The Aeronautical Rescue Co-ordination Centre (ARCC) notified the Air Accidents Investigation Branch (AAIB) of the accident at 1912 hrs on 18 February 2009 and the investigation commenced the following day.
In accordance with established international arrangements, the Bureau d’Enquêtes et d’Analyses pour la sécurité de l’aviation civile (BEA) of France, representing the State of Design and Manufacture of the aircraft, appointed an Accredited Representative and was supported by additional investigators from Eurocopter. The operator co?operated with the investigation and provided expertise as required.
Prior to this Final Report, the AAIB published Special Bulletins on 24 March 2009 and 23 June 2009.
Twenty-seven Safety Recommendations have been made.
The helicopter departed Aberdeen Airport at 1742 hrs on a scheduled flight to the Eastern Trough Area Project (ETAP). The flight consisted of three sectors, with the first landing being made, at night, on the ETAP Central Production Facility Platform. Weather conditions at the platform deteriorated after the aircraft departed Aberdeen; the visibility and cloud base were estimated as being 0.5 nm and 500 ft respectively. At 1835 hrs the flight crew made a visual approach to the platform during which the helicopter descended and impacted the surface of the sea. The helicopter remained upright, supported by its flotation equipment which had inflated automatically. All those onboard were able to evacuate the helicopter into its liferafts and they were successfully rescued by air and maritime Search and Rescue (SAR) assets.
The investigation identified the following causal factors:
1. The crew’s perception of the position and orientation of the helicopter relative to the platform during the final approach was erroneous. Neither crew member was aware that the helicopter was descending towards the surface of the sea. This was probably due to the effects of oculogravic and somatogravic illusions combined with both pilots being focussed on the platform and not monitoring the flight instruments.
2. The approach was conducted in reduced visibility, probably due to fog or low cloud. This degraded the visual cues provided by the platform lighting, adding to the strength of the visual illusions during the final approach.
3. The two radio altimeter-based audio-voice height alert warnings did not activate. The fixed 100 ft audio-voice alert failed to activate, due to a likely malfunction of the Terrain Awareness Warning System (TAWS), and the audio-voice element of the selectable 150 ft alert had been suspended by the crew. Had the latter not been suspended, it would also have failed to activate. The pilots were not aware of the inoperative state of the TAWS.
The investigation identified the following contributory factors:
1. There was no specified night visual approach profile on which the crew could base their approach and minimum heights, and stabilised approach criteria were not specified.
2. The visual picture on final approach was possibly confused by a reflection of the platform on the surface of the sea.
 An oculogravic illusion is a visual illusion that affects the apparent position of an object in the visual field. A full explanation is provided in Appendix A and B to this report.
 A somatogravic illusion is a non-visual illusion that produces a false sensation of helicopter attitude. A full explanation is provided in Appendix A and B to this report.