What happened
On April 23, 2005, a Siebel ATG Siat 223 K 1 departed from Grefrath-Niershorst airfield to perform a planned aerobatic program. The aircraft was occupied by a pilot and one passenger, both of whom were wearing parachutes. After an initial period of flight involving a tailwheel maneuver to familiarize the passenger with the aircraft's behavior, the pilot climbed to approximately 4,500 ft MSL to begin the aerobatic sequence.
During a loop maneuver performed at a speed of 140 kt, the left wing failed at an altitude of approximately 3,600 ft MSL. The aircraft's fuselage and right wing struck an open field near Straelen. While the pilot managed to egress the aircraft and deploy his parachute, the passenger remained inside the wreckage. The pilot sustained serious injuries during the bailout, while the passenger was killed upon impact.
The investigation
The BFU investigation focused on the structural integrity of the wing spar, specifically the area where the left wing separated from the fuselage. Microscopic analysis of the lower spar cap revealed that the failure originated from fatigue cracks (stress cracks) near a rivet hole at the junction of rib 1. These cracks had developed over the aircraft's service life due to repeated loading cycles, such as turbulence or landing impacts.
Following the initial fatigue cracking, the investigation found evidence of rapid fracture (brittle fracture) caused by high-load peaks, likely from aerobatic maneuvers or hard landings. Ultimately, the remaining material cross-section was too weakened to withstand the loads encountered during the recovery phase of the loop. The investigation also noted that the specific area where the cracks formed was not visible during standard external visual inspections.
Findings
- The primary cause of the accident was the structural failure of the left wing due to fatigue cracking in the spar.
- The fatigue cracks originated at a rivet hole in the lower spar cap and were undetectable via the prescribed visual inspection methods.
- The structural integrity was further compromised by subsequent rapid fractures caused by high-load-factor maneuvers.
- The pilot was able to escape because the wing failure had broken a section of the cockpit glazing, allowing egress, whereas the passenger was unable to exit due to high G-forces and the remaining intact glazing on the right side.