14 September 2010, by Adele Rackley
Tucked up snugly in the cabin of an airliner, it's easy to forget that outside the temperature is probably in the minus 50o Celsius – easily low enough for ice to form where it's not wanted.
Happily there are few incidents where ice is thought to have caused engine problems, but it remains a risk that's not yet fully understood. Now researchers believe they've come a step closer to understanding what's behind it.
The story begins with the crash landing of a Boeing 777 at Heathrow airport in 2008, after both engines failed almost simultaneously. The accident investigation cited ice crystals in the fuel system as the likely cause, reporting that in tests ice particles became 'sticky' and began to accumulate between -5 and -20oC.
Researchers working on ice formation in the atmosphere, in the School of Chemistry at the University of Leeds, and their contacts at ice crystallisation specialists Asymptote Ltd in Cambridge, realised their own research was directly relevant to this investigation. They were surprised to read that the test engineers' mock-ups of the incident had been based on the assumption that water freezes at 0oC.
Sounds sensible enough to most people, but it's not the full story.
Water does freeze at 0oC. But it's possible for droplets of pure water, with no solid particles to contaminate them, to remain liquid well below this – in fact if they're small enough they can 'supercool' to as low as -36oC. At this temperature they begin to freeze spontaneously – known as homogenous freezing.
But, if the droplets come into contact with solid particles or surfaces they begin to freeze before they get this cold - the solid catalyses the freezing process, producing what's called heterogenous freezing. (Follow the link to the Leeds team's YouTube clip to see this process in action.)
The collaborators suspected that supercooled water – not ice crystals – had been present in the crashed aircraft's fuel system. To put their theory to the test, they wanted to demonstrate clearly that micron-sized water droplets can remain liquid in aircraft fuel at very low temperatures.
'Because we could use the equipment designed for our atmospheric research, it was very straightforward,' explains Dr Benjamin Murray, now at Leeds' School of Earth and Environment, lead author of the team's report which is published in the journal Fuel.
The team submerged micron-sized droplets of water in drops of Jet A-1 fuel, enclosed them between two glass slides and cooled them to -41oC. During the cooling process they filmed the drops with a digital camera, so they could watch what was happening inside them as the temperature fell.
As they played back their recording they could see that droplets at -30.9oC were clearly still liquid. But once they were cooled to -36.6oC they had begun to freeze and at -39oC all the droplets had frozen.
So, instead of ice sticking together, the researchers think the problems could have begun when supercooled water droplets came into contact with solid surfaces in the aircraft's fuel system, instantly freezing and resulting in a build-up of ice that could have restricted the flow of fuel.
The work doesn't provide all the answers, but it's important if engineers are going to find the right solution. 'We need to define the problem properly,' says Murray. 'You really have to know if you're dealing with liquid or solid particles or you won't be able to understand exactly what's going wrong.'
It's an important insight for aviation engineers who are trying to find ways to prevent or engineer around the problem. And it's a nice example of basic science finding unanticipated, direct applications to real-life problems.
But, cautions Murray, further work is needed to understand the range of temperatures over which different solid materials found in aircraft fuel systems cause this kind of ice formation. 'This is critical if we are to find a solution to the problem,' he says.