Economist: THAT jet engines have evolved over the past few decades will be apparent to any seasoned air traveller. Early jet engines had narrow inlets and were very noisy, but as the diameter of the fans at the front increased, the engines became quieter. Compared with a rowdy 1960s jet, a modern turbofan is some 80% quieter and burns as little as half as much fuel–thus producing fewer greenhouse gases. But with regulation of aviation emissions likely in the coming years in order to combat climate …