The stability of MacCormack's method for the scalar advection equation

We show that MacCormack's method for the scalar advection equation ut = aux+buy is stable if (aΔt/Δx)2 + (bΔt/Δy)2 ≤ 2 1+18 −2≈18. This bound on the mesh ratios is not optimal, since numerical sampling shows that .3266 will do when (aΔt/Δx)2 = (bΔt/Δy)2.