Abstract A set of equations is derived for converting acoustic measurements taken in a free-jet flight simulation facility, such as the U.K. Noise Test Facility at Pyestock or the French CEPRA-19 wind tunnel at Saclay, to equivalent farfield flight conditions. The equations are based on the high-frequency geometrical acoustics approximation, whose application in the present context was justified in early studies by Morfey and Tester in 1977 and by Amiet in 1978. However, the present work differs by allowing the source to be positioned off the jet centreline, anywhere within the flight stream. The flight stream jet is modelled as an axisymmetric parallel shear flow, with a shear layer thickness which is small compared with the jet diameter. The model also permits the microphone to be located anywhere outside the flow, arbitrarily close to the open jet. The consequences of off-axis source location are illustrated by numerical calculations.