The altitude test facility at NGTE has been progressively developed over many years and now includes a test cell capable of testing large turbofan engines over a wide range of simulated flight conditions. The cell enables measurements to be made during rapid engine transients as well as at steady state conditions. Icing, freezing fog and tropical rain conditions can also be simulated. The cell is described and its basic test capability outlined. The methods used to measure the main engine performance parameters and the techniques for calibrating the measurement systems and defining their degrees of precision are described.