What happened
On November 5, 2002, a Eurocopter AS 332L2 helicopter, registration LN-ONI, was performing a routine offshore passenger flight from the Sleipner oil field toward Stavanger Airport Sola. The aircraft was carrying 14 passengers and 2 crew members. During the descent through 1,200 feet, the helicopter experienced intense vibrations that made it difficult for the crew to read flight instruments.
The crew issued a MAYDAY call and identified a nearby vessel to facilitate an emergency landing. To mitigate the vibrations, the pilot reduced the airspeed from 158 knots to 100 knots. The helicopter successfully performed a controlled landing on the helideck of the M/T Navion Anglia, approximately 17 nautical miles southwest of Sola. There were no injuries to the passengers or crew.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined the aircraft and found that a pendulum weight from the vibration absorber on one of the main rotor blades had detached. This lost weight had struck another rotor blade, penetrating the blade behind the main spar and causing significant structural damage. This impact destroyed the blade's rigidity and created a massive mass imbalance of 5,500 N.
Investigators analyzed the mechanical failure of the vibration absorber assembly and the fatigue characteristics of the component shaft. The investigation also reviewed maintenance records, manufacturer certification data, and the manufacturer's emergency procedures regarding severe rotor vibrations.