What happened
On June 5, 2006, an Interplane Skyboy ZK ultralight aircraft departed from the Neumagen-Dhron airfield for a VFR flight toward Hildesheim. After completing a left-hand pattern and climbing to approximately 2,500 ft MSL, the aircraft departed the pattern in a northeasterly direction. Witnesses near Kasholz observed the left wing of the aircraft break off in mid-air, causing the aircraft to plunge steeply into a forest clearing. The pilot had deployed the aircraft's emergency parachute system prior to the impact. The aircraft was destroyed, and the pilot sustained fatal injuries.
The investigation
The BFU investigation focused on the structural failure of the wing and the subsequent failure of the emergency recovery system. Examination of the broken aileron control tube revealed evidence of a fatigue fracture. Microscopic analysis showed that two-thirds of the tube wall had been destroyed by a fatigue crack, with the remaining third failing due to a final overload. The investigation also examined the wing's Y-strut, which had buckled under extreme loads.
Regarding the recovery system, investigators found that although the parachute was deployed, the lines became entangled with the propeller hub and the tail boom. This entanglement prevented the parachute from fully inflating. Furthermore, the investigation looked into the installation of the parachute system, noting that the placement of the device behind the propeller likely caused the lines to be deflected into the propeller arc during deployment.
Findings
- The primary cause of the accident was the failure of the aileron control tube due to fatigue.
- The failure was driven by superimposed vibrations, specifically torsional vibrations from the control mechanism and bending vibrations from the wing.
- The breakage of the control tube allowed the aileron to move freely, triggering a high-energy aeroelastic flutter phenomenon.
- This flutter induced violent oscillations in the wing, creating enough force to buckle the Y-strut and cause the left wing to detach.
- The severity of the outcome was significantly increased because the emergency parachute failed to deploy properly due to entanglement with the propeller and tail structure.
- Evidence of fretting corrosion and manufacturing notches on the control tube edges likely acted as precursors to the fatigue crack.