[dingaling]

Twins are actually overpowered compared to comparably-sized quads due to engine-out climb requirements. In terms of raw seat-km-litre they are therefore slightly less efficient.

A good example is the old A340-300 quad, often maligned for being underpowered. Now that it is mostly retired data has emerged that it was more efficient than the rival 777-200 twin.

Where twins are cheaper is in terms of capital costs and consumables, though ETOPs requirements mean you need more technicians to service a twin ( separate teams for each engine ).