A method for laterally avoiding a fixed zone in an aircraft. The contour of the zone to be avoided is modeled by a convex polygon shape. The planned route is located with respect to the modeled zone. The port and starboard sections of the route for exit and return of the planned route are computed. A predetermined angle is formed which meets with a corner point of the contour to obtain two avoidance routes formed by the exit and return sections of the route and by the portions of the contour. One of the two avoidance routes is then selected.