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A Super An AS332 L2 Super Puma. Image courtesy of Commander, U.S. 7th Fleet/Flickr.

Pilot error was most likely to blame for a 2013 North Sea helicopter crash that killed four UK oil and gas workers, a new government report found.

In a report published on Tuesday, the UK Air Accidents Investigation Branch found that the helicopter’s pilots deviation from cockpit procedures, failure to use checklists and inadequate monitoring of the aircraft’s instruments were behind the crash.

“It was considered that more use of the automated systems that were available on the aircraft might have prevented the accident,” the AIIB said.

The AS332 L2 Super Puma helicopter crashed into the UK North Sea around 6:00 p.m. on August 23, 2013 during a landing attempt at Sumburgh Airport with two pilots and 16 passengers on board.

The helicopter was making its final runway approach in cloud coverage when the flight commander failed to maintain a target approach speed of 80 knots, the AIIB found.

The craft’s engines failed to receive sufficient power and the helicopter’s airspeed “reduced continuously during the final approach,” falling to below 35 knots during the latter part of the approach.

The two pilots failed to notice the helicopter’s falling speed until “a very late stage” and could not recover, crashing the helicopter into the ocean about 1.7 nautical miles west of Sumburgh Airport.

The craft “rapidly filled with water and rolled inverted, but was kept afloat by the flotation bags which had deployed,” the investigators found.

The pilots and 12 of the passengers managed to survive the crash but four of the passengers were killed during the accident.

Three of the surviving passengers sustained serious injuries.

The AIIB did not identify any mechanical failures that could have caused the crash.

The AIIB also found that the pilots “had not developed a shared, unambiguous understanding of how the approach was to be flown,” did not make “optimum use” of the helicopter’s automated systems and did not alter their landing plan to account for the weather conditions.

Following the accident, the operator of the helicopter and the Civil Aviation Authority “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,” the report added.