We have studied site percolation on ballistic aggregates on a two-dimensional square lattice with a single seed particle. A percolating path is defined as the path connecting the seed to a surface particle. The fraction of the connected surface particles is found to obey a scaling relation. The percolation threshold is obtained at p≅0.97. This gives us an upper threshold for the impurity tolerance to preserve the connectivity of the structure in thin films with columnar growth where ballistic aggregation is known to be relevant.