Percolation and cosmic ray physics above $10^{17}$eV

We argue that the energy dependence of the average position of the shower maximum, $\bar X_{max}$, and of the number of produced muons, $N_{\mu}$, can be explained by a change, around $10^{17}$eV, in the energy dependence of the inelasticity $K$, which decreases with the energy above the string percolation threshold.