What happened
On 28 October 2009, a Eurocopter France EC120 B, registration SE-JME, was conducting a commercial air transport mission in the mountains of Z County, Sweden. The flight, operated by AB Jämtlands Aero, was intended to inspect several antennas. After a scheduled stop at Helags mountain station, the pilot was flying toward Funäsdalen when a sudden, loud bang occurred approximately one kilometer after passing the highest point at Flatruet.
Immediately following the noise, the helicopter experienced intense, low-frequency vibrations. The force of the vibration was so severe that cockpit instruments became unreadable and interior fittings were torn from the fuselage. Recognizing that the vibrations were tied to the main rotor speed, the pilot initiated an autorotation. The pilot managed to steer the aircraft toward a snow-covered moor, performing a controlled touchdown at an altitude of approximately 20 feet. The time elapsed from the initial structural failure to the landing was less than 30 seconds. While the aircraft sustained significant damage, there were no fatalities among the crew or passengers.
The investigation
The Swedish Accident Investigation Board (SHK) examined the wreckage and the main rotor hub. Technical analysis, including fractographic examination, revealed that the failure originated from a fatigue crack. Investigators found that the crack had initiated due to fretting corrosion under the bush shoulder on both sides of the FTi bush.
By counting macroscopic marks on the fracture surface, investigators determined that one mark corresponded to one engine start. Based on the operator's flight records, they estimated the propagation time from the crack's initiation to the final fracture was approximately 112 hours.
Findings
- The primary cause of the accident was a fatigue crack in the main rotor hub.
- The crack initiated in an area of significant fretting corrosion under the bush shoulder.
- The existing maintenance and inspection intervals were insufficient to detect the defect, as the time required for the crack to grow from initiation to final failure was shorter than the scheduled inspection period.
- The sudden structural failure caused severe vibrations that compromised the pilot's ability to monitor flight instruments.