Airlines are failing to take up the most efficient planes in sufficient numbers to make a significant dent in their carbon dioxide emissions, a new study has found. The most efficient new aircraft models, such as the Boeing 787-9 and Airbus A350-900 and A320neo, can achieve substantial carbon savings over older models, but no airlines have invested sufficiently in the new types to reach the top levels of energy efficiency, according to a ranking by Atmosfair, a German NGO. In the annual Atmosfair Airline Index for 2018, no airlines received an A for efficiency, and only two airlines were ranked in efficiency class B.