Some close-quarter flying has provided new insights into aircraft pollution.
US space agency-led scientists flew small, instrumented, chase planes directly in the exhaust plume of a big jet to measure the sorts of gases and particles being thrown out.
The data suggests aircraft burning a mix of aviation kerosene and biofuel could reduce their climate impact.
This would come from a substantial reduction in the production of the sooty particles that make contrails.
“Those soot particles serve as nuclei for water vapour in the very cold atmosphere to condense on and for the artificial-looking linear contrails that we see when we look out the window,” explained Richard Moore from Nasa’s Langley Research Center.
“You’ll then see those lines spread and form cirrus clouds that weren’t there before the plane flew through the airspace.
“We know these contrails and cirrus clouds have a warming effect on the Earth’s climate, and it’s currently thought the warming effect associated with those clouds is more significant than all of the carbon dioxide emitted by aviation since the first powered flights began.”
Dr Moore’s team describes its research in this week’s edition of the journal Nature.
It involved flying a DC-8 at cruising speed and altitude – to try to simulate real-world conditions.
Much of the data previously obtained in studies is the result of ground tests, where a jet has been locked down and its engines throttled up. But the team wanted to see what really happened at 30,000 – 36,000ft (9,000 – 11,000m), where the air temperatures and pressures are much lower.
The DC-8’s engines were fed either Jet A fuel, one of the conventional kerosenes used by the world’s airlines, or a 50-50 blend of Jet A and a fuel derived from the Camelina oilseed plant.
To be sure they were sampling only the exhaust plume from a particular engine, the chase planes – from Nasa, the German space agency (DLR), and the National Research Council of Canada – had to fly extremely close to the back of the DC-8, just 30-150m behind each engine and directly in the plume.
This called for military levels of skill and very good communication between the pilots.
“It’s very exciting,” recalled co-worker Bernadett Weinzierl from DLR and the University of Vienna.
“You have to imagine the plane in front is travelling at something like 200m/s and you are less than 100m behind. But in fact it’s quite safe to go very close or indeed very far away. It is in between where it is very dangerous: there is an area where the wave vortex is so strong it would destroy the following plane.”
What the team found was that the blended fuel, taking account of varying flying conditions, was producing 50% less black carbon by number and up to 70% by mass.
“We were testing in what we call the soot-rich regime,” Prof Weinzierl said.
“Models tells us if you reduce the number concentration of black carbon then you will reduce the number concentration of ice crystals. So this could be a way to mitigate the climate impacts of aviation,” she told BBC News.
The logical expectation might be that by increasing the proportion of biofuel in the blend, even bigger gains could be made. But Dr Moore cautioned that there were infrastructure and engineering reasons why such an approach would not be straightforward.
“Biofuels, because they’re derived from a different feedstock, have very low concentrations of the chemical compounds common in petroleum-based jet fuels,” he said.
“One of these is aromatic species which are ring-like carbon compounds. These have important impacts for compatibility with current aviation fuel systems, and some older planes rely on the properties of jet fuels to swell the seals in their fuel systems.
“You can imagine that if you changed the composition of the fuel drastically some of those older fuel systems are not going to behave as they were originally designed.”
Nasa is currently developing an experimental plane in which it hopes to demonstrate, among its capabilities, low-noise supersonic flight powered by biofuel.