What happened
On January 8, 2004, an AS 332L2 helicopter, registration LN-ONI, operated by Norsk Helikopter AS, was performing a takeoff from the Transocean Searcher semi-submersible rig at the Sleipner oil field in the North Sea. The aircraft was carrying two crew members and 19 passengers, along with 321 kg of luggage.
Due to heavy weather, including winds reaching 58 knots and significant wave action, the helideck was in motion. As the helicopter lifted into a hover, the aircraft moved backward with a low tail position. Simultaneously, the movement of the rig caused the helideck to rise toward the aircraft's rear. This relative motion caused the helicopter's tail guard to catch underneath the helideck's rope net. As the pilot continued to climb, the net remained snagged, being lifted approximately 1.3 to 1.5 meters above the deck level.
As the helicopter transitioned into a normal takeoff climb, the aircraft tilted increasingly toward the rear until the rope net finally slipped off the tail guard. The pilot flying experienced a sudden jolt and a nose-down pitch, initially mistaking the movement for accidental contact with the control stick by the co-pilot. The crew was notified of the entanglement by the Helideck Landing Officer (HLO) via radio only after the aircraft had departed the rig.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined the aircraft's configuration and the environmental conditions at the time of the incident. The investigation confirmed that the aircraft's weight and balance were within legal limits, though the center of gravity was positioned in the rear area. This specific loading configuration causes the AS 332L2 to maintain a low tail attitude during hover.
Investigators also reviewed the rig's motion-monitoring equipment and found it was unable to calculate the heave rate, a critical metric for pilots operating from moving decks. Furthermore, the investigation looked into the communication between the deck crew and the flight crew, noting that the HLO did not alert the pilots to the snagging net while the aircraft was still over the deck.
Findings
- The primary cause of the incident was the entanglement of the tail guard with the helideck rope net caused by the simultaneous relative movements of the aircraft and the moving rig.
- The rearward center of gravity caused the helicopter to hover with a low tail, increasing the risk of contact with deck equipment.
- The tail guard on this aircraft model was not designed for operations on decks utilizing rope nets.
- There was a lack of standardized procedures and communication regarding the reporting of immediate hazards by deck personnel to the flight crew during takeoff.
- The rig's motion-sensing equipment failed to provide the crew with essential heave rate data.