(Corrects "casual" to "causal" in 8th par)
LONDON, Oct 21 (Reuters) - The crew of a helicopter that crashed in the North Sea near Scotland's Shetland Islands did not notice the aircraft had been slowing until it was too late, a British air safety investigation team said.
Four oil rig contractors were killed when a Super Puma L2, made by EADS's Eurocopter subsidiary, crashed into the sea off Shetland's rocky southern coast on Aug. 23.
The helicopter, carrying 16 passengers and two crew, was operated by CHC Helicopter for France's Total and was heading to Sumburgh airport in Shetland.
In a report published on Friday, the Air Accident Investigation Branch (AAIB) said the aircraft had reduced its airspeed during its descent with the intention to maintain it at a certain level, but the helicopter had continued to lose speed, which went "unobserved by the crew".
"At some point the commander saw the sea, but he was unable to arrest the helicopter's descent and it struck the surface shortly thereafter," the report said.
CHC said that its pilot training followed strict regulations and industry standards and the airmen involved in the accident were well-trained and experienced, adding that it would not speculate on outcomes until the investigation was complete.
"The bulletin tells us what happened but not yet why it happened. We all want to understand why this happened and this will be a significant part of the ongoing investigation," CHC's Vice President of Safety and Quality, Duncan Trapp, said.
The AAIB repeated that it had found no evidence of a causal technical failure, a statement it first made on Sept. 5, and said the continuing investigation would focus on the operational aspects of the flight.
The crash was the fifth accident in four years in the area involving different models of Super Pumas, including a fatal crash of an L2 in April 2009 in which 16 people were killed. (Reporting by Brenda Goh; Editing by David Goodman)