What happened
On 23 August 2013, an AS332 L2 Super Puma, registration G-WNSB, was conducting a commercial passenger flight as part of a rotation between an offshore drilling platform and Sumburgh Airport. While on approach to Runway 09, the helicopter entered a critical flight condition that led to an impact with the sea approximately 1.5 nm west of the airport.
During the non-precision approach, the crew was managing a descent profile using autopilot vertical speed mode. While the commander intended to reduce airspeed to 80 kt, the aircraft's speed dropped significantly. At approximately 2.3 nm from the threshold, the airspeed fell below 80 kt and continued to decrease unobserved by the crew. As the aircraft descended, the co-pilot alerted the commander to the low airspeed, but the helicopter's descent rate increased sharply. The aircraft struck the water at 1717 hrs UTC. While the fuselage remained largely intact and the flotation system deployed, the impact and subsequent interaction with shoreline rocks caused the wreckage to break up.
Of the 18 people on board, 4 fatalities occurred among the passengers, while one crew member and three passengers sustained serious injuries.
The investigation
The AAIB investigation examined wreckage recovered from the seabed and shoreline, including the engines, main rotor gearbox, and the flight data recorder. Analysis of the Continuous Voice and Flight Data Recorder (CVFDR) provided a detailed record of the final approach. The investigation also reviewed the manufacturer's modelling of the aircraft's flight path and evaluated the effectiveness of the airport's surface rescue capabilities.
Findings
- The aircraft entered a vortex-ring state (also known as settling with power) during the final stages of the approach.
- The combination of a nose-high attitude, low airspeed, high power, and high rate of descent created the necessary conditions for this state.
- The reduction in airspeed went unobserved by the crew during the approach.
- The manufacturer's analysis indicated that the high rate of descent and limited altitude made the impact with the sea unavoidable once the condition was established.
- The airport's water rescue capability was limited by tidal conditions, which prevented the use of the primary slipway and delayed the launch of rescue craft.