What are the requirements of ferrying a spare engine? It was not uncommon in the 60s and 70s to have B747 ferrying a spare engine.


(source: Australian Aviation)

Was that allowed only on aircraft specifically designed with a pod, or was that possible to add the equipment on any B747. What is the regulation applicable? (in the US for instance).

Additional question: is this still commonly used today in civil aviation?
It's a manufacturer option on some larger planes - basically adding a bit of structure and some attachment points. And it's expensive enough that airlines think about it.

Today's engines are much, much more reliable than the ones used in the 1960s so the need to move engines around by bolting them on to the next thing going that way is not what it once was. There is also a larger air cargo market, and it's likely cheaper in general to put the replacement on a pallet rather than under a wing.