CNN  —  They’re massive pieces of engineering that seem to defy physics as they hang in the air during takeoff and landing, but double-decker passenger airplanes — the Boeing 747 and Airbus A380 — are still some of the most popular aircraft around. Perhaps it’s their whisper-quiet cabins and spacious designs. Or the availability of opulent amenities, like shower suites and onboard bars. For aviation enthusiasts, there’s the added novelty of flying on a plane’s upper deck or nose. However, it’s