1. Legacy airlines, on the other hand, are more likely to run old, carbon-spewing jumbos. Nearly half of BA's transatlantic capacity, for example, is accounted for by 747s. 2. On average, a nonstop transatlantic flight and back spews out about a tonne of CO2 emissions per passenger but that figure hides a big spread in the fuel efficiency measures of different airlines. 3. One reason for its impressive performance is that it predominately runs a fleet of modern, efficient Boeing 787-8s. The planes it uses are, on average, also only two years old and its high pax-km/l is also down to the fact that it squeezes in more passengers. 4. A new study of the 20 biggest transatlantic carriers by the International Council of Clean Transportation (ICCT) found that there was a 51% difference between the fuel efficiency of the best-performing airline, Norwegian Air Shuttle, and the worst, British Airways (BA). Norwegian, on average, flies at 40 passenger-kilometres per litre of jet fuel (pax-km/l).___