One-off accident: Technical director at Bonus Energy in Denmark, Henrik Stiesdal, says the unfortunate accident which befell this 300 kW prototype on March 24 could never happen to a standard turbine off the Bonus production line, 2000 of which operate worldwide. "The turbine is one of only three prototypes without an automatic braking system which were installed in 1989," explains Stiesdal. "Only these three have just a manual brake with hydraulically operated air brakes on the blade tips." It appears a brake failure was the cause of the accident. The turbine over-speeded in winds of 15-20 m/s and one of the blades hit the tower, bringing the whole structure down. It and its two sister machines, now taken out of operation, were sited in west Zealand, Denmark.