What happened
On October 16, 2004, at approximately 16:55, a Robinson R22 Mariner helicopter, registration OH-HTE, was conducting a reindeer management flight near Naruska, Salla. While the aircraft was hovering at an altitude of approximately 50 meters and initiating a right turn, the engine suddenly lost power.
The pilot immediately attempted to enter autorotation by lowering the collective to maintain rotor RPM. However, due to the low altitude and low airspeed, a successful autorotation was not possible. The aircraft continued its descent into the forest. Approximately ten meters before ground contact, the rotor blades struck trees and broke, rendering the helicopter uncontrollable. The two-person crew escaped the crash without injury, though the aircraft sustained significant damage.
The investigation
The investigation focused on the sudden loss of engine power and the mechanical failure of the ignition system. Investigators examined the condition of the engine's magnetos and the fuel system. The inquiry also reviewed the aircraft's maintenance history, specifically noting that the last 100-hour inspection had been completed roughly 45 flight hours prior to the accident. Additionally, the investigation looked into the fuel handling practices used during field operations.
Findings
- The primary cause of the engine failure was the failure of the distributor wheel on one of the helicopter's magnetos. Five teeth had broken off the wheel, which caused the ignition timing to advance prematurely, resulting in an engine backfire that shut down the motor.
- The pilot's ability to perform a controlled autorotation was severely limited by the low flight altitude and low airspeed at the time of the failure.
- Significant contamination was found in the fuel system. Rust and various impurities, including paint particles and debris, were discovered in the fuel strainer and fuel tanks. These contaminants originated from the improper storage and transport of fuel in unlined 200-liter oil drums and jerry cans.
- Maintenance records indicated that the specific service bulletin (SB 658) regarding magneto distributor wheel inspections had not been explicitly referenced in the recent 100-hour maintenance checklist.