Footprints in the Air

Reducing the carbon footprint

When it comes to cars, agreeing on a figure for carbon-dioxide emissions seems to be a straightforward thing: a car’s average mileage for in-town, highway and combined driving gives an overall average for carbon emitted per kilometre. With airplanes it is not quite that easy. And that is holding up an agreement for establishing a carbon dioxide certification standard for aircraft. The risk for airlines is that governments will end up regulating aircraft CO₂ emissions. One push in this direction is the EU’s plans to subject all airlines flying into or out of Europe as of January to the EU’s emissions trading system (ETS); one sign of resistance are the recent lawsuits filed against the US’s Environmental Protection Agency to introduce greenhouse gas emissions for aircraft. 

The conundrum for defining a standard lies with whether fuel burn, which equates directly to emissions, should be measured across a complete flight or just in the cruise phase (where fuel burn is lower), whether the standard should be based on aircraft gross weight or rather on payload and range, and whether it should be calculated for each individual airplane model or averaged across aircraft families.

Both airframers and engine makers have done great progress in making aircraft more fuel efficient in the past decade or so. On the airframer side, the use of composites and winglets, streamlined aerodynamics and greater reliance on electrical power for actuators and even tug duties have all contributed to making airplanes lighter and therefore more economical. Engine makers, in turn, have gone to great lengths refining their products with the use of clever electronics, composites, aerodynamic improvements and things like geared turbofans and very high-bypass ratios. Today’s engines burn but a fraction of the fuel of their predecessors. Biofuel blends are also starting to be used in some commercial flights.  

But that still leaves the problem of transforming this thriftier fuel burn into a carbon-emissions metric that can be either taxed or subject to the EU’s emissions trading system and, most importantly, that can provide a powerful incentive for further technological advances. Given that the commercial airframe and engine manufacturers have already set their product plans for the next decade, in the form of the Airbus A320 NEO and A350, and Boeing’s 737 MAX and the 787 Dreamliner, the CO₂ standard is taking on growing significance as a way to exert pressure to continue driving down fuel burn and emissions beyond what is already possible and into the territory of further technological developments.

Some experts argue that the most important factor is fuel efficiency in cruise, while manufacturers lean towards specific air range (distance travelled per weight of fuel) and maximum take-off weight, and the greener pundits advocate a standard based on mission fuel burn and payload/range performance.

A working group of the International Civil Aviation Organisation (ICAO) is seeking agreement on three elements that should form part of a standard: a fuel-efficiency metric, a point at which to measure this efficiency, and a means to compare the CO₂ emissions of aircraft ranging from small business jets to the megatransports now hitting the skies. But the group just came up with competing proposals and provided no definite date for one that can be palatable to all stakeholders.

The environmental groups propose measuring fuel burn at payload/range combinations during all phases of flight –climb, cruise and descent– representative of different missions, akin to the city and highway mileage metrics used for cars. This would mean that if you are transporting a heavier payload or flying a longer route, you would have to pay more carbon offsets. Proponents of this approach avowedly want to avoid creating a standard that merely leads to cruise-optimised designs.

Manufacturers prefer a simpler approach measuring the specific distance flown per unit of fuel at a percentage of maximum take-off weight, not least because cruise fuel-burn data is already specified in each aeroplane’s flight manual. 

Which standard to choose also bears upon the question of whether it will be applied to individual aircraft models or to entire families. In the second option,  performance against the standard would vary from the small A319 to the larger A321, or from the 737-700 to the 737-900. Not least, the impact the choice of standard would have on manufacturers' flexibility in designing aircraft and engines must be also taken into consideration.

Thus, agreeing on a standard is proving to be very difficult, both from a technical and a political perspective. The lack of progress in this regard reportedly pushed the EU to impose the ETS on foreign airlines, something that the EU Court of Justice has just ruled is permissible, raising the ire of airlines from America to China.

It is then in the best interest of both manufacturers and operators to agree soon on a standard. Technically, they have conquered far more difficult challenges in making their products so fuel-efficient. Devising a suitable carbon standard should be peanuts by comparison.

Politically, however, agreement seems more elusive. Maybe the financial impact of having to pay for their carbon footprint will work wonders to concentrate aviation minds.  

• CESifo Working Papers dealing with emissions
• CESifo Working Papers published in the past three months