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It was not uncommon in the 60s and 70s to have B747 ferrying a spare engine.

enter image description here
(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?

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    $\begingroup$ related: aviation.stackexchange.com/questions/8339/… $\endgroup$ – ratchet freak Feb 8 '15 at 14:30
  • $\begingroup$ @ratchetfreak: The image selected seems quite popular :-) Great addition, I didn't know one engine could be removed and replaced by an unused spare one. $\endgroup$ – mins Feb 8 '15 at 14:42
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    $\begingroup$ Hmm a brief search reveal nothing solid but most recently seen one is with Qantas and it is optional from production time. Most comments state speed limit (250knots) and range leads to decline in practice. Note the 400ER is not certified for 5th pod. Anyone got maintainence manual to confirm all this? $\endgroup$ – vasin1987 Feb 8 '15 at 17:23
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    $\begingroup$ @mins - although it's tricky to do in-flight $\endgroup$ – NobodySpecial Feb 9 '15 at 2:46
  • $\begingroup$ @NobodySpecial particularly if the engine to be replaced is on the other wing. $\endgroup$ – mgois May 28 '17 at 22:31
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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.

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