Technicians were aware of problems with a gearbox of a Super Puma helicopter a week before the tragic April 2009 crash off the coast of Aberdeenshire.
The information was revealed during the inquiry currently underway in Aberdeen.
Speaking at the Aberdeen Town House in front of Sheriff Principal Derek Pyle, Verner Hill, an engineer and deputy shift supervisor at Bond Offshore Helicopters at the time of the incident, said the helicopter’s health and usage monitoring system (HUMS) had signalled a problem on the main gearbox on 25 March 2009, about a week before the tragic accident.
The problem, spotted during a turnaround inspection, was attributed to a metal particle on the chip detector of the gearbox.
Although the issue was only discovered after the helicopter had taken off for another flight, the crew did not report any additional problems.
Hill explained that at the time of the accident, guidelines for helicopter operators hadn't required the HUMS data to be fully downloaded and examined between individual flights, something that became a norm later.
"I was asking whether we needed to recall the aircraft from flight, if I remember correctly," Hill said about reporting the problem to his in line manager. "I wasn't told to recall it."
After the helicopter returned back to the base, Hill and his colleague carried out proper inspection of the gearbox. Despite the system still showing the warning message, no particles were found, the inquiry heard.
"I wasn't happy with what I was seeing," Hill said. "I spoke to the oncoming supervisor about it. I can't remember who decided but we took it offline. It was due to go on a training flight but we took it off the training flight and put it into the hangar for an after-flight inspection to be carried out."
The inquiry heard that a particle was later found during the after-flight inspection.
Asked why it was not found earlier, Hill said: "I've thought about this and the only thing I can think of is I missed it because of whatever reason, bright sunlight, I don't know." He also suggested the particle could have been stuck in the chip detector’s O-ring when taken out of the gearbox.
According to a report issued after the fatal crash that killed 16 men including two pilots, the particle in the gearbox had been detected 94 times.
The inquiry previously heard the helicopter had been required to undergo recurrent inspections due to the gearbox problem in the days following the detection of the particle, with the magnetic chip plugs of the gearbox being inspected between every two flights.
The precautions were terminated on the morning of the disaster.
An investigation of the Air Accidents Investigation Branch (AAIB) into the crash revealed the aircraft suffered a ''catastrophic failure'' of its main rotor gearbox. The failure caused the main rotor on the AS332-L2 model to break away, severing its ''tail boom'' from the fuselage.
At the time of the disaster, the first in a string of fatal accidents involving the Super Puma aircraft in the past five years, the helicopter was travelling from the BP Miller offshore platform. All of the 14 oil industry workers aboard were killed in the crash.
The inquiry is expected to last about six weeks and will examine the circumstances of the crash in order to prevent any future tragedy.