What happened
On 23 August 2013, an AS3US L2 Super Puma, registration G-WNSB, was performing a commercial passenger flight from the Borgsten Dolphin drilling platform to Sumburgh Airport. While on approach to Runway 09, the crew was managing a non-precision approach using autopilot in vertical speed mode. The commander had intended to reduce the airspeed to 80 kt for the final stage of the approach.
As the aircraft progressed along the approach, the airspeed began to drop below the target speed. Although the crew attempted to maintain the profile, the airspeed continued to decrease unobserved. Automated cockpit warnings for "CHECK HEIGHT" were triggered as the aircraft's altitude dropped. In a final attempt to arrest the descent, the commander increased the collective pitch and engine torque, but the descent rate accelerated significantly. The helicopter struck the sea approximately 1.5 nm west of Sumburgh Airport. Of the 18 people on board, 4 passengers died and 1 crew member sustained serious injuries.
The investigation
Investigators examined the wreckage, which was recovered from the seabed and the shoreline. Analysis of the engines and main rotor gearbox showed no evidence of mechanical failure prior to impact. Data retrieved from the Combined Voice and Flight Data Recorder (CVFDR) provided a detailed timeline of the final moments of the flight.
Manufacturer analysis of the flight data concluded that the aircraft behaved as expected based on the control inputs provided. The investigation also looked into the rescue response, noting that the primary launch site for the airport's rescue craft was unusable due to tidal conditions, and an alternative launch attempt was hindered by soft sand.