An engine monitor (22;76;114) for a first engine configured to receive first control data and a second engine configured to receive second control data , the engine monitor (22;76;114) comprising: a first tuned modeling unit configured to model dynamics of the first engine by processing the first control data with a first engine model to provide modeled first engine parameter data , and configured to adjust at least partially the modeled first engine parameter data for model error in the first engine model to provide first tuned parameter data ; a second tuned modeling unit configured to model dynamics of the second engine by processing the second control data with a second engine model to provide modeled second engine parameter data , and configured to adjust at least partially the modeled second engine parameter data for model error in the second engine model to provide second tuned parameter data ; a monitoring unit configured to correlate the first tuned parameter data and the second tuned parameter data to monitor operation of the first and the second engines ; wherein the monitoring unit is configured to correlate the first tuned parameter data and the second tuned parameter data by processing the first tuned parameter data with measured first engine parameter data to provide first residual data , processing the second tuned parameter data with measured second engine parameter data to provide second residual data , and processing the first and the second residual data to provide engine correlation data ; and characterized in that the monitoring unit is configured to compare the engine correlation data to threshold data, and is configured to determine whether a fault has occurred during operation of the first and the second engines where one or more data points of the engine correlation data are at least one of greater than and less than one or more corresponding data points of the threshold data. a first tuned modeling unit configured to model dynamics of the first engine by processing the first control data with a first engine model to provide modeled first engine parameter data , and configured to adjust at least partially the modeled first engine parameter data for model error in the first engine model to provide first tuned parameter data ; a second tuned modeling unit configured to model dynamics of the second engine by processing the second control data with a second engine model to provide modeled second engine parameter data , and configured to adjust at least partially the modeled second engine parameter data for model error in the second engine model to provide second tuned parameter data ; a monitoring unit configured to correlate the first tuned parameter data and the second tuned parameter data to monitor operation of the first and the second engines ; wherein the monitoring unit is configured to correlate the first tuned parameter data and the second tuned parameter data by processing the first tuned parameter data with measured first engine parameter data to provide first residual data , processing the second tuned parameter data with measured second engine parameter data to provide second residual data , and processing the first and the second residual data to provide engine correlation data ; and characterized in that the monitoring unit is configured to compare the engine correlation data to threshold data, and is configured to determine whether a fault has occurred during operation of the first and the second engines where one or more data points of the engine correlation data are at least one of greater than and less than one or more corresponding data points of the threshold data. <EOS>