Pitch misalignment

Each blade on a modern turbine rotates about its own axis to control how much energy the rotor captures. Pitch is the primary control above rated power and a fine-tuning input below it. When the zero reference of one blade drifts from the others, the rotor becomes aerodynamically asymmetric even though it looks perfectly normal from the ground.

There are two distinct failure modes and they present differently. A collective offset, where all three blades sit away from the optimal angle, shows up as a uniform shortfall in the partial-load region of the power curve and looks very much like yaw misalignment in the energy record. An individual blade offset produces a once-per-revolution load imbalance that appears in vibration data long before it appears in production.

Pitch errors of one to two degrees are common after a blade replacement, a bearing service, or a controller software change, and they are easy to introduce because the reference is set during installation rather than measured continuously. The cost is comparable to a yaw offset of similar magnitude, and the correction is again a settings change rather than a component.

Detecting the collective case needs a production baseline; detecting the individual case needs the vibration and load channels that also drive condition monitoring. Because both are gradual and neither trips a threshold, they are typical of what a normal behavior model is built to find.