What happened
On April 28, 2012, at approximately 19:00 local time, a TL-2000 Sting, registration OK-NUA42, crashed near the village of Kosmová, approximately 6 km southeast of Toužim Airport (LKTO). The aircraft was being operated by Aeroklub Toužim for the purpose of a flight training mission, specifically a transition training flight for a pilot undergoing type rating.
During the flight, the aircraft entered a steep descent at a low altitude. Witnesses observed the aircraft flying at a height significantly lower than typical local traffic, between 30 and 90 meters above the ground. During this descent, the crew activated the aircraft's ballistic parachute system. Witnesses reported seeing the parachute deploy, but noted that the aircraft continued to fall in a spiral, nose-down attitude. The aircraft struck the ground and was destroyed by a subsequent fire, resulting in two fatalities.
The investigation
The ÚZPLN investigation examined the aircraft's technical condition, the flight profiles, and the structural integrity of the parachute deployment system. Investigators reconstructed the flight path using witness testimonies and analyzed the wreckage of the TL-2000 Sting and the parachute components.
The investigation also reviewed the regulatory history of the aircraft's certification. It was noted that the aircraft was certified under 2002 regulations, which utilized a different methodology for calculating the dynamic load on the parachute attachment cables compared to the updated 2004 standards. Furthermore, the investigation found that the specific parachute system installed in OK-NUA42 was different from the one used during the original 1996 strength tests for this aircraft type.
Findings
- The primary cause of the accident was a chain of events leading to a stall or loss of control at a speed below 120 km/h, which prompted the crew to activate the emergency parachute system at an altitude likely below the manufacturer's minimum recommended deployment height.
- The dynamic load from the parachute deployment caused the failure of the primary attachment cables. Specifically, both front steel cables and one rear stabilizing cable snapped due to the dynamic impact and the exceeding of the static load capacity of the cables.
- The aircraft's takeoff weight was approximately 10% above the maximum takeoff weight (MTOW), though investigators determined this was unlikely to be the decisive factor in the crash.
- The parachute deployment was partially unsuccessful in terms of full inflation due to the aircraft's rotation during the deployment process.
- The aircraft's engine exhaust from the ballistic parachute rocket motor was not diverted outside the airframe.