Helikopter Service Flight 451
| Company type | Private company |
|---|---|
| Industry | Aviation |
| Headquarters | Norway |
| Products | Helicopter services |
On 8 September 1997, Helikopter Service Flight 451, a Eurocopter AS 332L1 Super Puma operated by the Norwegian firm Helikopter Service, crashed into the Norwegian Sea roughly 100 nautical miles northwest of Brønnøysund, Norway. The helicopter was flying from Brønnøysund Airport to the Statoil-managed Norne FPSO vessel. The accident resulted from a fatigue crack in a spline within the power transmission shaft connector, which caused the shaft to fail. All twelve individuals aboard perished upon impact.
Aircraft
The aircraft involved was an AS332 L1 Super Puma produced by Eurocopter (currently Airbus Helicopters), bearing the registration LN-OPG.
Background
Flight 451 took off from Brønnøysund Airport at 06:00 local time (UTC+2), carrying two pilots and ten passengers. This flight operated as a daily shuttle service to the Norne FPSO during its commissioning phase, compensating for limited living quarters onboard.
Accident
At 06:50:07, the crew reported an engine overspeed warning but proceeded with their approach toward Norne. Routine communications were maintained with the Transocean Prospect platform and Bodø ATCC. At 06:55:37, abnormal indications appeared again, followed shortly by unusual sounds and loss of control at 06:56:30. The helicopter descended from approximately 1,800 feet before crashing into the sea; the wreckage came to rest at about 830 meters depth. None of the twelve occupants survived.
Summary of investigation findings
The Norwegian Accident Investigation Board (AIBN) concluded that multiple fatigue cracks had developed in the splined sleeve connecting the high-speed bendix shaft and the right-hand engine. This damage compromised the engine overspeed protection mechanism, causing the sleeve to fail and sever the high-speed shaft. The subsequent overspeed led to the failure of the right-hand power turbine, which in turn destroyed the left-hand engine and severed control rods, making the helicopter uncontrollable.
The investigation also revealed that the flight crew did not possess adequate understanding of the aircraft's control system to fully appreciate the seriousness of intermittent overspeed warnings. Moreover, no specific procedures or checklists existed to address this malfunction.
The AIBN noted that a HUMS accelerometer fitted on the helicopter could have detected abnormal vibration patterns prior to failure if it had been actively monitored. Analysis of historical HUMS data indicated vibration levels exceeded thresholds before the crash. However, HUMS information required manual extraction and evaluation between flights, and its application in offshore helicopters was still experimental and not mandated at that time. Since fatigue cracks developed over several days, systematic HUMS monitoring might have prevented this incident. Among eighteen recommendations in its final report, AIBN advocated regulatory adoption of HUMS for offshore helicopters. This report and HUMS’s significance were emphasized at the 2003 HUMS Conference in Melbourne, described as "a defining moment" for HUMS integration in offshore helicopter operations.
See also
Other notable helicopter accidents in the North Sea include: