How Aviation Can Come Clean

Advanced technology won't be enough for the industry to meet its own greenhouse-gas targets.

Last week the global aviation industry called on the United Nations to establish a single, worldwide policy for reducing aviation greenhouse-gas emissions, in an attempt to avoid a costly network of regional regulations. The industry proposed two primary goals--that by 2020 it should stop increasing its greenhouse emissions, and that by 2050 it should cut its emissions by 50 percent compared to 2005 levels.

Flying wing: The Boeing X-48B, an unmanned prototype with a 6.4-meter wingspan, has a blended-wing design that could one day replace that of today's commercial planes. Credit: NASA

These goals, while less stringent than the 80 percent reductions proposed for the rest of the world's economy, may nevertheless prove too ambitious, some experts say. Furthermore, an array of potential technologies that could significantly reduce emissions will be difficult to deploy quickly in an industry that is reluctant to take on the cost and risk of radical innovation and that can take decades to replace old airplanes.

Whichever new technologies do get implemented may not be enough to keep up with the industry's growth. Each year the industry reduces fuel consumption by improving efficiency by 1.5 percent to 2 percent. But each year people fly more--the industry is expected to grow by 4 percent to 5 percent--overwhelming fuel savings from efficiency.

Part of the problem is that it takes the industry as long as 20 to 30 years to replace planes. This means that the efficiency improvements of planes introduced in 2010 won't be seen throughout the fleet until 2025 or later. If things continue as they have been in recent years, by 2050 the industry will have to fly "three times as many airplanes with only half as many emissions," says Ian Waitz, a professor of aeronautics and astronautics at MIT and director of the Partnership for Air Transportation Noise and Emissions Reduction. "It's a tremendous challenge," he says. The challenge is so great that climate-change policies may force a tradeoff--requirements to cut emissions may increase prices and slow the growth of the industry.

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The aviation industry can limit its emissions in three basic ways--making airplanes more efficient, improving logistics to waste less fuel, and replacing fossil fuels with biofuels. But some potential technical improvements are limited because of the engineering requirements of airplanes. For example, it's conceivable that batteries and electric motors could one day replace internal combustion engines in cars. But batteries don't store enough energy to transport a commercial airliner across the Pacific.

With these limitations in mind, by 2020, new technologies could make aircraft about 20 percent to 35 percent more efficient, on average, than planes today. Fuselage coatings and adjustable wings, among other things, could reduce drag. Engines that run hotter and at higher pressures would use less fuel, as would engines that use gears to optimize the speeds of different parts of a turbine, and open-rotor designs that resemble and have some of the efficiency advantages of turboprops.