Investigators probing the crash of a Eurocopter AS332L2 Super Puma in the Shetland Islands in August say they have not yet found any evidence of technical failure onboard the helicopter.

Four oil workers died when the CHC Scotia-operated Super Puma ditched into the North Sea just minutes before it was due to land at Sumburgh Airport in the Shetlands on Aug. 23.

Twelve passengers and the two pilots survived, although one of the crew was seriously injured.

Reporting its preliminary findings on the accident on Sept. 5, the U.K. Air Accident Investigation Branch said that recorded data from the voice and flight data recorder (CVFDR), recovered from the water on Aug. 29, showed that when the aircraft was 3 mi. from runway and on the published approach to Sumburgh’s Runway 09, it was 240 ft. below the vertical approach profile, descending at 500 ft. per minute and losing airspeed.

The report states that the airspeed continued to drop and the helicopter entered a nose-up attitude. The rate of descent then increased rapidly, and moments later the helicopter struck the sea in a “near level pitch attitude with a slight right bank.”

Both engines were still delivering power until the moment of impact, investigators said.

“To date, no evidence of a causal technical failure has been identified; however, detailed examination of the CVFDR data and the helicopter wreckage is continuing,” the report says.

The Super Puma, registered G-WNSB, had been operating on behalf of oil company Total and was returning to Sumburgh after a flight from the drilling platform Borgsten Dolphin. Investigators have salvaged most of the wreckage from the water, including both engines and the main gearbox, which still had the main rotor head attached.

Members of the Scottish Parliament debated the accident on their return from summer break on Sept. 4. In a statement to the Parliament at Holyrood, John Swinney, the cabinet secretary for finance, employment and sustainable growth in Scotland, urged trade unions and the offshore community to learn lessons from the latest accident — the fifth in four years — adding that safety needed to remain the “first priority for those who service the oil and gas industry in the North Sea.”

The Helicopter Safety Steering Group (HSSG) lifted a suspension of Super Puma variant flights on Aug. 30, allowing the AS332L/L1 and EC225 helicopters to return to revenue operations. CHC-Scotia has not yet returned the AS332L2 to service in the U.K., although the type is back operating at locations overseas. Bristow Group has returned two AS332L2 aircraft back to operations in Nigeria.