What happened
On 15 September 2004, a Sikorsky S-61N, registration G-BDOC, was conducting a Commercial Air Transport task near Sullom Voe, Shetland. The crew, consisting of a commander, first officer, winch operator, and winchman, was tasked with transferring marine pilots between ships.
Upon arriving at the designated vessel, the crew identified the winching area but noted a potential obstruction in the form of a mast. To avoid this, the crew decided to shift their intended winching position forward by approximately 40 feet. The helicopter established a hover at roughly 20 feet above the deck. During the process of lowering a marine pilot, the commander noticed the aircraft drifting and attempted to correct its position. During this maneuver, a repetitive noise was heard, and the crew experienced a vibration through the airframe. The winchman on deck subsequently reported a possible blade strike, noting that the rotors had come very close to a yellow-topped mast riser.
The commander immediately increased the helicopter's altitude and returned to Scatsta Aerodrome. An inspection revealed that four of the five main rotor blades had sustained extensive damage on their lower surfaces due to contact with the ship's mast.
The investigation
The AAIB examined the aircraft's maintenance records, which showed no pre-existing defects, and reviewed the flight data recorder (FDR) and cockpit voice recorder (CVR). The investigation focused on the deck dimensions and the clearances available during the maneuver. It was determined that while the crew's decision to move the winching position was intended to increase safety, it actually reduced the margin of clearance between the rotor disc and the mast riser.
Investigators also analyzed the maritime regulations and the ship's deck markings. It was found that the markings on the vessel did not comply with the International Chamber of Shipping (ICS) guidelines for an aircraft of this size. Furthermore, there were discrepancies in the 'D' value (the diameter used to calculate safety zones) between aviation and maritime regulatory documents.
Findings
- The crew's decision to relocate the winching position to a different area of the deck reduced the safety margin between the rotor blades and the ship's mast.
- There were misunderstandings and insufficient information exchange between the helicopter operator and the maritime operator regarding the specific dimensions of the winching area and obstructions.
- The ship's deck markings were not in accordance with the established international guidelines for the S-61N helicopter type.
- The crew was transitioning from Search and Rescue (SAR) duties, which require high flexibility, to a more structured Commercial Air Transport (CAT) operation, which requires precise pre-flight information.