*2.2. Probability Density Functions*

The random input variables are introduced in the model using PDFs. Although this is not specifically required by the adopted aPC method, which is on the other hand able to operate on any kind of available data, in the present study PDFs were assumed based on an expert's opinion due to the lack of publicly available information regarding the studied parameters. In fact, the PDFs are based on the assumptions of Bortolotti et al. [20], who also attempt to deal with input uncertainties in aero-servo-elastic wind turbine models. Two beta functions are used for both LE Erosion Factor ε and TE Damage Factor τ. They are appropriately scaled to match the support these variables are defined upon. The values of the PDFs are reassumed in Table 1. The adopted PDFs are assumed as representative of cases where medium-low blade damage is present or of sites with challenging environmental conditions where regular maintenance is performed.


**Table 1.** Probability density functions for erosion factor andTrailing Edge (TE) damage factor.
