In the early seventes this was done frequently on trans-atlantic/contemetal flights. It was kept as a precaution incase of engine failure. QANTAS used to do this on there 707 and early 747s too. Pan Am has done this before. A fifth engine on regular flights is mainly kept as a precautionary engine. Heres a pic of a Honeywell B707 with a fifth engine during tests.
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i agree with Keltic and Mongu, but i took in consideration the fuselage leangth+width of the A310. Also the A310 cannot be compared with B767, B767 is bigger and better.
Of course its the greatest aircraft(still is) ever built in the history of civil aviation. Last year it was also rated as the most beautiful a/c ever built.