Greening the friendly skies

by Carolyn Gramling
Thursday, January 5, 2012

If you’re a frequent flyer, the script of plane travel is probably so familiar you may mumble it along with the flight attendant: “Please raise your tray tables and return your seatbacks to their full upright position. We’re beginning our descent.” The sounds of that descent are probably just as familiar: The whir of landing gear descending, the loud drone of engine power rising and falling as the plane makes a series of stair-step descents to lower and lower altitudes before landing on the runway.

But in the past couple of years, some flyers may have experienced a script rewrite. Since 2008, certain red-eye flights arriving at airports in Los Angeles and Atlanta have avoided the stair-step descents and instead tested “continuous descent arrivals,” in which the engines stay idle as the plane descends in a steady slope. Continuous descents are just one procedure that the Federal Aviation Administration (FAA) and the airline industry are investigating in a large-scale effort to conserve fuel and reduce aircraft emissions.

Growing aviation

Since 2000, there has been a dramatic growth in aviation, according to the 2007 Intergovernmental Panel on Climate Change’s (IPCC) Fourth Assessment Report: Despite the Sept. 11, 2001, terrorist attacks on the World Trade Center in New York City, passenger traffic increased by 38 percent between 2000 and 2007. Although the global economic downturn in the last couple of years has slowed the rate of growth — in 2008, FAA predicted that the U.S. commercial aviation industry would carry a billion passengers annually by 2016, but the 2009 forecast suggested the industry wouldn’t hit that mark until 2021 — it has not changed the overall upward trend.

Right now, according to IPCC, aviation contributes between 2 and 3 percent of the carbon dioxide emitted by humans to the atmosphere — a relatively small amount. But as the airline industry continues to grow rapidly in the next few decades, emissions are also likely to increase. Fuel combustion in airplanes produces not only carbon dioxide, but also nitrogen oxides, aerosols, sulfur oxides and hydrocarbons. The hydrocarbons, the product of incomplete combustion, can in turn react with nitrogen oxides in the atmosphere to produce ozone, contributing to smog.

And contrails — condensation trails, the artificial clouds of condensed water vapor left behind by planes — can also alter climate by trapping heat emitted by Earth and, to a lesser extent, by reflecting incoming solar radiation: Planes grounded for three days following the Sept. 11 terrorist attacks offered scientists an opportunity to study the effect of contrails on climate. A NOAA study using measurements at 4,000 meteorological stations across the United States from those three days found that without contrails, there was an increase in the difference between daytime and nighttime temperatures of 1 to 2 degrees Celsius — the largest recorded difference in 30 years.

A 2009 study in the journal Atmospheric Environment by David Lee, an atmospheric scientist at the Centre for Air Transport and the Environment (CATE) at Manchester Metropolitan University in England, and colleagues looked at how aviation, as a percent of total human activities, had affected radiative forcing between 2000 and 2005. Radiative forcing — defined by IPCC as externally imposed perturbations in the natural atmospheric balance between incoming radiation from the sun and outgoing heat radiating from Earth back into space — can have a significant effect on climate. During their period of study, Lee and his colleagues found, aviation was responsible for just under 5 percent of the total radiative forcing caused by humans.

Those percentages are likely to increase significantly in the next few decades, says chemist Bethan Owen, also of CATE. Owen and other researchers published a 2010 paper in Environmental Science & Technology in which they constructed a series of projections of future airline emissions based on emissions scenarios described in a special 2000 IPCC report, as well as on an aircraft movement and emissions model often used to calculate fuel use and emissions. The model, called FAST, incorporates data on global air traffic and fuel use related to aircraft performance. Using these data, Owen and her colleagues calculated that carbon dioxide emissions will grow by a factor of 2 to 3.6 from 2000 to 2050 and that nitrogen oxides will increase by a factor of 1.2 to 2.7. Although technology will also improve over that time, which will improve fuel efficiency, the study found that these improvements won’t keep pace with the rate of industry growth, Owen adds.

International disputes hamper emissions reduction

As air traffic grows, airplane technologies, fuel efficiency and operational efficiency will also evolve; government and industry officials alike hope these changes will help to offset the increase in emissions due to increasing traffic. The European Union’s Advisory Council for Aeronautical Research in Europe (ACARE) — a public-private partnership designed to improve the EU’s competitiveness in aviation technology — is working to design new technologies based on what Owen calls “ambitious” targets for greenhouse gas emissions reductions. Such targets include cutting carbon dioxide emissions by 50 percent and nitrogen oxide emissions by 80 percent by 2020.

Owen and colleagues used ACARE’s technology targets for fuel efficiency and emissions technology to construct aircraft emissions scenarios post-2020. The study assumed the EU’s fleet would transition to the ACARE-compliant airplanes slowly at first and then more rapidly, until 75 percent of the fleet is ACARE-compliant by 2050. That’s an optimistic scenario compared to IPCC projections, Owen notes, which assume that efficiency improvements will taper off with time. Yet despite these technological advances, the researchers found, EU aviation will still become a significant contributor to climate change and global greenhouse gas emissions, unless international aviation emissions are included in reduction targets set for climate change mitigation.

