Helicopter's height alerts failed on rig approach that ended in sea
The crew of a Bond Offshore Eurocopter EC225 Super Puma that collided with the sea surface during a poor visibility night approach to a North Sea oil rig received no automatic height callouts or terrain awareness system alerts, according to an initial factual report by the UK Air Accidents Investigation Branch.
The low-speed sea impact took place on 18 February, and all 18 people on board survived because the aircraft's flotation equipment was automatically deployed and they were able to disembark into emergency rafts.
When the crew departed Aberdeen at 17:42 for the 245km (130nm) flight to the ETAP platform the forecast weather for arrival was quite good, but conditions worsened markedly during the transit, which was made at a height of 5,500ft (1,675m). At about 24km from the ETAP the captain could see the rig, but when the crew learned that the forecast cloudbase had lowered from 800ft to 500ft and the visibility at the rig to 0.9km, they abandoned plans for an airborne radar approach in favour of descending to 500ft to enable a visual approach.
As they neared the rig they encountered extremely challenging visual conditions and elected to descend to 300ft to stay clear of the cloudbase. But meanwhile sea fog was building up and the helicopter entered it at 400ft, so the captain elected to climb back to 500ft. As they began the approach the rig, its gas flare could be seen clearly, but the rig's helideck, which - at an elevation of 166ft - was lower than the flare, was obscured by the fog.
At a distance from the rig of 1.4km the captain disconnected the autopilot and carried out a left turn to begin the final approach. The rig flare was in sight. The crew elected to disable the "check height" audio warning, and the co-pilot believed he could see the helideck, but the captain could only see the rig lights.
The AAIB says: "Shortly after that, at about 18:35, the helicopter landed heavily on the surface of the sea." There was no callout from the radio altimeter at 100ft - an alert that cannot be suspended by the crew - nor from the terrain awareness warning system, says the AAIB. The impact took place with a rate of descent of about 1,000ft/min (5.08m/s) and an airspeed of about 24kt.
