  We suggest a mathematical model for computing and regularly updating the next
preventive maintenance plan for a wind farm. Our optimization criterium takes
into account the current ages of the key components, the major maintenance
costs including eventual energy production losses as well as the available data
monitoring the condition of the wind turbines. We illustrate our approach with
a case study based on data collected from several wind farms located in Sweden.
Our results show that preventive maintenance planning gives some effect, if the
wind turbine components in question live significantly shorter than the turbine
itself.