Unfortunately, Owen says, the Kyoto Protocol and other international agreements on emissions reduction do not cover international aviation emissions — which currently amount to 60 percent of the total emissions from aviation, she says. The issue of how to regulate such emissions has long been a contentious one. In 1997, the Kyoto Protocol — an international treaty that went into effect in 2005 and set binding targets for reducing greenhouse gas emissions for 37 countries (all nations classified as industrialized, minus the United States) — assigned the United Nations' International Civil Aviation Organization (ICAO) the task of reducing aircraft engine emissions. ICAO developed emissions standards for international aviation in 2007, but these are not yet mandatory. The issue continues to be contentious: Representatives of the European Union attending the 36th Assembly of ICAO in 2007 sought to impose EU domestic aviation emissions standards on foreign airlines, but the Assembly has so far only agreed to make the standards voluntary — with much of the opposition coming from the United States, which does not yet have a carbon-trading scheme that could work with the EU scheme.

The issue arose once again at the 2009 U.N. Framework Convention on Climate Change (UNFCCC) in Copenhagen, Denmark, but the conference again did not set international aviation emissions targets, due to disagreements over who should set them — ICAO or the UNFCCC — and what those targets should be.

U.S. plans to CLEEN the air

In 2008, meanwhile, FAA announced its own plans to create a government-industry partnership, the Continuous Lower Energy, Emissions and Noise (CLEEN) program, to develop new aircraft technologies, particularly engine and frame design.

Last June, FAA awarded $125 million in CLEEN contracts to five companies — Boeing, GE Aviation, Honeywell, Pratt & Whitney and Rolls-Royce North America — to develop technologies that could be introduced into commercial aircraft by 2015. FAA also awarded $66 million to Alaska Airlines and three other aviation companies through the CLEEN program to develop new computer technologies to improve efficiency. Alaska Airlines began conducting test flights this year to determine the baseline for its commercial aircraft fuel usage; over the next five years, the airline will run more trial flights to test the new technologies.

Some of the primary goals of the CLEEN program, FAA Administrator Randy Babbitt announced in June at a conference sponsored by the airline industry media group Air Transport World, include reducing fuel burn by 33 percent, reducing nitrogen oxide emissions during takeoff and landing by 60 percent, and using more sustainable jet fuels.

The U.S. Department of Defense (DOD) has helped lead the way in domestic investigation of alternative jet fuels. A huge energy consumer — the department consumes more than 80 percent of the U.S. government’s energy use and about 1 percent of the country’s total energy use — DOD has been investigating alternative jet fuels for the past decade. The U.S. Air Force has already set a goal to fuel half of its North American fleet with fuels made from alternative feedstocks such as camelina, algae and cellulosic biofuels by 2016.

Now, alternative fuels may be moving into the commercial airline market. In May, United Airlines was the first U.S. commercial airline to conduct a flight using natural gas-derived synthetic jet fuel, which is, so far, the only alternative fuel type certified by the FAA for aviation use in already-existing engines. During the test flight, which used a mix of 40 percent synthetic fuel and 60 percent conventional fuel, engineers collected data on the fuel’s performance during takeoff, climbing, cruising and descent. The flight was a success: The synthetic fuel was deemed “safe for use” by the international technical standards organization ASTM.

The synthetic fuel, produced by Los Angeles, Calif.-based Rentech, is called RenJet and is derived from natural gas converted to liquid fuel by the Fischer-Tropsch process. Rentech says its fuel has several advantages over conventional jet fuel: For one thing, it is less dense, allowing aircraft to be lighter on takeoff and thus conserve fuel; for another, it produces dramatically fewer particulate emissions while on the ground. In 2009, Rentech agreed to provide its synthetic jet fuel to 13 airlines, including United. Rentech also signed a multiyear deal with eight airlines at Los Angeles International Airport to provide renewable synthetic diesel for ground service equipment starting in 2012.
The NextGen of reducing emissions

FAA is also looking into more immediate ways to improve efficiency and reduce emissions. The agency’s Next Generation Air Transportation System (NextGen) is a multipronged plan to redesign the national airspace and improve technology, updating the system in stages between 2012 and 2025, so that planes can travel closer together along more direct routes. Continuous descent, for example, has been shown to reduce nitrogen oxide emissions by a third, because a plane’s engines aren’t continuously throttling up and down during arrival; it also shaves off several minutes during approach and landing, further reducing emissions.

Critics, however, say that continuous descent is risky. Currently, FAA has strict guidelines about minimum spacing for landing aircraft; certain factors, ranging from pilot behavior (when they lower landing gear, for example) to wind patterns over an airport, can also affect this spacing. Continuous descent wouldn’t allow for as much wiggle room, and would require a highly sensitive air traffic control system — one that doesn’t yet exist.

Nonetheless, FAA Administrator Babbitt announced in March that this kind of traffic control system may arrive soon. As of last December, a new satellite tracking system monitoring the airspace over the Gulf of Mexico came online, and the plan suggests that this system will be nationwide by 2013. Since 2003, through a program called PARTNER, a research partnership between FAA and multiple universities, scientists have been investigating other ways to improve efficiency, from turning off engines while planes are waiting to take off to cruising longer at higher altitudes to changing flying routes so that the minimum amount of fuel is burned.

Up in the air

NASA is getting involved as well. In September, NASA hosted a green aviation summit at the Ames Research Center in Moffett Field, Calif., to discuss emerging technologies for the reduction of emissions and fuel consumption. And last year, NASA restructured its aeronautics research and created the Integrated Systems Research Program to accelerate the transfer of its technologies to the aviation community. Environmentally Responsible Aviation (ERA) — the program’s first project, working with FAA’s CLEEN program — began in October 2009 and focused on reducing, among other things, nitrogen oxide emissions during takeoff and landing as well as fuel burn. Like CLEEN, ERA is intended to engage industry in the effort to green aviation, according to ERA program manager Fay Collier. And that is likely to be the best way to more quickly get greener planes up in the air.

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