Immense airplanes continue to be proposed and occasionally even built. It is difficult to select a single measure of an airplane’s size, but wingspan is a strong candidate, and by that measure we seem to be near some sort of limit. Nothing has yet come close to a 500-foot wingspan, let alone a 1,250-foot one. The sizes of airports are fundamental constraints with which even the comparatively small — 262-foot span — A380 had to struggle. The record for wingspan is still held by Howard Hughes’ sexagenarian Hercules, better known as the Spruce Goose, at 320 feet. Wingspan, however, is a poor indicator of weight for airplanes of different eras. The Spruce Goose, powered by eight Pratt & Whitney four-row, 28-cylinder radials of 3,000 hp each, had a design takeoff weight of a mere 400,000 pounds; that of a C-5 Galaxy is 840,000 pounds, a 747-8 975,000 pounds, an A380 1.2 million pounds and the An-225 1.3 million pounds.