Super Puma fatal accident inquiry: Repairs debris 'could have caused crash' 23 January 2014 Last updated at 14:57 A North Sea helicopter crash in which 16 men died could have been caused by debris falling into the helicopter gearbox during repair work, an inquiry has heard. The 13th day of the fatal accident inquiry heard a defective helicopter part was ruled out as it would have failed not long after it was fitted. Debris or steel breaking away due to contact pressure were suspected. The inquiry heard that sophisticated tests were carried out in a bid to establish what went wrong. Eurocopter transmission design specialist Emmanuel Mermoz said experts identified three possible scenarios which could have caused the crack. With a defective part ruled out, the inquiry heard that they believed the accident could have been caused by two things - spalling or debris getting into the gearbox part. The gear tooth showed classic signs of spalling, which is when a piece of steel has broken away as a result of contact pressure caused by other gearbox components. Chip detectors Mr Mermoz said the helicopter manufacturer had only dealt with a few cases of spalling in helicopter parts before. But he said maintenance staff had always identified the problem because of chip detectors installed in helicopter gearboxes which were designed to collect the metallic particles. The Air Accidents Investigation Branch (AAIB) pinpointed a catastrophic failure of the gearbox for the crash.