Efforts to establish the first carbon dioxide certification standard for aircraft are proving difficult, and the recent failure to agree on a metric for CO2 emissions risks delaying completion of the standard. Any delay will empower environmental groups already putting pressure on governments to regulate aircraft CO2 emissions in the absence of a global standard.

Last year, the International Civil Aviation Organization's (ICAO) Committee on Aviation Environmental Protection (CAEP) set 2013 as the target for completing the CO2 standard. But last month the working group tasked with developing the metric on which the standard will be based failed to reach an agreement, instead forwarding rival proposals to a steering group meeting scheduled for September. The working group also proposed a timetable that would delay completion of the standard to 2015.

Differences center on whether a standard based on a simple measure of cruise fuel flow will put sufficient pressure on manufacturers to drive down CO2 emissions. Proponents of a more complex measurement scheme want to minimize the possibility that manufacturers and operators could “game” the standard.

With commercial-airliner manufacturers' product plans now essentially in place for the next decade or more, the CO2 standard is taking on increasing significance as a way of exerting pressure to continue driving down fuel burn and emissions. But because emissions equate directly to fuel efficiency, which is at the heart of competition between manufacturers, developing a global standard is proving contentious.

Among the issues are whether fuel consumption should be measured across a complete flight or just in the cruise phase, whether the CO2 standard should be based on aircraft gross weight or payload and range, and whether it should be calculated for each individual model or be averaged across aircraft families.

The concern for manufacturers is that a poorly designed standard could have unintended consequences for aircraft design. Environmental groups are worried that, like ICAO's noise and nitrogen oxide (NOx) limits, any CO2 standard will simply reflect what is already possible instead of forcing the development of technology to reduce fuel burn and emissions. Manufacturers argue that rising fuel prices will provide more than enough pressure to keep improving fuel efficiency.

The key to progress with the global standard is reaching agreement on the equivalent for aircraft of the miles-per-gallon (mpg) fuel-efficiency metric used for cars. “It is still Boeing's desire to get a metric decision as soon as possible, and get to a standard in this CAEP cycle, in 2013,” says Billy Glover, managing director for environmental strategy. The goal should be to “provide transparency and set a reference point, the same way things started for noise and NOx,” he says.

“As industry and regulators get experience, we can review the standard to make sure it keeps up with technology development and does not allow us to slacken off,” he says. This is the approach taken by ICAO on noise and NOx standards, but has been criticized as simply setting a minimum baseline and preventing backsliding rather than actively pushing technology for quieter and cleaner aircraft.

“Noise and NOx are technology-following standards that have lagged the capability of industry. The line is drawn above where engines are today,” says Juan Alonso, associate professor of aeronautics and astronautics at Stanford University. A “technology-forcing” standard is required to drive improvements in the absence of a strong pull from customers for lower CO2 emissions, he says. “Market-driven forces will not push until the demand is there.”

Glover, who argues that market forces will drive subsequent improvements, says: “Boeing believes very strongly that we need to set a baseline. Current design practices by manufacturers are so tied to meeting the fuel-efficiency requirements of airlines that for any design to be attractive it will have to do extremely well in fuel and CO2, because they are exactly correlated.”

Glover maintains that fuel efficiency in cruise “is the most important factor by far” in determining how to regulate carbon emissions. His comment highlights the difficult debate under way within the working group responsible for agreeing on the fuel-efficiency metric.

Of the rival proposals, manufacturers prefer an approach based on specific air range and maximum takeoff weight, while a “green” grouping led by the U.S. Environmental Protection Agency (EPA), the U.K. and nongovernment organizations favors a more “robust” multidimensional standard based on mission fuel burn and payload/range performance.

The working group is seeking to agree on a standard that has three elements: a fuel-efficiency metric; a certification test point, or “evaluation option,” at which to measure efficiency; and a “correlating parameter,” or means to compare the CO2 emissions of aircraft ranging from business jets to the Airbus A380. A “stringency line” would be developed that regulators can move down over time to tighten limits on CO2—the approach already used with noise and NOx.

Environmental groups propose measuring climb, cruise and descent fuel burn at payload/range combinations representative of different missions—equivalent to the city and highway mpg metrics used for cars. Manufacturers propose the simpler approach of measuring specific air range—distance flown per weight of fuel—at a percentage of maximum takeoff weight. “If you do well in the cruise you will do equally well on takeoff and approach, so why measure all three?” asks Glover.

Proponents of the mission-fuel approach want to avoid creating a standard that drives aircraft to cruise-optimized designs. Instead, they want to encourage robust designs able to perform a range of missions. Similarly, opponents of using gross weight rather than payload/range argue it could encourage productivity rather than efficiency and favor the design of larger and faster aircraft.

Boeing's position is that data for a cruise fuel measurement “is already in the aircraft flight manual,” says Glover, while “using payload as a parameter could reduce the utility of the airframe and make aircraft less efficient.” The choice of parameter is also tied to whether the standard will be applied to individual models or averaged over a family of the aircraft like the 737NG, where performance against the standard would vary from the smallest 737-600 to the largest 737-900.

There is disagreement over which approach will give manufacturers more flexibility to meet the standard. Manufacturers argue that a multidimensional standard would “decouple the levers” they use to optimize aircraft design and limit their flexibility, says an industry source close to the process.

Agreeing on a more-complex approach is expected to take longer, but manufacturers believe adopting the simpler metric based on specific air range could keep development of the CO2 standard on track for completion by late 2013. As it will take another 3-4 years to translate into law in ICAO member states, Glover says the standard could apply to new aircraft type-certified from 2016 onward.

A delay in developing and promulgating the standard would provide ammunition for U.S. environmental groups, which have launched legal action intended to force the EPA to introduce greenhouse-gas emissions standards for aircraft. This moved ahead in July, when the U.S. District Court denied a motion by the EPA to dismiss the lawsuit, filed by Earthjustice, challenging the agency's “unreasonable delay” in determining whether aircraft CO2 emissions are harmful to humans.

“The ICAO standard faces big hurdles, in development and political acceptance. And so far they are not talking about stringency, just about how to measure CO2,” says Sarah Burt, an attorney with Earthjustice. “We are concerned that any final standard is going to be pretty weak, and will be set at a level that will include all aircraft now in production.”

An endangerment finding under the Clean Air Act would set in motion action to regulate aircraft carbon emissions in the U.S., and one aim of the lawsuit is to force Washington to push for more stringent limits during development of the CO2 standard. “The U.S. is a big player at ICAO. If they go there with legal obligations hanging over their heads, the U.S. can move things forward,” says Burt. The lawsuit is also an effort to force the U.S. to act unilaterally, if the ICAO process is delayed. “The EPA has a stand-alone obligation to act under U.S. law,” she says.