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Recent letters (November 1998) continue the debate about the failure of wind turbines in the recent cyclone in India. Were the wind speeds much higher than could have been anticipated or were some of the wind turbines simply not sufficiently robust?

Your report of the cyclone (September 1998) describes the wind as gusting to 70 m/s. In the context of UK wind farm sites, the maximum design wind speed (three second gust likely to occur once in 50 years) could exceed 70 m/s for many Scottish sites and could approach 65 m/s on some English and Welsh sites with significant topographic features.

I believe that a critical factor in the failures in India is that the grid also failed. If this happened at the height of the cyclone, then turbines would move to the downwind position as the cyclone raged, with blades feathered in the case of pitch regulated machines. Yawing could be slow enough that the full force of the wind would hit the side of the nacelle and across the full face area of the feathered blades. In this case, the tower top loading could be significantly higher than for the "following wind" condition and the robustness of any wind turbine would be tested to the full.

Wind turbine manufacturers would be well advised to check that this load case has been included in their design calculations.