The performance of an aircraft fuel system ball valve has been assessed using various modelling techniques and experimental methods. The performance largely splits into two parts. The first part deals with the steady state characteristics and internal flow field for fixed valve positions. The second part looks at the dynamic performance of pressure surge and flow overshoot by closing the valve using a linear profile. For the steady state flow, a 3D CFO model was used to characterize the flow coefficient to within 10% of the measured data. The variation of the flow coefficient with angle is largely dominated by viscous effects, pressure recovery and jet contraction in that order with increasing angle. An accurate and efficient 20 model of the flow coefficient used 1/12th of the computing time of the equivalent 3D model. The 20 model was then used to look at the effect of orifice radius and it was shown that the Coanda effect has little influence on the trend in the flow coefficient. Following this, a steady state CFO model was used to predict the maximum velocity from LOV data to within 12.5%. With the LOV measurements, the presence of shear layers in the flow field creates jet contraction and is a possible source of cavitation. For the dynamic flows, a method for determining the valve fluid inertance was presented, which followed an analogy between a porous medium and the valve fluid inertance. However, the effect of this inertance on the measured pressure surge was small due to the small percentage of the total system fluid inertance that the valve fluid inertance represents. For flow rate measurement, the two transducer technique isproposed and this method is an accurate post processing technique and can predict overshoot to within less than 2% of the modelled valve. A curve fitting method was also used to characterize the surge pressure and volume overshoot across a range of test conditions, the characterized surge pressure being within 6% of the measured values.