Disclosed is test apparatus (100) for testing at least part of an aircraft system. The test apparatus (100) is configured to obtain first data (210) comprising a plurality of input values and process the input values using a classifier (230) configured to model an operation of the at least part of an aircraft system (220). Second data is obtained, comprising a set of output values (240b). The output values (240b) from the second data are processed with output values (240a) generated by the classifier to identify differences between operation of the at least part of an aircraft system (220) and the operation as modelled by the classifier (230).