Fail over

There is a joke (by De Bono) about the airline pilot who apologized to the passengers for having to shut down one engine. He explained that it meant they would arrive two hours late into New York. A second engine failed and he explained that they would be four hours late. Then a third engine failed. At this point the co-pilot leaned across to him and said:” I hope to goodness the last engine doesn't fail or we shall be up here all night!"