Degenerate fibers of three-dimensional manifolds fibered into rational surfaces

It is proved that the components of degenerate fibers of three-dimensional algebraic manifolds fibered into rational surfaces are rational or irrational ruled suriaces. An example is constructed of a three-dimensional algebraic manifold, fibered into rational surfaces, whose degenerate fiber contains an irrational ruled surface which cannot be eliminated by birational transformations that do not alter the common fiber.