Why wouldn't one simply transport a "spare" in the cargo hold?

Marc


"jfp" wrote in message
...
Hi all last week at antigua airport i am sure i saw a 777 with a 3 rd
engine on the wing on a BWIA i know they used to carry a 5 th on a 747,
anyone know if this is standard on a 777 to transport spare engine mebe
regards Frank