By Wg Cdr N K Pant (Retd) Scientists estimate that the effect of aviation emissions on the climate is up to five times the impact of emissions occurring on the ground. The primary gas emitted by jet aircraft engines is carbon dioxide, which, scientists believe, can survive in the atmosphere up to 100 years. It means denying the coming generations from living in a world where they can breathe clean air, enjoy diverse ecosystems and eat healthy food. Some of the major airlines of the world seem to be lately realising the adverse role of their big passenger jets on climate changes on account of emissions of carbon dioxide and other greenhouse gases from the engine-exhausts into the earth's atmosphere. In this context Virgin Atlantic's first flight of a commercial aircraft powered with bio-fuel from London to Amsterdam on February 24, 2008 to show it can produce less carbon dioxide than normal jet fuels was an appreciable endeavour. This particular flight was partially fueled with a bio-fuel mixture of coconut and babassu oil in one of its four main fuel tanks and is expected to produce much less CO2 than regular jet fuel. Since aircraft engines emit gases and particulates that reduce air quality, spearheading the test flight was indeed a potentially useful experiment aimed at cutting down emission of greenhouse gases. A few weeks prior to this, an airbus A 380 too had taken off on a similar test sortie, powered on a blend of regular fuel and liquid fuel processed from natural gas with the hope that the super jumbo will become a centerpiece of efforts to develop the next generation of cleaner fuel. Several airlines have also pledged to clean up their fleets as fears about global warming and fuel costs have mounted. The petrol and diesel driven vehicles leave harmful trails of smoke on the roads. Thousands of air turbine fuel (ATF) guzzling aeroplanes release tonnes of hazardous fumes into our biosphere daily. The increasing size of aircraft, the emission of black smoke during take-off, and the density of air traffic at major airports, have drawn environmentalists' attention to pollution by aircraft. A loaded jumbo 747, for instance, uses tens of thousands of litres of fuel on merely take-off. Pollution by aircraft arises from the jettisoning of spare fuel after being airborne. It must be released at a height sufficient to allow it to vaporise so that it does not reach the ground in liquid form. Commercial jet planes make a significant contribution to the problem of global warming. According to a UN report, aviation is responsible for over half of the pollution caused by transportation on the surface. In India the number of passenger and freight aircraft flown by private airlines is fast multiplying. The Indian Air Force is likely to increase its flying operations in the coming years. Its increasing use of supersonic combat aircraft flying at high altitudes may lead to increasing pollution of the upper air, where pollutants may accumulate since natural dispersion at such heights is not very effective. It is estimated that one multi-engine passenger aircraft is being added every fortnight to the fleets of India's domestic carriers that are engaged in cutthroat competition to fly to the remotest corners of the country. As a result, the number of aircraft flying the Indian skies has gone up considerably. Air Force, Army, Navy and the coast guard has more than 1,000 fighter jets, transport planes and helicopters kept aloft in routinely scheduled day and night flying.