An apparatus for measuring erosion of an edge of an aerofoil portion of a fan blade includes a laser source to direct light at a position on the edge of the fan blade. A detector detects light reflected from the position on the edge of the fan blade. A computer measures the distance from the laser source to points on the edge of the fan blade to produce a profile of the edge in terms of x and z coordinates. The computer calculates the distances of points on the edge of the fan blade from a centroid of the profile. The computer detects peaks in the distance of points from the centroid of the profile. The computer selects two peaks with the greatest distance therebetween. The computer determines the distance between the two peaks and compares the determined distance and a predetermined distance to determine if there is unacceptable erosion.