North Sea crash helicopter's gearbox 'failed'
Thursday 24 November 2011
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 aboard, an air accident report said today.
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."
The AAIB said that the particle had been discovered on March 25 2009. The operator's engineers had sought the assistance of the manufacturer, Eurocopter, to deal with with what they considered to be a complex main rotor gearbox problem.
The AAIB added: "The use of verbal and email communication between the operator and manufacturer on March 25 led to a misunderstanding or miscommunication of the issue."
The discovered particle was initially identified as a piece of scale.
But further visual examination led the engineers to misidentify it as silver or cadmium plating which, according to the maintenance task card guiding maintenance staff, was "unimportant" and did not require the gearbox to be removed from service or to be put on "close monitoring".
The AAIB said that after March 25, the existing detection methods did not provide any further indication of the degradation of the second stage planet gear.
It added that the possibility of a material defect in the planet gear or damage due to the presence of foreign object debris "could not be discounted".
The report said that after March 25 the maintenance task to examine the ring of magnets on the helicopter's oil separator plates was not carried out.
It added that the ring of magnets reduced the probability of detecting released debris.
The AAIB also said this helicopter did not provide an alert to the flight crew when the magnetic chip detector detected a particle.
The aircraft was at 2,000ft when the accident happened. The first indication to the crew of a problem was loss of main rotor gearbox (MGB) oil pressure and the triggering of a master warning.
Two and half seconds before this indication, the co-pilot had made a radio transmission stating that the helicopter was serviceable.
Immediately after the loss of MGB oil pressure, the helicopter began to descend and failed to respond to control inputs. The main rotor system separated from the helicopter approximately 20 seconds after the loss of MGB oil pressure.
During separation the main rotor blades struck the helicopter's tail boom in several places, severing it from the fuselage.
"The fuselage fell into the sea at high vertical speed and the impact was non-survivable for all occupants," the AAIB said.
In today's final report, which followed three interim reports, the AAIB listed 17 recommendations that it had made during its investigation.
They included recommendations to Eurocopter, the European Aviation Safety Agency, the UK's Civil Aviation Authority and America's Federal Aviation Administration.
Scotland's Crown Office and Procurator Fiscal Service (COPFS) welcomed the publication of the report on this "tragic incident, following a technically complex and challenging investigation".
It went on: "The findings contained therein will now be fully considered by the health and safety division of COPFS.
"The division and Grampian Police have been engaged in this investigation since the tragedy occurred and will continue to progress lines of inquiry and carry out such investigation as is necessary in order that a decision may be taken in relation to the form of any proceedings.
"The liaison with the nearest relatives of the 16 men who lost their lives will also continue and the division will keep them advised of significant developments."
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