Turbofan engines are normally bench-tested with a standard flared bellmouth intake. This is different from the aircraft situation. As a result, an engine installation may experience a degree of inlet flow distortion not normally present during tests. It is, therefore, very desirable to understand the effect of any radial inlet total pressure loss on turbofan engine performance. Steady-state radial inlet distortion may occur, for example, as a result of boundary layers. An early awareness on distortion tolerance is very important to enable the prediction of surge margin. However, synthesis of turbofan performance with distortion is currently not available. This work therefore, investigates in detail the modelling of the fan component of low bypass-ratio turbofan engines within an engine performance simulation program. For example, the air flow in turbofan engines is split after the fan between the core gas generator and the bypass flow. A fan model must be able to simulate the required flow and thermodynamic parameters to the core and bypass flows at fan exit. Conventional fan models, however, are restricted to a fixed bypass ratio versus non-dimensional speed schedule at which the fan has been rig-tested. The fan component also experiences a varying degree of inlet total pressure distortion. Existing engine simulation fan models are unable to quantify this effect on fan performance and on engine performance. The turbofan modelling work conducted here is preceded by an analysis of rig data of Low Bypass Ratio (LBPR) turbo-fan engines to give a firm background basis. The engine modelling uses the component-based iterative solution method for gas turbine performance calculations. Two key outcomes of the work are the following. Firstly, LBPR fans have large circumferential fan exit flow variations as well as radial variations. This includes total temperature profiles which are an order of magnitude higher than those for High Bypass Ratio Fans (HBPR) fans. Secondly, it is inconclusive, at a given non-dimensional speed and flow function, as to whether fan exit profiles are independent of BPR. The fan radial profile modelling starts from an existing modification of a conventional compressor characteristic but also models in 2-D with dependency on the fan exit radial position. The inlet distortion fan model uses a throughflow streamline curvature for radial performance prediction coupled to the 2-D-LBPR fan model. Against this background, a new fan characteristic model has been devised for LBPR fans. In addition, a new inlet distortion performance model has been developed which is able to predict engine performance changes with radial inlet total pressure distortion.