The incidents are not a result of too much elasticity in the blade design and construction, but were caused by an unlikely combination of wind strength, temperature and wind direction, says Laurids. In New Zealand, the blades struck the corner of the lattice tower used in the Tararua project. So unusual were the conditions that it took a "major and expensive" investigation to localise the cause of the accidents, he adds.
"On the other hand, the solution has been both simple and cheap to carry out," says Laurids Pedersen. He explains that in unusual wind conditions, the wind can be stronger at the bottom of the rotor's circle than at the top. If the machine is hit by a gust at the same time, possibly while it is yawing, the blade can clip the tower. To avoid the problem, the blades have been tipped slightly forward-a solution which increases the structure's strength, adds Laurids Pedersen. The problem has only occurred in New Zealand and Wisconsin.