What happened
On July 25, 2021, a Cessna U206G, registration HK-2803, was operating a humanitarian medical flight between Mitú and San Miguel in the Vaupés department of Colombia. The aircraft, configured as an air ambulance, was transporting a medical professional and a patient.
Approximately ten minutes after departing San Miguel, the pilot noticed a strong smell of burning oil. Monitoring the engine parameters, the pilot observed a significant increase in oil and cylinder head temperatures alongside a drop in oil pressure. As the engine lost power, the pilot attempted to reach the San Miguel airstrip but found the dense jungle terrain made a conventional landing impossible.
With the engine eventually failing completely, the pilot performed an emergency tree landing (arborizaje). The aircraft struck tree branches and slid through the vegetation before the nose section collapsed following a 15-meter drop to the ground. The pilot sustained a minor head injury, while the medical professional and the patient were uninjured. Following the impact, the crew evacuated the aircraft and trekked for two hours through the jungle to reach a river, where they were eventually transported by boat to the Sonaña Indigenous Community.
The investigation
The GRIAA investigation focused on the engine's internal condition and maintenance practices. Investigators examined the engine components and found that the electrodes of the number five spark plug were heavily fouled with carbon, suggesting improper combustion.
Technical analysis revealed that this carbon buildup likely triggered detonation in the number five cylinder. This event caused extreme temperatures that damaged the cylinder head and lower section, leading to material erosion. The resulting leakage of combustion gases into the crankcase caused excessive pressurization, forcing engine oil out through the vents and drains, which ultimately led to the loss of engine power.
Findings
- The fracture of the number five piston was the primary cause of the engine failure and subsequent loss of power.
- Excessive carbon buildup on the number five spark plug electrodes likely initiated the detonation process.
- The operator's maintenance manual lacked specific procedures to limit the service life of spark plugs.
- There was a lack of adequate inspection procedures within the operator's maintenance manual regarding ignition system tasks and required inspection items (RII).
- Deficiencies were noted in the operator's maintenance manual regarding guidance for maintenance personnel during critical inspection processes.
- Potential improper engine operation by the crew, specifically regarding incorrect fuel mixture settings for different flight phases, may have contributed to the engine detonation.