Helicopter that ditched in North Sea had 1ft crack in gearbox
THE SUPER Puma helicopter that was at the centre of a dramatic controlled ditching in the North Sea in May had been placed on a “close monitoring” alert before the crash, a new report by the government’s Air Accident Investigation Branch (AAIB) has revealed.
The Bond-operated aircraft, with two crew and 12 passengers on board, ditched 25 miles east of Aberdeen on 10 May after the pilots noticed a low oil pressure warning in the cockpit.
An initial bulletin by the AAIB has already revealed that its gearbox shaft was cracked.
But a special bulletin by the branch, issued yesterday, has now disclosed that the shaft failed after only 167 flying hours.
It also said the helicopter had been placed on a system of ten-hourly close-monitoring checks after two previous “amber” alerts – one the day before the ditching and the second on the day of the crash.
The initial bulletin disclosed that AAIB investigators found a 360-degree crack in the “bevel gear vertical shaft” in the main gearbox, near the manufacturing weld.
But the new bulletin reveals that investigators have now found a total of three cracks – one more than a foot long.
The report states: “The main gearbox (MGB) was fitted on 18 March, 2012, following overhaul at the helicopter manufacturer’s facility where a new bevel gear vertical shaft was fitted. A strip examination of the MGB established that the bevel gear vertical drive shaft had failed across the 4.2 mm diameter hole in the area where the two parts of the shaft are welded together.” The report continues: “Examination of the fracture surface on both parts of the bevel gear vertical shaft revealed the presence of three cracks. Two started from the 4.2 mm diameter hole in the weld.”
One of the cracks was 336mm long. The second was 106mm long and the third, 42 mm long.
But, a review of the Super Puma’s health and usage monitoring system showed there had been no indication of any significant rising vibration trends until some six hours before the aircraft’s final flight.
Bond’s engineers followed the fault diagnosis chart in the aircraft maintenance manual but found that magnetic chip detectors were free of telltale debris.
The report continues: “Thirty-six other indicators were checked and no significant trends were detected.
“In accordance with the guidance in the aircraft maintenance manual, the aircraft was placed on ten-hourly close monitoring and released for flight.”
The AAIB investigators said: “The investigation will continue to review the results from the fatigue tests, with other data and evidence, to establish the mechanism that caused the initiation and propagation of the fatigue cracks in the bevel gear vertical shaft.”
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Thursday 20 June 2013
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