What happened
On August 16, 2009, a Cessna FR 172H, registration OK-AKM, was performing a local flight near Strakonice Airport (LKST). The flight, operated by Air Service CS, included the pilot and three passengers. The intended route was a sightseeing flight over Orlík Castle and Zvíkov Castle.
During the approach to runway 13, the aircraft's engine sound was reported by witnesses to be irregular and fluctuating. The aircraft struck the treetops on the right bank of the Otava River at an altitude of approximately 10 meters before impacting the left bank of the river at water level. The impact resulted in three fatalities (the pilot and two passengers) and one serious injury to a minor passenger. The aircraft was destroyed.
The investigation
The ÚZPLN investigation focused on determining the cause of the engine's irregular performance and the aircraft's descent. Investigators examined the wreckage, which was partially submerged in the river. The engine was disassembled by an authorized maintenance organization, and no mechanical defects were found that could have caused a power loss. However, a red synthetic rope used for river buoys was found wrapped around the propeller shaft, suggesting the engine was running until the moment of impact.
Investigators also examined the cockpit controls. The flaps were found set to 30°, and the mixture control knob was found broken on the floor. The mixture control linkage was found bent, indicating the knob had been pulled into a lean position. The investigation also noted that the pilot had experience flying other aircraft types, such as the Z-37A, which required different mixture management procedures during landing.
Findings
- The engine was mechanically sound, and there was no evidence of fuel exhaustion.
- The pilot had extended the flaps to 30°, which increased aerodynamic drag and decreased airspeed.
- The pilot likely failed to enrich the fuel mixture according to the Cessna FR 172H flight manual procedures for the approach phase.
- The lean mixture setting, combined with the increased drag from the flaps, likely caused a significant loss of engine power when the pilot attempted to adjust the throttle.
- The pilot's failure to follow recommended procedures led to an insufficient descent profile, resulting in the impact with trees and the riverbank.