Abstract: The method allows to control a test apparatus for a gas turbine engine; WI values of one or more tentative fuel gas mixtures are predicted by calculations and the predicted WI values are used for setting the composition of a fuel gas mixture to be supplied to a combustor of a gas turbine engine under test. The test apparatus comprises: a first supply flow line for fuel gas; a second supply flow line for inert gas; a mixer with a first inlet for fuel gas and a second inlet for inert gas, and with an outlet for supplying the mixture of fuel gas and inert gas to the combustor; a set of meters; and a flow control device for the inert gas.