Method of performing service work on a horizontal axis wind turbine

A method of performing service work on a horizontal axis wind turbine having a tower carrying a nacelle with a rotor, the rotor having a rotor hub and three wind turbine blades equally distributed about the rotor hub, each wind turbine blade being arranged rotationally adjustably on the rotor hub about a pitch axis between at least one operational pitch angle corresponding to a normal operational angle of attack of the wind turbine blade and at least one non-operational pitch angle corresponding to a non-operational angle of attack of the wind turbine blade, wherein an operational pitch angle is defined as an angle which deviates not more than 10 degrees from a theoretical pitch angle of 0 degrees and wherein a non-operational pitch angle is defined as an angle which does not deviate more than 20 degrees from a theoretical pitch angle of 90 degrees, whereby, before performing the service work, a pitch angle adjustment of a first one of said three wind turbine blades is performed so that a pitch angle difference between respective pitch angles of the first one and a third one of said three wind turbine blades is between 45 degrees and 135 degrees or between 225 degrees and 315 degrees, whereby before performing the service work, the respective pitch angles of the first one and a second one of the three wind turbine blades are adjusted by rotation to one of their at least one non-operational pitch angles, and the pitch angle of the third one of the three wind turbine blades is maintained in or adjusted by rotation to one of its at least one operational pitch angles, whereby, after performing the service work, a pitch angle adjustment of the first one and the second one of the three wind turbine blades is performed so that there is at least substantially no pitch angle difference between pitch angles of the three wind turbine blades of the rotor, and whereby, after performing the service work, the respective pitch angles of the first one and the second one of the three wind turbine blades are adjusted by rotation to one of their at least one operational pitch angles, wherein, during the service work, the rotor is maintained in a rotational position in which a longitudinal axis of the third one of the three wind turbine blade forms an angle of not more than 5 degrees with a horizontal axis.