“Fly by Wire” isn’t muckraking, exactly. Mr. Langewiesche doesn’t dispute the events of Jan. 15, 2009, when US Airways Flight 1549 successfully ditched on the Hudson River. Nor does he dispute that the flight’s pilot, Chesley B. Sullenberger III, a k a Sully, is,...a “superb pilot.” But Mr. Langewiesche does bang a few light dents into Sully’s hero aura. What the public doesn’t understand, he writes in “Fly by Wire,” is the extent to which advances in aviation and digital technology have made pilots almost superfluous, perhaps even “the weak link in flight.” Mr. Sullenberger’s airplane, an Airbus A320, was...