The European giant Airbus has given itself the objective of placing on the market by 2020 aircraft that emit half as much CO2 and 80% less nitrogen oxide (NOx) and that make only half as much noise as aircraft designed in 2000. The goals are those of the Advisory Council for Aeronautics Research in Europe (ACARE), made up of representatives of the European Commission, member states and industry. This ambitious research agenda is seen as unrealistic by some. One thing is certain, though: fluctuating oil prices, the depletion of oil resources and public policies such as ETS (emissions trading scheme – see other article in this edition) make research to develop lighter, more aerodynamic aircraft that consume less fuel a vital economic challenge.

There are many ways forward. Research teams are working on reducing aircraft drag (i.e. all the forces of resistance to movement, which require higher fuel consumption to take the aircraft forward), lightening structures (replacing traditional materials by composite materials), improving engines and optimising air traffic management (the aircraft’s descent technique has an impact on fuel consumption). Work on substitute fuels has also been stepped up in the last few years.

Clearly, however, the scientific community is not necessarily aiming for an absolute revolution. «In terms of lightening up structures, we’re mostly at the stage of cutting pounds,» explains Christophe Mathieu, Director of Aeronautics Development at ONERA, the French aerospace research centre, leader in its field in France (1,500 engineers and scientists). On the wide-body Airbus, the A380 for example, switching from aluminium alloys to carbon composites - a world first - saves around 1.5 tonnes on the central wing box (which connects the two wings and connects the wings to the fuselage). This figure may not seem particularly impressive but it has to be remembered that every kilo saved reduces fuel consumption and consequently polluting emissions. Small savings are also sought when it comes to reducing drag, a fundamental aspect for saving fuel. «On a medium-haul aircraft like the A320, we’re trying to save a few percentage points with the next generations,» notes Christophe Mathieu.

Will these percentages be enough to meet ACARE’s ambitious goals and to develop a really less polluting means of transport? «It’s ambitious,» admits Christophe Mathieu, «but the entire industry and research community is involved and everything is being done to meet this challenge. I wouldn’t say it’s unrealistic. We still have 11 years of research ahead of us.» His colleague Paul Kuentzmann, Senior Scientific Adviser at ONERA and specialist in alternative fuels, agrees: «We will be very close in 2020».

Not everyone is that optimistic. In May 2008, the Technical University of Berlin published a study commissioned by the German environment ministry estimating that aircraft would not meet the ACARE objectives until 2050. Real efficiency gains will require technological breakthroughs, completely new concepts in aircraft design that will not be available in the short term.

Christophe Mathieu even admits that «a time comes when you cannot go on indefinitely. You feel the curve of scientific progress slowing simply because it is clear that classic aircraft are already fairly optimised». Many think that current aircraft are about as close to the best that can be hoped for with classic solutions and that new aircraft configurations will be needed to achieve significant efficiency gains. There is regular discussion, for example, of a «flying wing», a concept of an aircraft without fuselage or a tail unit, considered the most efficient from the standpoint of aerodynamics. Manufacturers are working on this concept.

WHAT ABOUT THE POST-JET FUEL AGE?

This is another subject of widespread concern among aircraft and engine manufacturers, particularly in the past few years as a result of surging oil prices and projections on the depletion of oil reserves. «We are no longer in a cyclical movement, but in a fundamental change,» sums up Christophe Mathieu. Paul Kuentzmann shares that view, even though «it’s an old story that comes up with nearly every oil crisis».

What the sector seeks is solutions that could be used to replace traditional jet fuel in existing aircraft with only minimal changes to planes and engines (known as «drop-in»). Certain options have already been ruled out, including ethanol, whose calorific value is well below that of jet fuel (requiring bigger fuel tanks), and unprocessed vegetable oils that freeze between 0° and -10° (the temperature drops below 50°C at 11,000 metres). Work is fairly advanced on synthetic fuels manufactured from fossil resources – coal, natural gas and biomass (see interview with Paul Kuentzmann). However, their environmental performance leaves room for improvement because refining such products produces a lot of CO2.

That leaves biofuels. A number of highly publicised test flights have taken place in recent months, suggesting that solutions are within reach (see box), but the fact is that it will still be quite a while before aircraft are fuelled by biofuels, not before 2020 or 2030. It is an option to which aircraft and engine manufacturers are actively committed, however.

«A wide range of plants can be used as a starting point,» explains Paul Kuentzmann. «What we are aiming for in aeronautics is second-generation biofuels manufactured from agricultural residues (branches, leaves, etc.). We have already demonstrated that such fuel can be manufactured on a small scale. Forest resources – wood chips, sawdust, etc. – are another interesting option for Europe.» There is also seaweed, which offers the advantage of growing much faster than terrestrial plants (between 30 and 300 times as fast) and which can be grown in tanks, lagoons, fresh water or salt water. «For now, little is being manufactured but it is a very interesting area of research.»

What is certain is that aircraft manufacturers believe in biofuels. To avoid the debate on flying versus eating, the scientific community has committed to developing biofuels that do not compete with food production, do not endanger drinking water reserves and do not cause deforestation. All they have to do now is to put those commitments into practice...

Who is flying with what?

- February 2008: Virgin Atlantic, in partnership with Boeing and GE, operated a commercial flight (without passengers) between London and Amsterdam of a Boeing 747-400 fuelled partly by biofuel manufactured from palm oil and coconut oil. One of the tanks contained traditional jet fuel blended with biofuel. Neither the aircraft nor its engines were adapted for the test. The flight was a world first.

- February 2008: Airbus and Rolls-Royce conducted a similar test on an A380, with a liquid synthetic fuel produced from natural gas (GTL). The aircraft flew from Filton (near Bristol) to Toulouse. Three of the engines operated on kerosene and the fourth on gas. The test was carried out under an agreement signed in November 2007 by Airbus, Rolls-Royce, Shell and Qatar Airways, which plans to become the first airline to use this type of fuel (Qatar owns 15% of the world’s gas reserves).

- December 2008: Air New Zealand operated a test flight of a Boeing 747-400 with biofuel (50% jatropha). The test results show that, for a 12-hour flight, CO2 emissions could be reduced by 60 to 65% compared to traditional fuel.

- January 2009: Continental Airlines flies a Boeing 737-800 for one and a half hours with a jet engine fuelled by a blend of biofuel (algae and jatropha) and traditional jet fuel.

- January 2009: Japan Airlines ran a test flight of a Boeing 747-300 fuelled partially by second-generation biofuel. One of its four engines was fuelled by a blend of biofuel (camelina and jatropha) and traditional jet fuel.