The accident was reported by the helicopter operator at approximately 1756 hrs on the day of the accident.
In exercise of his powers, the Chief Inspector of Air Accidents ordered an investigation into the accident be carried out in accordance with the Civil Aviation (Investigation of Air Accidents and Incidents) Regulations 1996. The sole objective of the investigation of an accident or incident under these Regulations is the prevention of accidents and incidents. It shall not be the purpose of such an investigation to apportion blame or liability.
The AAIB despatched teams of investigators and support staff to Aberdeen and the Shetland Islands early the following morning, to commence the investigation.
In accordance with the provisions of ICAO Annex 13, France (the State of aircraft design and manufacture) appointed an Accredited Representative from the BEA , assisted by Advisers from the helicopter and engine manufacturers. Advisers from the European Aviation Safety Agency (EASA) and the UK Civil Aviation Authority (CAA) also participated in the investigation.
At 1717 hrs UTC on 23 August 2013, an AS332 L2 Super Puma helicopter with sixteen passengers and two crew on board crashed in the sea during the approach to land at Sumburgh Airport. Four of the passengers did not survive.
The purpose of the flight was to transport the passengers, who were employees of the UK offshore oil and gas industry, to Aberdeen. On the accident flight, the helicopter had departed the Borgsten Dolphin semi-submersible drilling platform in the North Sea, to route to Sumburgh Airport for a refuelling stop. It then planned to continue to Aberdeen Airport.
The commander was the Pilot Flying (PF) on the accident sector. The weather conditions were such that the final approach to Runway 09 at Sumburgh Airport was flown in cloud, requiring the approach to be made by sole reference to the helicopter’s instruments, in accordance with the Standard Operating Procedure (SOP) set out in the operator’s Operating Manual (OM). The approach was flown with the autopilot in 3-axes with Vertical Speed (V/S) mode, which required the commander to operate the collective pitch control manually to control the helicopter’s airspeed. The co-pilot was responsible for monitoring the helicopter’s vertical flightpath against the published approach vertical profile and for seeking the external visual references necessary to continue with the approach and landing. The procedures permitted the helicopter to descend to a height of 300 ft, the Minimum Descent Altitude (MDA) for the approach, at which point a level-off was required if visual references had not yet been acquired.
Although the approach vertical profile was maintained initially, insufficient collective pitch control input was applied by the commander to maintain the approach profile and the target approach airspeed of 80 kt. This resulted in insufficient engine power being provided and the helicopter’s airspeed reduced continuously during the final approach. Control of the flightpath was lost and the helicopter continued to descend below the MDA. During the latter stages of the approach the helicopter’s airspeed had decreased below 35 kt and a high rate of descent had developed.
The decreasing airspeed went unnoticed by the pilots until a very late stage, when the helicopter was in a critically low energy state. The commander’s attempt to recover the situation was unsuccessful and the helicopter struck the surface of the sea approximately 1.7 nm west of Sumburgh Airport. It rapidly filled with water and rolled inverted, but was kept afloat by the flotation bags which had deployed.
Search and Rescue (SAR) assets were dispatched to assist and the survivors were rescued by the Sumburgh-based SAR helicopters that attended the scene.
The investigation identified the following causal factors in the accident:
The helicopter’s flight instruments were not monitored effectively during the latter stages of the non-precision instrument approach. This allowed the helicopter to enter a critically low energy state, from which recovery was not possible.
Visual references had not been acquired by the Minimum Descent Altitude (MDA) and no effective action was taken to level the helicopter, as required by the operator’s procedure for an instrument approach.
The following contributory factors were identified:
The operator’s SOP for this type of approach was not clearly defined and the pilots had not developed a shared, unambiguous understanding of how the approach was to be flown.
The operator’s SOPs at the time did not optimise the use of the helicopter’s automated systems during a Non-Precision Approach.
The decision to fly a 3-axes with V/S mode, decelerating approach in marginal weather conditions did not make optimum use of the helicopter’s automated systems and required closer monitoring of the instruments by the crew.
Despite the poorer than forecast weather conditions at Sumburgh Airport, the commander had not altered his expectation of being able to land from a Non-Precision Approach.
AAIB Special Bulletins S6/2013 and S7/2013 , published on 5 September 2013 and 18 October 2013 respectively, provided initial information on the circumstances of the accident. Special Bulletin S1/2014, published on 23 January 2014, highlighted a safety concern relating to pre-flight safety briefings given to passengers, on the functionality of emergency equipment provided to them for UK North Sea offshore helicopter flights.
The AAIB investigation found similarities between this accident and previous accidents resulting from ineffective monitoring of the flight instruments by the flight crew.
Following this accident, the operator of G-WNSB and the Civil Aviation Authority (CAA) took safety actions intended to prevent similar accidents in future and to increase the level of safety of UK offshore helicopter operations in the North Sea.
During the investigation a number of additional safety concerns were identified. In addition to the Safety Recommendations issued in the aforementioned Special Bulletins, this final report contains further Safety Recommendations concerned with the certification of rotorcraft, Helicopter Flight Data Monitoring and offshore helicopter survivability.