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 1 April 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 AAIB said that the particle had been discovered on 25 March 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 25 March 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 25 March, 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 25 March 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.
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.
“Eurocopter has been working closely with the AAIB throughout the investigation both in ascertaining the circumstances of the accident and in proposing improvement measures. Eurocopter has proactively initiated the implementation of a number of these modifications before the safety recommendations were issued.
“In addition, Eurocopter has full confidence in the various measures that have been implemented following the accident, including methods of information sharing, maintenance and inspection procedures, and technical improvements in particle detection capabilities, and are convinced that they are sufficient to strengthen the means already in force to prevent accidents.”
Eurocopter went on: “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.
“The Super Puma and its successor, the EC225, have accumulated more than four million flight hours to date. It is a reliable aircraft capable of flying missions at all latitudes and often under very difficult conditions, such as those encountered off the coast of Aberdeen, and to this day remains one of the oil and gas community's preferred helicopters, particularly due to the high level of safety it offers.”