneuvers and speeds.
- The manufacturer should identify and rectify all D4-B aircraft potentially affected by similar manufacturing defects in the wing structure.