Cool running

 作者:嵇逯     |      日期:2019-03-07 04:02:08
By Paul Marks A SIMPLE idea could make flying much safer. Tests in the US have shown that cooling fuel before it is put into aircraft prevents explosive fumes building up in the fuel tanks. The explosion of TWA flight 800 off Long Island in July 1996, which killed 230 people, is thought to have been caused by the ignition of fuel vapour in one of the jet’s fuel tanks. The US Federal Aviation Administration has since been looking at many ways of improving fuel tank safety, including eliminating any potential sources of sparks. But Terry Koethe, a Texan inventor, believes he has hit on a way of making planes safer without the need for any expensive modifications to the world’s 12 000 commercial jets. “We do not alter the composition of the aviation fuel in any way—so it meets all specifications as there are no magic additives,” he says. “All we are doing is cooling the fuel.” The idea is to minimise vaporisation, so there is no chance of an explosion even if static electricity or faulty wiring creates a spark. The temperature of the fuel delivered to an aircraft from an underground tank or a tanker ranges between 15 °C and 33 °C, depending upon the ambient temperature. But since the TWA 800 crash, industry research has revealed that fuel can get far hotter than that before take-off, especially if a plane has been sitting in the sun. And as an aircraft climbs, says Koethe, the drop in pressure draws more fumes into the tank headspace. In such cases, he reasons, safety will depend largely on the absence of sparks. Koethe thinks it is better to severely reduce the formation of combustible vapour. His company, Fuel Dynamics of Arlington, Texas, has developed a system that cools fuel to –1 °C or below before it is put into an aircraft. “This is the ideal safety temperature, where we get fewer hydrocarbon vapours, and keeps it out of the flammability range,” he says. The system also injects nitrogen gas during fuelling, flushing out oxygen and thus reducing the chance of an explosion even further. Last week, Koethe, an FAA observer and a team from the University of North Dakota tested the Polarjet cooling system at an airbase in Fort Worth, Texas. They loaded one wing tank of a Learjet with cooled fuel and the other with fuel at a normal temperature. After the plane sat on the tarmac for an hour, the university team found that there was only 20 per cent as much fuel vapour in the space above the cooled fuel as in the other tank. Levels of hydrocarbon pollutants escaping from the cooled tank’s vents were similarly reduced. “I was sceptical about this cooled fuel approach, but the benefits are significant,” says Ted Aulich, project manager at the University of North Dakota’s Energy and Environmental Research Centre. “You will be way below the lower explosive limit of jet fuel vapours in air for a long, long period of the flight.” Tom Haueter, a safety expert with the US National Transportation Safety Board,