Abstract: An engineering approach to the determination of the inertia properties of the fuel in external tanks is presented. Systematic experiments were conducted for a range of slenderness and scale of the tank,depth of the fuel and vibration frequencies. Simple equivalent inertia coefficients of the fuel in different motions were obtained. Applying these coefficients to the calculation of the structural dynamic characteristics of the wing\|external tank of an aircraft, good correlation with the results of ground resonance tests was obtained.