I've seen the "piggybacked" shuttle twice. Once I was at the El Paso airport, and it was sitting atop the 747 at the adjacent Air Force base. On another occasion it was deliberately flown over the Washington Beltway - actually that might have been when the Enterprise was being delivered to Dulles Airport for inclusion in the Air & Space Museum annex out there.

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Everything I need to know I learned through Googling.