An apparatus comprising: an aircraft gas turbine engine controller comprising: a baseline controller structured to generate a command to affect an operation of an engine; and a delta controller operable to generate an offset command to be summed with the command generated by the baseline controller, the delta controller comprising: a module structured to determine a model of engine performance; and an optimization routine, wherein the module and optimization routine together determine the offset command, characterized in that the aircraft gas turbine engine controller further comprises: an observer in which the states of its output relationship are resolved by assuming the dynamic states in the observer to be in steady state, wherein the optimization routine comprises an objective function derived from the output relationship; a piece-wise linear state variable model defined at a flight condition and arranged as a function of a gas turbine engine pressure, the piece-wise linear state variable model determined by: - correcting a first condition pressure to a reference pressure; - evaluating the piece-wise linear state variable model based upon the reference pressure to produce a reference linear relationship; and - adjusting the reference linear relationship to a first condition linear relationship to create the model of engine performance; wherein the observer is structured to receive residuals representing a difference between actual outputs of the engine and outputs of the piece-wise linear state variable model and to send performance tuners to the piece-wise linear state variable model. an aircraft gas turbine engine controller comprising: a baseline controller structured to generate a command to affect an operation of an engine; and a delta controller operable to generate an offset command to be summed with the command generated by the baseline controller, the delta controller comprising: a module structured to determine a model of engine performance; and an optimization routine, wherein the module and optimization routine together determine the offset command, a baseline controller structured to generate a command to affect an operation of an engine; and a delta controller operable to generate an offset command to be summed with the command generated by the baseline controller, the delta controller comprising: a module structured to determine a model of engine performance; and an optimization routine, wherein the module and optimization routine together determine the offset command, a module structured to determine a model of engine performance; and an optimization routine, wherein the module and optimization routine together determine the offset command, characterized in that the aircraft gas turbine engine controller further comprises: an observer in which the states of its output relationship are resolved by assuming the dynamic states in the observer to be in steady state, wherein the optimization routine comprises an objective function derived from the output relationship; a piece-wise linear state variable model defined at a flight condition and arranged as a function of a gas turbine engine pressure, the piece-wise linear state variable model determined by: - correcting a first condition pressure to a reference pressure; - evaluating the piece-wise linear state variable model based upon the reference pressure to produce a reference linear relationship; and - adjusting the reference linear relationship to a first condition linear relationship to create the model of engine performance; wherein the observer is structured to receive residuals representing a difference between actual outputs of the engine and outputs of the piece-wise linear state variable model and to send performance tuners to the piece-wise linear state variable model. an observer in which the states of its output relationship are resolved by assuming the dynamic states in the observer to be in steady state, wherein the optimization routine comprises an objective function derived from the output relationship; a piece-wise linear state variable model defined at a flight condition and arranged as a function of a gas turbine engine pressure, the piece-wise linear state variable model determined by: - correcting a first condition pressure to a reference pressure; - evaluating the piece-wise linear state variable model based upon the reference pressure to produce a reference linear relationship; and - adjusting the reference linear relationship to a first condition linear relationship to create the model of engine performance; - correcting a first condition pressure to a reference pressure; - evaluating the piece-wise linear state variable model based upon the reference pressure to produce a reference linear relationship; and - adjusting the reference linear relationship to a first condition linear relationship to create the model of engine performance; wherein the observer is structured to receive residuals representing a difference between actual outputs of the engine and outputs of the piece-wise linear state variable model and to send performance tuners to the piece-wise linear state variable model.