Dr. John Green, who chairs the science and technology sub-group of industry environment group Greener by Design, would like to outlaw the sale of aircraft that are capable of ranges greater than 3,000 nm.

His reason is a compelling one. As he told last week’s NACRE conference in London, flying 8,000 nm in three hops instead of one uses just half the fuel. “So why aren’t people doing it,” he wondered.

Hardly any aircraft actually fly 8,000 nm, of course, and half the total fuel consumed by commercial aircraft is used on routes of 1,000 nm or less. But much of the other half is burned by 747s and other long-range aircraft carrying around the bigger, heavier wings and landing gear they need only for the extreme ranges.

More efficient air traffic management would reduce the fuel burn of the entire existing fleet as well as new designs, so it would have the greatest immediate impact. But in terms of new aircraft, he said, “reducing the design range is by far the most powerful way of getting fuel burn down.”

Contrails and contrail-induced cirrus cloud formation, meanwhile, are emerging as a major concern. The means of avoiding them–reducing air traffic in the regions of cold, super-saturated air where they occur–would increase fuel burn and associated climate-harming emissions, exacerbate airline disruption and complicate air traffic control. “But that may be the price we have to pay,” Green said.

Another conundrum highlighted by Green is the impact of noise reduction on other environmental targets. Increasing engine bypass ratios reduces noise but means bigger fans, bigger nacelles, more weight and higher drag, he said, citing the example of the small fuel burn penalty the A380 pays to comply with Heathrow’s noise limits.
The good news when looking ahead to 2050, Green believes, is that the world fleet fuel burn and consequent CO2 emissions per passenger kilometer could be reduced by a factor of three, oxides of nitrogen at altitude by a factor of 10 and contrail cirrus by as much as 15.