Back in the days before airfriegt was as common as it is today, an airline had to be able to move a large engine (JT9, CF6, RB211) very quickly if they had a problem in an area where there was no engine available.
The manufacturers decided to add an option to their airframes which would enable an engine to be carried as exterior payload. This engine was non functioning and could be carried on revenue flight. So, your recovery aircraft could also carry your engine. We had one ready to go at PanAm over in one of the hangars at JFK
. It only took an experienced crew between 2 and 3 hours to install. I believe the option was available on B747, L1011 & DC10.
Now, with all this heavy lift (B747, MD11, DC10) cpability in frieghters readily available, the practice has fallen out of use. Though I wonder if a Trent or GE90 could be carried on the deck of a B747 or such.