An inquiry into the deaths of 16 men killed in a Super Puma crash has heard that it was impossible to tell whether the 2009 tragedy could have been avoided.

Divers failed to recover missing pieces of the shattered gearbox during underwater searches of the sea bed. Experts believe the missing parts may have been destroyed during the crash or could have left the aircraft as it broke up over the water.

On Monday a fatal accident inquiry was told that it was impossible to determine whether helicopter engineers could have prevented the accident.

Air Accident Investigation Branch (AAIB) senior engineering inspector Mark Jarvis, 50, led the recovery operation in the hours following the crash on April 1, 2009. Solicitor advocate Tom Marshall, who represents the families of the victims, asked: "This accident was avoidable, is that right?"

Mr Jarvis replied: "I don't know. We don't fully understand the failure mode.

"What we do know is that if the gearbox had been removed from service and sent to the manufacturer then during their overhaul procedure this gearbox would have been rejected."

Extensive stress tests carried out on other gear models suggested it was possible the cracked gear had a material defect which had weakened it. This could have led to it breaking apart and may have resulted in catastrophic failure.

Murdo MacLeod QC, who represents Super Puma manufacturer Eurocopter, suggested a material defect would be "unlikely".

Fourteen oil workers and two pilots died when the Super Puma crashed into the sea. The inquiry continues.