OVER THE GREAT PLAINS — Glance down from the ageless expanse of blue sky into the cockpit of the Air Force’s largest bomber, and the panorama is decidedly more dated — banks of steam gauges quiver above aluminum levers built during the Eisenhower administration, obsolete knobs and dials unused in decades gather dust. And much of the rest of the mammoth B-52 bomber is just as antiquated. Vacuum tub