Fair dues to QANTAS for milking this on social media but spare-engine ferrying was common from the dawn of the jet age.<p>The 707, 747, DC-8, DC-10, TriStar and VC-10 all had an underwing hardpoint for the extra engine and carrying it was fairly routine; just apply the compensations laid-out in the flight manual.<p>Four-engined TriStars looked funny...<p>It is rare nowadays due to the ubiquity of twin-engined airliners which have neither the underwing space nor the take-off engine-failure margin to ferry a spare.  Instead their spare engines are stripped and sent in freighters, usually by the engine manufacturer who provide power-by-the-hour contracts to the airlines.