This paper describes an engineering method developed for calculating the water spray and the precipitation drag for aircraft on water-contaminated runways. The spray is considered to consist of a large number of individual particle trajectories. A Monte Carlo approach is followed to obtain realistic spray characteristics. Precipitation drag is calculated from the impingement of this spray on the aircraft. Empirical constants were being determined using data from available experiments and flight test results. The calculated precipitation drag compares well with the available flight test data. Results are described among which the contribution of bow spray, the effects of lift variation, geometry variation, tyre pressure and pool height variation. Paper presented at The 48th CASI Annual Conference, Toronto, April 29 - May 2, 2001.