A method and apparatus for determining the operating condition of a turbine engine. An eddy current sensor is provided having a sensing field in communication with a plurality of rotating turbine blades during a revolution of a turbine engine. A threshold point value is defined for the signature data. A plurality of positive threshold point pairs and negative threshold point pairs are identified. A plurality of positive threshold widths from a plurality of times elapsed between the positive threshold point pairs and a plurality of negative threshold widths from a plurality of times elapsed between the negative threshold point pairs are determined. A plurality of threshold ratios are determined from a plurality of pairs of positive and negative threshold widths. A variance of the threshold ratios is correlated with an operating condition of the turbine engine.