At around 9.10 a.m., an air brake was detached from a wind turbine blade in a wind farm. The wind turbine stopped automatically. The manager on duty decided not to try restarting the machine by remote control. Since the operating instructions for the farm did not provide for standby duty, the intervention of the technicians was scheduled for 3 days later. When the maintenance team went to the site, it discovered the equipment 155m away in the field alongside the wind turbine. The area was taped off and the air brake was evacuated. The whole farm was shut down as a preventive measure pending an expert appraisal. The operating loss was estimated at €10,000.

A similar incident had occurred on the same machine and the same blade four years earlier (ARIA 50291), due to the breakage of a retaining washer. However, the cause of the incident seemed different in this case, because the helical part was missing. Following this incident, the operator had scheduled an inspection every five years, and the next one was to be performed in the summer of 2022.

The operator explained this new fall of an air brake by the combination of the breakage of a tensioner and the weakening of the air brake’s bonded joint. The tensioners were inspected every six months. The bonding was designed to hold for the certified lifetime of the wind turbine (20 years). The weakening was apparently due to the previous fall. After this incident, the bonding had not been able to be inspected ultrasonically, because the air bubbles present returned an echo.

The operator decided to replace the farm’s 15 tensioners. It was recommended that it should:

• specify the expectations of the half-yearly inspections and the action to be taken;
• step up maintenance and the frequency of inspection and replacement of the tensioners;
• check that the five-yearly inspection programme was still suitable in light of experience feedback;
• deploy the experience feedback obtained to other farms using this same type of machine.

The operator changed the procedure concerning visual inspection of the cables and jacks of the air brake system at a half-yearly frequency. The five-yearly inspection programme was considered satisfactory in terms of its content and frequency.