25 July 2002
Faulty rotor blade blamed for Sikorsky's fatal plunge
By David Sapsted
Crash investigators believe that metal fatigue in a rotor blade - possibly the result of a lightning strike three years ago - caused last week's helicopter crash in the North Sea that killed all 11 men on board.
The Department of Transport disclosed yesterday that one of the four blades on the Sikorsky S-76A had snapped as it ferried workers between gas rigs off the East Anglian coast.
Data recovered from the two black boxes show no "significant abnormality" during most of the Bristow helicopter's 10-minute flight from the Clipper platform to the Monarch platform, 30 miles off Cromer, Norfolk.
"During this short flight, the pilots discussed a small increase in vibration but this increase appears to be so slight as to give them no cause for concern," said the department.
"A few minutes later a catastrophic event occurred during the early stages of the helicopter's approach to land on the Monarch platform."
That "catastrophic event" was the snapping of a rotor blade about six feet from its base as the helicopter travelled at 100mph at 400 feet.
As the broken section fell away, it caused such a massive vibration that the gearbox and the rotor head were torn away from the helicopter's fuselage. The Sikorsky immediately plunged into the sea and yesterday's preliminary findings made it plain there was nothing the pilots could do.
"On Tuesday, evidence of fatigue was found that enabled the investigation team to conclude that the blade fracture had initiated the catastrophic event," the department explained.
"The gearbox had separated from its mountings due to the severe imbalance created by the separated blade section.
"At this stage, there are a number of variables that are being evaluated to determine the likely origin of the fatigue. One variable may be related to a lightning strike suffered by the subject blade in 1999 but there are other variables under active consideration."
The faulty blade was returned to the manufacturers for evaluation before it returned to service. "Currently the investigation team is gathering more information about the history of this blade. At this stage, there is no evidence to link the lightning strike to the fatigue failure: however, this is one of the variables still under active consideration."
More than 90 per cent of the helicopter has now been recovered from the seabed by the Air Accidents Investigation Branch. The end of the rotor blade that broke off, however, is still missing.
The bodies of all but one of the men who died in the crash have been recovered.