The principles behind gas turbine design and off-design operation are important for engineers to understand the performance of the engine. The appreciation of gas turbine design will also allow engineers to be more perceptive towards the constraints on design characteristics, the testing and evaluation of engines prior to their installation the potential hazard of external factors. While knowing about the design and performance issues of engines is a necessity, understanding diagnostics of gas turbine performance is critical. Failure of engines may cause aircraft downtime or, on more serious repercussions, loss of passenger lives.