A terse statement by the UK Air Accidents Investigation Branch has provided basic facts downloaded from the cockpit voice and flight data recorder of the CHC Scotia AS332L2 Super Puma helicopter (G-WSNB) involved in a fatal crash on approach to Sumburgh, in the Shetland Isles, on 23 August.
Crucially, no technical failure has been detected, the statement says, noting that both engines continued to deliver power until the helicopter impacted the sea. Four passengers were killed during the accident.
It adds: "To date, no evidence of a causal technical failure has been identified; however, detailed examination of the [combined cockpit voice and flight data recorder] and the helicopter wreckage is continuing."
During the localiser/DME non-precision approach to Sumburgh's runway 09, the aircraft was on the correct vertical descent profile at three [nautical] miles, but was descending faster than appropriate at a time when the crew would still not have been able to see the runway through the thin mist present.
When air traffic control at Sumburgh provided the CHC crew with radar vectors to join the localiser/DME approach to 09, visibility was 1.5nm (2.8km), and the wind was 17kt from the southeast.
On such an approach lateral guidance is provided, but the crew must set and monitor their own vertical profile by plotting DME distance from the runway against the height they should be passing at that point.
The statement says that at three miles from the threshold the helicopter was "on the published horizontal and vertical profile of the approach to runway 09, with the airspeed decreasing steadily".
However, a mile later it had descended some 240ft (73m) below the vertical approach profile, and its rate of descent had hit 500ft/min, with an airspeed of 68kt (126km/h).
The statement adds: "The airspeed continued to reduce to below 30kt, and as it did so the helicopter pitched increasingly nose-up. The rate of descent remained constant for a period before increasing rapidly.
"Shortly thereafter the helicopter, which was intact, struck the sea in a near level pitch attitude with a slight right bank. Both engines were delivering power until impact."
Suspicions that the crash was not due to an issue with the airframe or engines began to surface shortly after the Aberdeen-based Helicopter Safety Steering Group lifted its voluntary flight ban covering all Super Puma variants on 29 August.
A statement was released the following day by the UK Civil Aviation Authority which backed the HSSG's position, stressing that it did not believe "the accident was caused by an airworthiness or technical problem, and consider[ed] that the decision by the operators to resume Super Puma flights is appropriate".
"We would not allow a return to service unless we were satisfied that it was safe to do so. We will review the position if any new evidence comes to light," it said.
Source: Flight International