Among commercial aeroplanes, this facility of carrying a spare engine, known as the fifth pod in aviation circles, is the most effective and fastest way of transporting a huge aircraft engine by air, though due to the extra weight of the engine the speed of the airliner gets reduced to some extent.

While the Boeing 747 is a four-engine aircraft, it was uniquely designed with the capability to carry a fifth, non-functioning engine as cargo. This "fifth pod" is located under the port (left) wing, between the fuselage and the inner functioning engine. This feature was created to help airlines transport massive spare engines to "grounded" aircraft in remote locations without needing to charter a separate heavy-lift cargo plane. When a 747 is "ferrying" a fifth engine, the spare is strictly for transport; it is not connected to the fuel or control systems and does not provide thrust. To maintain aerodynamic balance, the plane usually flies at a slightly lower speed and altitude, and once the destination is reached, the spare engine is removed and installed on the broken aircraft. While modern engine reliability has made this less common, it remains a legendary piece of aviation engineering that allowed the "Queen of the Skies" to act as its own support vehicle.