What happened
On October 20, 2014, a Cessna F172G, registration YU-DSD, was participating in a group flight operation to disperse wildlife vaccines over the Golija mountain region near Rudno, Serbia. The aircraft was the final plane in a formation of nine, performing specialized aerial tasks for the operator "JAT Privredna Avijacija" d.o.o. Beograd.
While flying at approximately 8,000 feet, the pilot noticed irregular engine performance, characterized by intense vibrations, changes in engine sound, and a drop in RPM of approximately 200-250. Suspecting carburetor icing, the pilot activated the carburetor heat. Although the engine stabilized briefly, the instability returned.
Unable to maintain flight, the pilot attempted a forced landing on terrain consisting of a slight uphill then downhill slope. During the landing, the engine surged to full power, causing the aircraft to overshoot the crest of the slope. The aircraft subsequently struck a wooden telephone pole with its right wing, causing significant structural damage, including the breakage of the wing spar. The aircraft then entered dense vegetation, where the nose gear collapsed upon impact. The aircraft came to rest on its right side, approximately 100 meters from a residential house. The crew sustained minor injuries and exited the wreckage safely.
The investigation
The CINS investigation examined the aircraft's technical condition, the pilot's training, and the environmental factors. Investigators analyzed the engine's fuel system, the air filter, and the pilot's adherence to emergency procedures. The investigation also reviewed the operator's training programs and the aircraft's certification regarding the specialized vaccine dispersal equipment.
Findings
- The primary cause of the engine instability was a misdiagnosis of the engine issue by the pilot; the pilot applied carburetor heat, which enriched the fuel mixture, further reducing engine power.
- A significant contributing factor was a heavily contaminated air filter, which was coated with dust and grease, restricting the necessary airflow to the carburetor.
- The pilot's lack of specific experience in aerial vaccination operations over mountainous terrain led to high stress levels and the failure to follow established emergency procedures, such as shutting down the engine during the forced landing.
- The aircraft was performing at the upper limits of its performance capabilities, as the engine was slightly less powerful than those in the rest of the formation.
- The aircraft's specialized equipment installation was not in alignment with EASA Supplemental Type Certificate requirements for this specific Cessna model.
Safety action
- The operator was recommended to develop more robust technical preparation programs and include in-flight training specifically for aerial vaccination operations over high-risk mountainous terrain.
- The Serbian Civil Aviation Directorate was advised to verify all Air Service Certificates for Cessna aircraft to ensure compliance with EASA Supplemental Type Certificate EASA.A.S.03983.