What happened
On September 3, 2007, at approximately 14:30, a Robinson R-44 Astro helicopter, registration ES-HRW, was returning from a photography flight near Valkeala, Finland. While cruising at an altitude of 800 feet, the pilot noticed an unusual odor in the cockpit. Shortly thereafter, a loud bang occurred, followed by intense vibrations throughout the aircraft.
In response to the sudden loss of power and the onset of vibrations, the pilot initiated an autorotative landing in a nearby field. The landing was executed successfully, and the aircraft sustained no additional damage during the touchdown. The pilot and passenger were uninjured. Following the landing, the pilot notified Utti Air Traffic Control, which subsequently dispatched police and emergency services to the scene.
The investigation
The investigation focused on the mechanical failure of the transmission system. Investigators examined the drive shaft connecting the main and tail rotors, specifically looking at the clutch actuator's lower bearing. The investigation included a lubricant analysis to ensure the correct grease, Mobilgrease 28, had been used during the last maintenance interval.
Technical examination revealed that the lower bearing of the clutch actuator had failed. This failure caused the drive shaft to break due to extreme frictional heat, which subsequently damaged the clutch actuator for the belts, the fan, and the surrounding housing. Furthermore, the engine itself suffered mechanical failure because it reached an excessive rotational speed once the load from the rotors was removed.