Environmental concerns have resulted in consideration of the effects of condensation trails from gas turbine engines. By provision of a condensation stage in a heat exchanger arrangement for a gas turbine engine the level of condensation within a final exhaust gas flow G from an engine is reduced. Furthermore, by cooling of the exhaust gas flows for mixing with by-pass air flows a water partial pressure at an exhaust gas exit temperature can be provided which is below the eutectic liquid to vapor phase transition and therefore avoid condensation (contrail) formation.