What happened
On August 6, 2009, at approximately 15:30 UTC, a BLANIK L-13 SW motor glider, registration S5-KAH, was performing a flight from Ptuj (LJPT) to Lesce-Bled (LJBL). The flight was conducted under visual meteorological conditions following a 100-hour and annual inspection.
After takeoff, the aircraft climbed to an altitude of approximately 50 meters when the air brakes underwent an uncontrolled, automatic deployment. The pilot, unaware that the brakes had deployed, attempted to stabilize the aircraft using full engine power, attributing the degraded performance to turbulence. While maneuvering to avoid obstacles, the pilot attempted to fly beneath overhead power lines to avoid losing airspeed. During this maneuver, the aircraft's main landing gear struck a roadside embankment, causing the aircraft to bounce nearly vertically and rotate approximately 180 degrees around its longitudinal axis. The aircraft subsequently became entangled in the power lines, which were damaged in the collision. The aircraft was 100% destroyed.
The investigation
The investigation examined the aircraft's mechanical systems, including the engine, propeller, flight controls, and the air brake and pitot-static systems. Investigators also reviewed the pilot's training records, maintenance documentation, and meteorological conditions. The investigation found that the aircraft's permit to fly had expired on July 20, 2009, and that the recent maintenance check had been performed using a checklist intended for a different model (the L-13 SEH) because the owner had not provided the correct documentation. Furthermore, the maintenance organization utilized for the inspection was not authorized to perform work on this specific aircraft type.
Findings
- The primary cause of the accident was human error, specifically the pilot's failure to follow manufacturer-prescribed procedures to verify that the air brakes were fully locked in the forward position before takeoff.
- The uncontrolled deployment of the air brakes occurred because the brakes were not properly secured in the locked position.
- The pilot failed to recognize the deployment of the brakes, misinterpreting the resulting loss of performance as turbulence.
- The pilot did not perform the required physical check of the air brake lever to ensure it was pushed fully forward into the locking notch.
- The aircraft was operating with an expired permit to fly.
- The maintenance organization lacked the proper authorization to service this specific aircraft model.