The efficiency of the aerial drop of firefighting agents (water and retardants) is extremely dependent on pilot skills in dealing with complex atmospheric conditions, mostly because on-board systems for computer-assisted drops have not yet been used operationally. Hence, numerical modelling tools can be of primary importance for the optimization of firefighting operations and in the testing of new chemical products. The current work addresses the development of the operational Aerial Drop Model. This numerical tool allows a near real-time simulation of aerial drops with fixed-wing aircraft, while covering the fundamental stages of the process. It copes with a wide range of product viscosities, from water to highly thickened long-term retardants. The Aerial Drop Model simulates the continuous stripping of droplets from the liquid jet by the action of Rayleigh-Taylor and Kelvin-Helmholtz instabilities applying the linear stability theory. The subsequent secondary breakup and deformation of the formed droplets due to aerodynamic forces is based on experimental correlations defined in terms of the dimensionless Weber number. Droplet trajectories are computed by applying a Lagrangian approach, in which a dynamical drag module accounts for the effect of deformation. This operational tool provides an improved understanding of the behavior and effectiveness of aerially delivered firefighting liquids.