11/23/2011 04:01 pm ET Updated Jan 23, 2012

Crash Helicopter's Gearbox 'Failed'

Operators of a North Sea helicopter had been planning to replace its main rotor gearbox a week before the unit suffered a "catastrophic failure" leading to a crash which killed all 16 people on board, an air accident report has said.

A magnetic particle had been found on the chip detector in the gearbox of the Eurocopter Super Puma, a final report by the Air Accidents Investigation Branch (AAIB) said. 

This had led the operator to initiate a plan to remove the main rotor gearbox and replace it with a unit from another helicopter undergoing heavy maintenance, the AAIB said.

But actions taken following the discovery of the particle meant it was not recognised as an indication of the degradation of a part of the gearbox known as the second stage planet gear.

"The gearbox was declared serviceable by the operator and its planned replacement cancelled," said the report. It was this second stage planet gear that failed as a result of a fatigue crack, causing the failure of the main rotor gearbox.

As the helicopter was flying to Aberdeen from the Miller Platform in the North Sea on the afternoon of April 1 2009, the main rotor separated from the fuselage and the aircraft crashed into the sea. All 14 offshore workers and the two crewmen died.

The report told how, six seconds after expressing alarm, the captain had transmitted "Mayday Mayday Mayday" followed one second later by the co-pilot transmitting "Mayday Mayday Mayday, this is Bond 85 November, emergency, currently on the 055."

The report went on: "One second later, one of the flight crew uttered an expletive; this was the final radio transmission."

Eurocopter said: "The Eurocopter team was deeply saddened by this tragic event and our thoughts, deepest concerns and sympathy continue to be with the families, friends and loved ones affected by the accident.

"At Eurocopter, safety is and has always been the number one priority and the group continuously works to improve its safety standards, requiring suppliers to do the same and sharing this culture with customers. Eurocopter remains committed to working closely with the regulatory authorities, investigators and its operators to prevent the risk of accidents."