Transporting a 747 engine

Most don't realize it, but 747 engines were so large when it first hit the market that only a 747 could most easily transport them where needed by airlines. There wasn't a "freighter" version of the aircraft at the time, the Interstate Highway System was not yet complete, and military transports were out of the question to assist civilian airlines, so wise Engineers at Boeing installed a "5th" pylon on the wing, between the Pilot and the #1 engine, which carried a "spare" engine for transportation to and from remote locations. This "airline option" was only available on 747-100/200/300 models. The company I worked for made the Flight Management System (FMS), which was how I learned of this capability. The aerodynamics and fuel burn were so impacted while carrying a 5th engine that our software required the crew respond to a prompt whether a 5th engine was installed in order for proper trajectory, remaining fuel, etc. be calculated for cockpit display.

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Transporting a 747 engine - GIF