As a sidenote, the L-1011 is still one of my favorite planes. It was one of the most technologically advanced airliners ever created when it was unveiled; lots of things about it were way ahead of its time. It's a shame that the competition with the Douglas and the DC-10 nearly bankrupted both Lockheed and Douglas. Airlines played both companies against each other and nearly killed both. Lockheed ended up exiting the commercial business while Douglas merged with McDonnell.
There's a great long article on the the L-1011 published a few months ago: http://www.airlinereporter.com/2015/09/requiem-trijet-master...
> Qantas has used the Boeing 747’s ability to ferry an extra engine in the past, most recently in 2011. Qantas used the method often with their Boeing 707s when engines were less reliable, but the procedure has become quite rare.
What does this mean?
In the case of a twin engine aircraft, if one engine fails the other has to be able to compensate so the flight can continue safely, especially at takeoff when the plane is at its heaviest. Twin engine aircraft can easily fly with one engine out but the additional drag imposed by the ferried engine can reduce the ability for the flight to continue under the power of one engine.
There are other factors too, like increase adverse yaw which would become more pronounced if the ferried engine were on the same side as the failed engine.
The 707, 747, DC-8, DC-10, TriStar and VC-10 all had an underwing hardpoint for the extra engine and carrying it was fairly routine; just apply the compensations laid-out in the flight manual.
Four-engined TriStars looked funny...
It is rare nowadays due to the ubiquity of twin-engined airliners which have neither the underwing space nor the take-off engine-failure margin to ferry a spare. Instead their spare engines are stripped and sent in freighters, usually by the engine manufacturer who provide power-by-the-hour contracts to the airlines.