It has been shown that since the performance of a turbo-jet engine can be expressed theoretically as a function of the two variables and , the performance of a turbo-jet aircraft can also be expressed as a function of two variables, and Two methods can then be used for the reduction of the performance to standard conditions. In the “non-dimensional” graphical method, flight test measurements of, for instance, are plotted against and , and the standard value of is read from these curves at the standard values of and . In this way, the relation between the parameters is established during the tests. The alternative method is an analytical one in which corrections are applied to, for example, the level speed for differences from standard in the independent variables. For this method the effect on the performance of changes in these variables must be predetermined.